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NATIONAL  COMPANY,  INC.

 HRO  - COMMUNICATION RECEIVERS - "The Cream of the Crop"

  Design and Production History  ~  The HRO Models

Serial Numbering Analysis  ~  Dating by Serial Number 

   Engineering Changes  ~  HRO Coil Details, PS Details, Accessories 

  Servicing the PW-Gear Drive & the PW-D Micrometer Dial
 
 Restorations in Detail  ~  Collector's Photo Gallery HRO Receivers

  Competition Comparisons  ~  Performance Evaluation


by:  Henry Rogers WA7YBS


 W9DXX, Alice Bourke, Chicago, IL ca.1935 -  from Frank C. Jones "Radio Handbook" 1936. This interesting photo shows a very early HRO receiver with a Peak Pre-selector to the left and a 1934 AGS-X receiver to the right. Alice Bourke was a reporter for the Chicago Tribune along with being very active in ham radio.

NATIONAL  COMPANY,  INC. 
HRO  -  COMMUNICATION RECEIVERS

"The Cream of the Crop"

Without a doubt the finest creation to come from National Company, Inc. was their HRO Communications Receiver. Introduced in October 1934 (first production run January 1935 with first deliveries in March 1935,) the HRO featured incredible performance capabilities coupled with an anachronistic, almost scientific instrument appearance that certainly appealed to the technically-minded ham. The host of accessories necessary for complete operation included a power supply, a speaker and four plug-in coil sets. The HRO main tuning dial was unlike any other - a non-illuminated micrometer device that displayed numbers behind small openings as the dial was rotated. Each of the coil sets had graphs that could be correlated to the micrometer readout to determine tuned frequency. Much of the HRO's engineering seemed to contradict everything that was happening with contemporary receiver design. Why did a receiver that seemed to defy then-modern communications receiver evolution become such a favorite of hams, the military and commercial users? By providing absolutely the best low noise front-end resulting in high sensitivity coupled with an incredibly well-designed tuning system along with tremendous bandspread capability, giving the user the ultimate advantage when it came to working rare DX or coping with challenging band conditions. - H. Rogers, April 2007

History of the HRO Design

AGS Receiver

 The HRO design owes much to its predecessor, the AGS receiver. The AGS was developed to fulfill a contract with the Department of Commerce for modern receivers for airports in 1932. The initial receiver designed for the contract was designated the RHM. It is likely that Herbert Hoover Jr. and his design team were involved to a certain degree in the electronic design of the RHM while National provided the mechanical design and assembly. The final product was a high quality, high performance receiver built from the best parts available. The accuracy of the Type-N vernier dial was excellent and the receiver's sensitivity quite good. Since the contract was only for a handful of receivers, National decided to also produce the RHM as a civilian communications receiver called the AGS. Though expensive, National felt there must be a market for a high performance receiver, even during the Depression. Within a short time, National upgraded the AGS with different tubes and different calibration procedures in an effort to make the receiver easier to produce. By late 1933, the ham version of the AGS, the Single Signal AGS-X, made its appearance. The AGS-X had a crystal filter, amateur bandspread coils (optional) and a front panel adjustable BFO. At nearly $300, these receivers were for affluent enthusiasts and very few were sold. By 1934, AGS coils for 10 meter operation were being offered. Meanwhile, the commercial  users (mainly airlines) were complaining  about the design quirks of the RHM-AGS. Three coils were difficult to handle during a band change and the receiver had to be turned off to facilitate a coil change since there was no standby switch. Additionally, the Type-N dial was beginning to slip on these receivers as the lubrication dried up on the rack and pinion drive to the tuning condensers. A redesigned AGS appeared for commercial users as the AGU, featuring a coil assembly that carried all three coils as a unit for easy band changing. The vernier dial was changed to a National Type-BX (similar to the dial used on the SW-3.) Again, tube line up was changed but it was becoming apparent that the design was aging rapidly and a new receiver was going to be necessary. In fact, some of the airlines never used the AGS-AGU receivers because they felt the design was not sophisticated enough for their requirements (Transcontinental and Western Airlines in particular.) A modern superheterodyne with double preselection was needed. >>>

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photo above: The original National Co., Airport Superheterodyne - the 1932 RHM sn: 4

>>>  Herbert Hoover Jr. was selected by the Bureau of Aeronautical Commerce to coordinate the job of designing a replacement for the AGS receiver. He was living in Pasadena, California and teaching part-time at Cal Tech. Hoover, of course, contacted James Millen at National Co., since the creation of a "sophisticated" design was going to require the expertise that National had gained building the AGS receivers. Hoover setup a lab in his garage, employing Howard Morgan from Western Electric Co. and a few of his technicians to develop the new receiver circuitry. The new receiver would be a team effort with engineers working on the project on both coasts. 


photo above: Artwork for the HRO Prototype published in National Bulletin No.240 showing the receiver with AGS style knobs, a Volume control, a single coil frequency graph, no S-meter switch and many other differences from the actual production HRO receiver. Bulletin No.240 was published in the October 1934 issue of QST

HRO Design

In order for the "new" AGS replacement to meet the requirements of commercial users, Millen and Hoover thoroughly examined the shortcomings of the AGS/AGU receivers and additionally, examined all of the necessary requirements for an ultimate communications receiver. The two design teams - James Millen, National's Chief Engineer and General Manager, headed the mechanical design team which was located on the East Coast in Malden, Mass. While on the West Coast, Herbert Hoover Jr. headed the electrical design team. Both men agreed that the greatest sensitivity and a low noise floor came from using plug-in coils, eliminating the many types of signal loss found in most bandswitching circuits at that time. Also, to keep other kinds of noise (like hum) out of the receiver an external power supply would be used. An added benefit of the external supply was the additional heat was not in the receiver cabinet. Maximum efficient operation of each tube stage would keep the number of tubes to a minimum and resulted in a good signal to noise ratio and low drift due to reduced thermal problems.

Double shielding would be used on the coils for frequency stability and the coil set would be located at the bottom of the receiver, away from heat. For the additional ham market, a bandspread option on the plug-in coils that had been popular with the SW-3, FB-7 and the AGS and would be continued with the HRO - National was not going to exclude the very profitable ham market. Dial accuracy was a difficult problem to solve and required tight specifications on many of the components used in each receiver.  >>>

 The tuning condenser drive used a spring-loaded split-gear driven by a spring-preloaded worm gear eliminating any backlash. The dial itself was based on a Sperry Gyroscope design that had the main dial rotate an internal readout dial via an elliptic hub. The 0-500 readout dial was viewed through the top window of the external dial's five windows and, with ten revolutions, the micrometer dial mechanism had the equivalent resolution of a linear dial 12 feet long. Accuracy of the reset ability would be incredible. More design necessities were double preselection on all coil sets, a front panel adjustable BFO, a crystal filter, a S-meter and a front panel stand-by switch (B+ switch.)

Towards the end of development, Millen personally delivered a prototype HRO to Hoover's lab in Pasadena for final revisions and final testing. At this time, a drift problem turned up when the HRO was used in the bandspread mode. The corrections utilized combinations of brass, steel and aluminum mounts for the trimmers in the coil assemblies to reduce the problem. However, with the resolution the micrometer dial had in bandspread a minor drift of a few kilocycles meant that drift showed up as several divisions on the micrometer dial. The drift problem was reduced to an acceptable level but it could not be eliminated due to the extreme resolution the HRO had in bandspread. The HRO was designed to use mostly parts that National made. Very few purchased parts were utilized for the HRO. As National stated, "The National HRO receivers are not an assembly of broadcast receiver parts, they are completely designed from antenna to output." In all, the HRO design leapt ahead of any other receiver being built in 1934.


Delayed Release
- James Millen had earlier started a monthly "open letter" to all hams, published in each issue of QST. Usually the letter told about projects that National was working on or sometimes interesting problems were written about. Generally, the letters were used for fomenting ham interest in new National products. Millen announced that National was working on a new receiver in the October 1934 issue of QST, however the HRO wasn't even close to release yet. In the December 1934 issue, Millen told his ham audience about the instability problems facing the new receiver and that the release was delayed until January 1935. Millen candidly related that the problem would not have been noticed except for the accuracy of the micrometer dial. Apparently the problem was more difficult to solve than anticipated for the January release date was also missed.  >>>


photo above:
  This 1935 ad is from Leeds, a New York radio dealer, showing the high regard that was accorded the new National HRO. This artwork shows the production receiver.

 >>> Production HRO receivers were being built as early as January 1935 but these units weren't shipped out until all of the "bugs" had been worked out. HROs finally began shipping to awaiting customer orders around March 1935. The first HRO receivers were eagerly purchased by National fans because they knew the receiver was going to be the best available since James Millen had been keeping all of the hams informed of the HRO's progress by way of his monthly open letter to hams that was published in QST.

Prototype HRO Receivers -  When National published their bulletin No.240 (a catalog that was inserted in the October 1934 issue of QST) it contained details on the HRO and revealed artwork of a "prototype" HRO (which is shown in a photo above.) The "HRO Prototype" receiver did have a micrometer dial but note that the knob is the type used on National's Type N dial (used on the AGS.)  The receiver had AGS style knobs installed. Also note the prototype had a "Volume" control and a BFO switch on the front panel but no front panel BFO adjustment. Also, the coil sets only have one graph which implies that they don't have the band spread function. Absent also is an S-meter switch. Though the artwork of the interior shown in Bulletin No.240 doesn't show the internal BFO knob on the BFO coil can, it was shown in subsequent artwork and photos of other prototypes. These prototypes were never production receivers and were built for testing and development purposes, for artwork or for subsequent photographs.

How the HRO became the HRO and not the HOR

The original published story for the origin of  the HRO designation related that all of National's inter-departmental paperwork for the receiver project was stamped "H.R.O." which stood for "Hellva Rush Order" since the time table for the receiver development was a "rush order" type of project. For many years this was the story related in National advertising and it sounded believable. However, after James Millen left National in 1939, he corrected the story as follows:

The original development paperwork was usually marked "H.O.R." - for "Hell Of a Rush" but during the finalization phase, someone at National decided they didn't want their new receivers to be referred to as "HORs" so the letters were rearranged and became HRO - then the "Hellva Rush Order" story created to explain the HRO designation.

Well,...that's Millen's story anyway.   

 

The HRO Models

 

HRO

When first introduced, the receiver was the "HRO." No other designation was necessary since it was the only version available. The receiver used nine, 2.5vac heater tubes providing two RF amplifiers, two IF amplifiers, separate Mixer and Local Oscillator functions, a Duplex Diode-Pentode for AVC, Detector and First Audio Amplifier functions, a Beat Frequency Oscillator and a pentode Audio Output Amplifier. The separate power supply utilized a type 80 tube rectifier that actually brought the tube total to ten.

Typical of most communications receiver manufacturing, progressively later models will have several minor improvements added as production continued. During early production both rack mounted versions with 3/16" thick aluminum panels finished in a textured "crackle finish" that resembled leatherette were produced along with the standard table model that was painted with standard wrinkle finish. It's likely National anticipated that a large number of the initial orders for the HRO would be from commercial users and thus many of the first production run receivers were the rack mount variety. In fact, 60% of the reported serial numbers to WHRM that are "first production run" HRO receivers are from rack mount receivers (see Serial Number Log in section below.)

A specific power supply wasn't available for the early HRO receivers, so many receivers were sold with the FB-7 power supply, type 5897AB with the data plate emblazoned "Designed Especially for the FB-7." Many rack mounted power supplies were also produced for the commercial users. Speakers were not specifically available in a table cabinet but National advertising stated that a "loud speaker could be provided, if desired." The first advertised speaker shown was a rack mount version.

Shown in the photo to the right is a rack mount HRO from the first production run with the serial number of D-65. Note that this receiver has all of the features that are found on the earliest production. D-65 has the pearl-button push switch for the S-meter and the small red "NC" dial pointer mounting screw. Also obvious are the white background frequency charts on the coil sets. Notable is the leatherette finish on the panel and on the coil set panels, although this finish was used on the rack mounted receiver up until around 1940. This receiver was probably built around January or February of 1935.
 


photo above: HRO D-65, first production run built in January 1935



photo above:
  HRO  sn: H-103 was built in September 1935 (5th production run.) The "H" production run was the last to use the plated PW-D, the 0-5 scale S-meter (later 0-5 scale) and the early-style cabinet.

Compare D-65 from the first production run to fifth production run receiver, H-103, shown in the photo to the left. H-103 is a table model rather than a rack mount so it has the standard black wrinkle finish. H-103 has several production upgrades incorporated with the most noticeable being the black push-pull S-meter switch that replaced the pearl-button switch after the second production run. The original push-button was inconvenient to use and also was noisy in operation. Also apparent is the pilot lamp. It was noted that there was no visual indication that the receiver powered-up so the pilot lamp had to be added by the third run. The frequency charts became black background after the middle of the third run. Moisture may have caused staining on the white charts that prompted the change to black charts. Internally, a Remote Standby had been added and there was an improvement to the plug-in coil contacts. The sixth production run, run-J, would bring several changes to the HRO. Gone would be the plated PW-D dial in favor of a lacquer painted version. The cabinet would also now feature improved ventilation with louvers on the side and enlarged holes on the rear. The S-meter scale was changed from the old QSA 0-5 scaling to the RST scaling of 0-9. These "J-run" receivers were advertised as the "1936" HRO.

The HRO's design was much more advanced than any other communications receiver available in early 1935. James Millen, using his monthly QST letter to hams, had the ham interest piqued and both commercial users and hams rushed to buy the first HRO production available. Of course, the HRO performance became legend quickly and, if the receiver was affordable to a depression-era enthusiast, it was purchased. The production runs were very small and by the introduction of the HRO Senior in 1936, about 1100 HROs had been produced and sold.

Features Found on the Earliest HRO Receivers - Shown below are some of the external parts and manufacturing variations that are found only on the early HRO receivers, that is, the receivers built in 1935. Photo "A" shows the German Silver plated Micrometer dial. This dial was used for the first five production runs (D, E, F, G & H.) Photo "B" shows the "NC" emblem that is used with the dial pointer. This emblem was exclusively used on the initial production run (run-D.) It was then used for most of run-E and sporadically on runs-F and G alternating with the "diamond" emblem (see HRO Senior below for close-up photo.) Photo "C" shows the pearl push-button S-meter switch that was used only for two production runs, runs-D and E, along with the "raised-rounded flange" S-meter only used for the E-run. Additionally, the S-meter shown has the early 0-5 S-Units scaling that was used on runs D & E. By run F, the "PLUS" was dropped and the NC diamond was changed to red with black "NC."  Photo "D" shows the white background frequency graphs that were used for three production runs, runs-D, E and F. Photo "E" shows a small dial-knob with the short boss (left) used on run-D through part of run-J. The small dial-knob to the right has the taller boss used from production run-J-on. Photo "F" shows the black painted chassis that was found on production runs-D through L. This photo also shows the round IF and BFO shields (cans) that was used from runs-D through P. Photo "G" shows the early cabinet ventilation holes that were used from production run-D through J.  (photos B and C are by Gary Halverson K6GLH of his E-run HRO)

A

B

C

D

 

E

F

G

 

HRO Junior

The HRO was an expensive receiver, priced at about $200 with all of the accessories necessary for operation. In the January 1936 issue of QST, James Millen indicated that he had been in contact with many hams who were HRO owners - either through letters or actual visits to their ham stations. He came to the conclusion that most hams didn't use the HRO to its full capabilities. Most hams never used the Crystal Filter. Many never switched the coil sets to amateur bandspread. Most hams were on CW at that time and never used the S-meter. Millen also indicated that some letters from hams actually inquired if they could order the HRO - minus specific circuits they believed they wouldn't need - at a reduced price, of course. Millen thought that offering a budget priced HRO, that eliminated these unused circuits and parts might be a good seller. Certainly, it would be a way for the Depression Era ham on a strictly limited budget to get an HRO. National announced the reduced-cost HRO in February 1936, dubbed "HRO Junior." From that point on, the standard HRO became known as the "HRO Senior."

The HRO Junior didn't have a crystal filter. This modification required that the Crystal Filter assembly be replaced with a standard IF transformer. Elimination of the S-meter also eliminated the meter push-pull switch and meter adjustment pot circuitry.

photo right: HRO Junior sn P-116 from August 1936 with its original P-116 JD coil set installed. Note that this receiver has the early gun-metal gray lacquer on the PW-D micrometer dial. Later dials will have a dark bronze color lacquer paint. Note that the pilot lamp location is different than the HRO Senior. Also, the location of the phone jack is moved since there is no S-meter switch (or S-meter.) Also obvious is the centrally located frequency graph on the coil set. Since the Junior coils didn't bandspread only one graph was necessary.

Naturally, a different front panel was going to be required since the meter wasn't used. National also moved the location of the pilot lamp slightly to the right and moved the phone jack slightly to the left. The Junior used the same chassis as the regular HRO, obvious because the S-meter pot mounting hole is present as are the mounting holes for the Crystal Filter assembly.

The coil sets for the "Junior" did not have the bandspread feature and were identified with a "J" proceeding the regular coil set letter designator. Since "J" coils couldn't be set to bandspread only one tuning chart was needed and it was centrally mounted on the coil panel. The "Junior" was usually offered for $99 from most of the discount dealers but this price only included one coil set. Millen recommended equipping the "Junior" with two coils sets, the JA and the JC for the CW operator giving coverage of 20 and 10 meters on the JA coil and 40 and 80 meters on the JC coil. For the AM phone op, the JA and JD coils gave 160 and 80 meters (JD) and 20 and 10 meters (JA) - at that time (pre-WWII) 40 meters was a CW only band. Of course, a purchaser could order as many coils as they wanted although the idea of the Junior was to keep the cost down by utilizing only what was going to be absolutely necessary.
 

photo left: The chassis of HRO Junior P-116 showing the absence of the Crystal Filter and S-meter. Note that the chassis is punched for the S-meter adjustment pot which was located next to the Antenna-Ground terminal insulator. This Junior has the gray chassis paint which started to be used in early 1936. The IF cans had just changed to the square type with the "P" production run.

The HRO Junior was not a "reduced cost" HRO in the sense that it would have been built using cheaper parts. Rather, the Junior used all of the standard Senior parts and assembly techniques - just the S-meter, the Crystal Filter and the bandspreading coil sets were not used. The cost was less than the standard HRO because certain circuits and parts were not used - circuitry and parts that some hams or other users might find superfluous to their needs. Since the Junior had almost everything a Senior had it was natural that National offered to upgrade a Junior after purchase (at a later date - perhaps when the owner had saved up the money) for a very reasonable price. Perhaps some Juniors were sent back to National for conversion to an HRO Senior though there doesn't seem to be much evidence that it happened too often. A new front panel would have been required along with the Crystal Filter assembly and S-meter with the S-meter switch. Then there was the problem of the J coil sets. Although the conversion was offered, it must have been done at a "loss" for National, who probably realized that very few Juniors would ever be returned for the conversion to Seniors.

The HRO Junior wasn't very popular and today it is quite a rare set in its original configuration. Most thirties-era hams probably realized that purchasing the Junior wasn't that good of a "deal." The HRO Senior was around $170 with four bandspreading coil sets included. Most dealers would sell the HRO Senior with some money down and monthly payments to follow. The Junior was $99 with one general coverage coil set. Just adding the three coil sets to provide full frequency coverage would run the price up to about $150 and then these coils would not bandspread. The price would be even more if bandspreading coils were desired. For only a few dollars more, the HRO Senior provided many more features. Power supplies and speakers were considered extra accessories for both types of receivers.

Most "Juniors" encountered today are really RAS or RBJ Navy receivers - rack mounted HRO Juniors with some modifications for military use. Like Millen had observed with the hams, the Navy found their radio operators had little use for an S-meter, Crystal Filter or Bandspread coil sets.



photo above:
1936 advertisement for the Junior's introduction.

 

HRO Senior


photo above:  HRO Senior sn: 463-K was built in 1940 and has most of the later characteristics.

From February 1936 on - coinciding with the HRO Junior introduction - the standard HRO was referred to as the "HRO Senior." From this point up to just before the WWII, the HRO went through several minor physical changes. Most noticeable was the change from the German Silver plated PW-D micrometer dial to a black lacquer dial. The black paint color on the PW-D micrometer dials is actually a dark gun-metal gray on early painted dials and later is a very dark bronze color. Under intense light the PW-D will show its true color shading. The inner dial readout also changed from black numerals on silver to white numerals on black paint. The cost savings were realized by Doehler (the manufacturer of the PW-D) in that they could now use castings that had pits and other defects that would be "filled" by the lacquer paint. Also, since the index lines were going to be filled with paint, they were widened so the paint fill would appear the correct width when finished (early painted dials had the narrow index lines.) Other changes were to the chassis paint, probably to conform with the paint used on the new gray paint being used on the 1936 NC-100 receivers. This cost savings was realized in that only the small quantity SW-3 continued to have the black chassis paint. Most of production now used gray paint for all chassis.

In the December 1936 issue of QST, National ran an ad on the back-inside cover showing a rack mount HRO finished in gray "leatherette" finish. The photo below shows a table rack designated MRR and a combination speaker and coil storage box designated SPC. The ad states that the rack mount HRO is available, "Your choice of finish, either rich grey or black leatherette." (note the British spelling of gray.) A close-up of the receiver shown in the ad is to the left. This ad ran several times in QST from 1936 up to around 1938. Note the bound QST magazines on the book shelf.
For 1937, the IF transformers were changed to different adjusting screws with slots instead of hex heads and the shields were changed to square rather than the earlier round shields. Again, the cost savings was that the the HRO would conform with the NC-100 series, although better quality IF transformers were probably also involved in the changeover. Another minor change at the time was to the remote standby terminals which now placed the "BSW" terminals in series with the panel B+ switch requiring a jumper be installed across the terminals if the receiver was used without a remote T-R switch. This seemingly minor change actually now provided the operator of a transmit-receive station the ability to remove the B+ independent of the T-R switch - a function required for safe coil set removal and installation.

The next change (still in 1937) was to the S-meter - going from a non-illuminated white scale unit to an illuminated light-yellow scaled meter built by Marion Electric Instrument Company. The S-meter also added "db above S-9" to the scale in red printing. This change was probably more to have the HRO conform with the competition. Finally, an identification tag was added to the upper right corner of the front panel in 1938. This was the first time that the actual receiver model is physically identified in any manner. This completed the physical evolution of the HRO from 1936 to 1938. From then up to WWII, the HRO looked the same except that some very late HRO Seniors will have a bar knob for the selectivity adjustment.

By the later thirties, most of the HRO receivers were being equipped with 6.3vac tubes. One of Millen's QST letters (1937) had pointed out that the 2.5vac heater tubes were preferred by National since the 6.3vac heater tube were subject to producing modulated hum. In 1939, the 697 power supply was introduced featuring a 6.3vac heater winding with sufficient current to operate the HRO and packaged in the "dog house" style cabinet. At this time, the older 2.5vac tubes were all but eliminated from the production HRO receiver. Millen also recanted his former opinion of the 6.3vac tubes in 1939, when the 697 power supply became available. There was some speculation by former National employees that the actual reason for the preference for 2.5vac tubes was that National had over-stocked the 2.5vac filament winding HRO-type power transformers and these really weren't useable anywhere else. The story goes that National kept up the 2.5vac tube performance "myth" active until they depleted their over-stocked transformers. Many HRO owners did re-tube their older 2.5vac HRO receivers with 6.3vac tubes. More than likely this was due to tube availability rather than performance improvement. About this time, May 1939, James Millen resigned from National. See "HRO Production & Engineering Changes" section (1939) below for various reasons sited for Millen's departure from National.  



photo above
: Gray HRO Rack Mount from 1936 QST Advertisement

Features Found on Later HRO Senior Receivers - Probably the most obvious difference between the early HRO receivers and the later HRO Senior is the lacquer finished micrometer dial that made its appearance with production run-J (1936). The painted micrometer dial is shown in photo A (this is the later "bronze" paint.) Photo B shows the "raised diamond" pointer-mounting screw that started to replace the "NC" emblem mounting screw as early as production run-E (1935) but was intermixed with the "NC" emblem until run-H. Photo C shows the black background coil frequency graphs that were installed during the latter part of the third production run (run-F.) Photo D shows the Marion Electric bakelite and illuminated S-meter that was installed beginning with run-T (mid-1937.) Typical of the Marion S-meters, this example shows considerable fading of the red scale, "DB OVER S-9" and the scale itself has darkened somewhat from the original light-yellow color. Also, note that this photo shows the black pull-switch that replaced the pearl push-button with run-F (1935.) Photo E shows the gray painted chassis (beginning run-L - 1936) and the square IF and BFO enclosures (beginning run-P.) Also, note in this photo the polystyrene Antenna-Ground terminal mount which was introduced very late in production, around run-suffix F (1939.) Photo F shows the improved ventilation of the new cabinet introduced with run-J (1936.) Note: All photos are of 1940 HRO Senior SN: 463-K

A

 

B

C

 

D

E

F

 

The HRO During WWII

Prior to WWII, the Navy was buying some HRO receivers for various uses. These receivers generally will have a National Audio Output transformer installed in the chassis area behind the S-meter and adjacent to the antenna terminals. This area of the standard chassis already had mounting holes and lead thru holes for an audio transformer, implying that National anticipated some customers requiring this option. This would especially be true for many commercial users as well as the military. In fact, some National catalogs do mention that any audio output configuration could be provided. The Navy wanted 500 ohm Z outputs for their requirements and the National transformer usually installed is a National Type S500. Also, these pre-WWII Navy HROs will usually have an "anchor" ink stamped somewhere around the chassis or chassis mounted parts. Other than the audio configuration changes, the pre-WWII Navy HROs are standard production types and even have the standard serial number placement and format which implies that the Navy purchased them "as needed" rather than by a large quantity contract.

Perhaps the HRO's most famous use during WWII was in England where banks of HROs were set-up as intercept receivers at various sites. Generally, most reception stations were separated from the transmitting sites and the decoding sites were separate from either the reception or transmitting sites. At first, Britain couldn't buy the HRO receivers directly, so various methods were used to purchase the receivers. Usually, British officials in the US on business would purchase an HRO receiver from a dealer and hand carry it back to England. This rather tedious method lasted until Lend-Lease was passed at which time then the British were able to have a steady supply of HROs direct from National.

The HRO-M & HRO-MX - The HRO Senior was given a few engineering upgrades at the end of 1941 or early in 1942. The most obvious was the modification of the crystal filter to use an internally mounted crystal, thus eliminating the "easy to remove" crystal that plugged into the top of the filter assembly. There was always questions that arouse regarding the plug-in crystal in that the "air gap" required for the crystal seemed to be interpreted as something "loose" and "rattling around" in the crystal. The new design crystal eliminated that unwarranted concern and didn't change how the Crystal Filter operated. After that change, National referred to the new HRO Senior as the HRO-M or MX. During WWII, the HRO-M was improved with ID rings added to the AVC and B+ toggle switches. Many HRO-M receivers will have a 0-1mA scaled S-meter made by Marion Electric. Initially, these mA meters were only for the receivers being sent to England but later many military HROs had this meter installed. The S-meter "pull-switch" was replaced with a black-finish, ball-handle toggle switch.

photo right: Inspection tag from WWII HRO-M sn PP-988 showing the various signatures and dates for the operations to complete the receiver. The HRO-M is owned by G3UWP- Robin Pickering, who found the tag wedged between the chassis and the cabinet of the receiver. Full dates show June 30, 1943


photo above: The U.S. Navy RBJ-2 receiver featured 50kc to 400kc and 480kc to 30mc coverage with nine coil sets.

The RAS & RBJ - The U.S. Navy wanted a simple to use receiver and National supplied HRO Juniors in fairly large numbers as the RBJ, RAW and the RAS. The RAS was a special receiver that had a 175KC IF to allow tuning through 400KC to 500KC range without interruption. The RAS models also have special coil sets for 175KC IF operation. The Navy receivers were normally rack mounted and usually were supplied with anywhere from five to nine coils that were housed in a coil storage box that was rack mounted. The RAS came with seven coil sets while the RBJ was supplied with nine. The power supply was a rack mounted type and loud speaker panels were generally not included in the rack since nearly all Navy operations required headset reception by the radioman. During the RBJ, RAW and RAS production the plug-in coil set panels were changed from 3/16" thick aluminum to 1/8" steel panels. Small ID tags are mounted between the frequency graph and the logging chart for coil identification. Actually, the Navy bought a lot more NC-100A (RAO series) variations than they did HRO variations.

Other Designations - The Coast Guard also purchased HRO receivers with designations of R-105. These are HRO Seniors and probably date to shortly before WWII. Additionally, the British designated some of their HRO-M receivers as R-106. To further add to the confusion, the U.S. Army Signal Corps also identified their HRO-M receivers as R-106. The Signal Corps versions have a specific data plate mounted in the upper right corner of the panel. There was an HRO-12-S that operated on a 12vdc battery system with the 1286 power pack which may have been built for Canadian use. There are certainly many more designations and variations. During WWII, the many variations of the HRO and its accessories were necessary for the various uses the receiver was put to and for the various end-users of the receiver. Some end-users modified their HRO receivers to their specific needs and these receivers were sometimes given new designations.

The HRO-5 & HRO-W - Near the end of WWII, the HRO was upgraded to all octal metal tubes (except the 6V6GT "glass" audio tube) and most of the components became JAN standard values. This receiver was dubbed the HRO-5. The HRO-5 was identified as HRO-W if it was going specifically to the Signal Corps. They are virtually the same receivers. The HRO-W will have a military data plate installed in the upper right corner of the panel that specifically identifies the receiver as "HRO-W" and the contract number is "49906-PHILA-45-03." The HRO-5/W receivers generally have a Marion Electric S-meter that is non-illuminated and has a white metal 0 to 1.0mA scale. The National manual is not very specific about the S-meter and some HRO-5/W receivers have turned up with the standard amber S-meter scale installed. The common belief was that the all white scale mA meters were exported to England but there were many exceptions to this and the HRO-W is commonly found with the mA meter. Additionally, the S-meter ball-handle toggle switch was replaced with a "bat handle" toggle switch. The HRO-5/W cabinet was changed to have no ventilation holes in the rear panel of the cabinet or louvers on the sides. The HRO-5/W was sometimes powered by the 697W heavy duty power supply. Sometimes receivers were given a heavy moisture and fungus proofing for severe service depending on the intended location. Audio output transformers are shown as optional for the HRO-5 in the National manual but are shown on the HRO-5 schematic. The HRO-W didn't have an audio output transformer installed in the receiver. With the HRO-5/W, coil sets were changed to have a large aluminum plate with silk-screened frequency graph and logging chart along with the coil set identification mounted onto the front panel of the coil set.

photos left& below: The Signal Corps version, the HRO-W sn: K-127, from mid-1945. All HRO-W receivers were heavily coated with MFP which is a yellow lacquer with a fungicide added. Even the knobs are MFD'd. Note the gold appearance of the knob skirts and the olive drab appearance of the PW-D dial which is due to the heavy MFP coating. Under the lid is silk-screened data with a place for stamping an application date. In this receiver's case, the date is JUL 29, 1945. Also note the use of metal octal tubes that began with the HRO-5/W receivers, the non-ventilated cabinet and the silk-screened coil ID plate.

HRO Copies from Other Countries - National Co., Inc. published a pamphlet in 1964 that celebrated their 50th anniversary in business. Inside that pamphlet, National mentioned that both Germany and Japan had produced "knock-offs" of the HRO. Shown in one of the photos in the pamphlet was a technician testing one of the German HRO copies. Also shown was one of the Japanese copies. Both photos are shown below. The Germans built at least two HRO copies during WWII, the Korting KST and the Siemens R4. After WWII, a few other copies were built in Eastern Germany. Later versions of the East German copy used Czech tubes and Russian resistors.
photo left: A National engineer testing one of the WWII German copies of the HRO. Note the HRO-5 receiver to the left of the German HRO. Also, note the incredibly huge General Radio 805 Signal Generator being adjusted.
 

photo right: A Japanese copy of the HRO from WWII. Note that the nomenclature is entirely in Japanese. These copies were not as "literal" as the German copies.

 

Both photos are from "National's Anniversary Photo Album" - published in 1964 for National's fifty years 1914-1964 in business. 

Kingsley Radio Company - AR7 - In addition to Axis-created copies of the HRO, some of our allies also created "knock-offs." Probably the best known HRO "knock-off" is the AR7 receiver built during WWII by Kingsley Radio Co. of Melbourne, Australia. The Australian Army referred to the AR7 as "Reception Set No.1" while Kingsley's original designation was K/CR/11. Though the AR7 has a micrometer dial and uses plug-in coil sets, that's about as far as the HRO copying went. The AR7 uses eight tubes (plus two in the original PS) and covers 138kc up to 25mc using five coil sets. Rather than using a separate Mixer and Local Oscillator, the AR7 uses a Converter Stage. The tubes employed are standard "American" tube types. The receiver uses a stainless steel overlay on the front panel although some of the Australia Army receivers have the front panels painted green. Interestingly, the S-meter on the AR7 works "backwards." Full scale is "0" and mechanical zero is "9." Additionally, the micrometer dial also works "backwards" (when compared to the HRO) with 0 being the highest frequency tuned and 500 being the lowest. All AR7 receivers were rack mounted with a rack mounted power supply and a rack mounted speaker. The audio output impedance was approximately 1750 Z ohms and 600 Z ohms and the panel jacks provided both audio outputs. Probably around 3000 AR7s were produced. After WWII, the AR7 was used extensively in airports around Australia as a communications receiver. Some of the receivers had the LO coil removed from the coil sets and a Crystal controlled oscillator installed for "fixed frequency" operation. It was common to find "banks" of modified AR7s in Australian airports post-WWII. See "Collector's Gallery" below for the Amalgamated Wireless Australasia AMR-100 "HRO Copy" from K6DGH.      

photo right: Kingsley Radio Co., Melbourne, Australia - AR-7 receiver. Most receivers left the stainless steel front panel overlay unpainted with the nomenclature slightly polished to improve its readability. Note that the coil graphs are also stamped stainless overlays. Two graphs are used to improve the accuracy of the graph. The coil sets are general coverage only.

 

The Post-WWII HRO Receivers

 

HRO-5A, HRO-5A1 and HRO-6

At the end of WWII, almost all manufacturing had been for the war effort since 1942. Most companies were ready to start civilian production by September 1945. National, like most other radio companies, offered what had been late WWII receivers as the initial, post-war product line. The first post-WWII receivers offered were standard WWII HRO-5/W models with general coverage coils and a aluminum silk-screened panel with graphs and ID mounted to the coil assembly. These coils sets will be ID'd as JA, JB, etc., to indicate they are general coverage only. Since the military HRO-5/W came with nine coil sets, this may have been offered with the civilian HRO-5/W (at an extra cost.) It's also likely that some of the civilian HRO-5/W power supplies were the heavy duty 697W. See 1946 Radio Shack Boston ad below showing the HRO-W being offered for $217.35 including power supply and four coil sets. Note that the other five coils sets are offered at an extra cost. No wonder it's so hard to find an HRO-W with its complete original nine coil sets. Additionally, the power supply shown in the ad's artwork is the standard 697 and not the heavy-duty "W" version. As expected, the coils sets are "J" version non-bandspreading types. Note that the meter shown is the 0-1mA Marion Electric that was standard for the HRO-W.

Updates from National seem to come at a leisurely pace and by early 1946 the HRO-5 had been upgraded to have the A,B,C and D coils feature the bandspread function. National designated this receiver the HRO-5A. National replaced the white scale 0-1mA scaled meter used in the HRO-5/W with the round S meter with yellow-amber scale made by Marion Electric (as used on the pre-war HRO Senior) although some HROs were also fitted with a white S meter scale with the "NC" diamond logo (probably left-over stock from earlier manufacturing.)


photo above: The HRO-5TA1 sn: 184 1054. This is a very late version of the receiver (made well after June 1946) featuring the square S-meter made by Marion Electric which was used to conform with the S-meter used on the NC-240D receiver. Also, this receiver has the later cadmium-plated chassis and the six-step Selectivity control for the Crystal Filter operation. Note the increased spacing between the two Crystal Filter controls.

The HRO-5A1 was introduced around March 1946 although this receiver is still basically built from mostly "left-over" stock. The 5A1 featured a Noise Limiter circuit that added two more tubes to the receiver bringing the total to twelve tubes. There was no ideal place to add the Noise Limiter control so National mounted it about the only place there was room for it - to the left of the PW-D dial. The Noise Limiter potentiometer somewhat blocks easy accessibility to the 1RF adjustments when performing an alignment. The NL circuit is built onto a small chassis on these early receivers. The hum resistor on the tube heaters was eliminated and instead one side of the heater line was connected to the chassis. Typically, the National ID plate is mounted over the NL control. Many of these early versions are MFP coated for some reason.

On all HRO receivers built between September 1945 up to about June of 1946, it's apparent that National was trying to clear out any of the old stock along with the WWII stock and just about anything they thought they could put into a receiver and successfully sell it. National had two warehouses in the Malden area with lots of "left-over stock" to work through. Consequently, any HRO-5, HRO-5A or HRO-5A1 made in that time period is likely to have parts that seem to come from either different receivers or a different time period. It's difficult to list all of the variations that could be possible. Just suffice it to say that just about any combination of parts are likely to be found on the HRO receivers built between September 1945 and June 1946. These early HRO-5A1 receivers were offered up to July1946.

photo left: This is a 1946 Radio Shack Boston advertisement showing the HRO-W for sale with power supply and only four coil sets included. Note that the remaining five coil sets would cost extra. Also note that these are the military "J" coils as would be expected immediately after WWII. Another note is that the power supply shown in the ad is the standard 697 type. Ad copy provided by: Michael Everett
 

photo right: This is an early HRO-5TA1 from March 1946 showing that at this time National was still using the round Marion Electric S-meter and "left-over" HRO-W front panels. Note that the ID tag is over the Noise Limiter and there are mounting holes in the upper right corner for the HRO-W data plate. This receiver also has the small chassis NL circuit and for some reason is MFP coated.     photo: from eBay

The photos to the right show an HRO-5TA1 probably built around April or May of 1946. Although this receiver appears to have the National tag installed back in its proper place in the upper right corner of the front panel, note that the S-meter scale is an earlier style scale from pre-WWII but it is installed in the typical WWII Marion bakelite meter case. Note on this version that the Crystal Filter is still the older style with the close spacing of the two small dials and with the Selectivity control as the upper dial.

Looking at the photo far right of the chassis it is obvious that this receiver is still using WWII "left-overs" in the IF transformers and BFO coils. Note that the last IF transformer shield and the BFO coil shield are MFP'd. The BFO coil shield still has the WWII ID printing on it. Also apparent is the gray painted chassis.   >>>

photos right:  from eBay

>>>  Though difficult to tell, the Noise Limiter is still being built onto a small chassis mounted on top of the main chassis. Note the cable routed across the chassis going to the NL chassis. The cable exits the chassis to the right of the 6V6 audio output tube.                      

A new version of the HRO-5TA1 was introduced around July, 1946. It featured a new Noise Limiter circuit that was now built onto the receiver chassis which was now cadmium-plated. At the same time, the Crystal Filter was changed to a more conventional circuit that National had used on the WWII NC-100A series (RAO USN receivers and NC-100ASD Signal Corps receivers) with a six position switch controlling Selectivity and a variable capacitor to adjust Phasing. The position of the two controls were interchanged and the spacing between the two controls increased when compared to the earlier HRO Crystal Filter. Another change was to the S-meter which became a Marion Electric meter with a square housing (to conform with the meter used on the new NC-240D.) Additionally, on the later HRO-5A1 receivers the serial number format is changed from the old letter designation for production run identification to a three digit number identification. The specific receiver is identified with a four digit number which results in a seven digit serial number that was then relocated to the rightside of the chassis about midway back. Although dealers began offering the newer version HRO-5A1 as early as July 1946, National didn't feature it on a back cover ad in QST until February 1947. The construction of these later HRO-5A1 receivers is very consistent and will use all new parts in the assembly.

photo right: Inside a very late version HRO-5TA1 showing that the chassis is now cadmium plated (post-July '46 units,) two more tubes are added next to the left side of the tuning condenser for the chassis mounted Noise Limiter. Also, note the NATIONAL decal has returned and is installed under the lid. These decals began to be installed on cabinets very early in production - by run-G (1935) but were not applied to the cabinets during WWII.  Also, note rear wall of cabinet has no ventilation holes as the pre-WWII HRO Senior cabinets did.

HRO-6 - Are the Late HRO-5TA1 Receivers Actually HRO-6s? - In late-1946 or early 1947, a short lived HRO-6 was produced with an "improved" Noise Limiter but its physical appearance is identical to the HRO-5A1. Some National manuals will have an advertisement in the back pages for the HRO-6. These ads date from 1946. Very few HRO-6 models were sold and it is seldom encountered today. It's known that HRO-6 receivers were built with-in the same production run as some late versions of the HRO-5TA1 receivers. Note in the "HRO Serial Number Log" (below) that the HRO-6 and two of the HRO-5TA-1 receivers are all from run 184. Interestingly, one of the HRO-5TA-1 receivers (which is the "very late" HRO-5TA1 shown in the photos above) is apparently a higher serial number than the reported HRO-6. It might be possible that National considered identifying the later, post-July 1946 HRO-5A1 receivers as the "HRO-6" since these late HRO-5A1 receivers incorporated so many changes when compared to the HRO-5, HRO-5A and the highly variable (construction-wise) early versions of the HRO-5A1 receivers. In comparing photographs of the HRO-6 to the late version HRO-5A1, there are no apparent differences and perhaps only the ID tag is the sole change. Perhaps the "improved" NL was the fact that these "late" HRO-5A1 receivers had the NL built onto the chassis rather than the separate small NL chassis used in early HRO-5A1 production. National may have "jumped the gun" on the HRO-6 designation since basically the post-July 1946 HRO-5TA1 (or HRO-6) looked very much like all of the proceeding HRO models. National had certainly started design work on the HRO-7 which was going to have significant changes to its external appearance, fitting a new model receiver designed after WWII. Maybe to keep the "older" looking HRO-6 from being mistaken for National's "new post-WWII design," the designation of HRO-6 was quickly dropped and returned to HRO-5TA1. National didn't want the late-1946 (1947 model year) prospective customers to think that National was still just "revamping" the old HRO as their new model receiver. By stopping the HRO-6 designation, the introduction of the HRO-7 would then have been seen as National's "new post-WWII design" and that their "new" receiver would have a "new" look, completely different from the older HRO receivers.
 

HRO-7

For the 1947 model year a complete "make-over" was given to the old HRO. Introduced in mid-1947, the HRO-7 had a new cabinet with slightly rounded corners, a smooth light-gray paint job, levers for easy coil removal, S-meter viewed though a panel "cut-out," different knobs with light gray grips, a light gray PW-D....however, the HRO-7 circuit remained basically the same as the early HRO-5TA1. There were some tube changes and a tube addition with the local oscillator changed to a miniature tube (6C4) and an added voltage regulator (0A2) but the HRO-7 was really not much more than a cosmetic upgrade. It's introduction did spell the end of the rectangular, black wrinkle finish box though. The HRO's exterior had remained basically unchanged for the past 12 years. The HRO-7 was also offered in a rack mount configuration called the HRO-7R and it was very different from the table model with black wrinkle finish panel, external S-meter mounting, a black PW-D and different knobs. Generally, the rack included a coil storage unit and a rack mounted speaker. The HRO-7 was available for about three years but National's next upgrade was going to change almost everything about the old HRO receiver.


photo above: 1948 advertising artwork for the National HRO-7, note that the speaker style was also changed to a more "modern" look.

 

HRO-50, HRO-50T1 and HRO-60

The new HRO-50 was going to bring the HRO design into the mid-twentieth century. The separate power supply was first on the list of things to go. The new HRO power supply was built-in, though on a separate chassis that was thermally insulated from the receiver chassis and bolted behind the main chassis. Relying on the micrometer dial versus graphs was also gone. Now a linear slide-rule dial would provide direct frequency readout. Changeable plastic scales were mounted to a front panel controlled, rotating drum, providing band-in-use scaling. Sockets were provided for the optional 1MC/100KC crystal calibrator and NBFM adaptor. Voltage regulator, push-pull audio - almost everything necessary to update the old HRO. The plug-in coils had to remain along with the PW-D micrometer dial (now for logging) - otherwise it wouldn't even have been an HRO. The HRO-50 was introduced in 1950 and was followed quickly by the HRO-50-1 that added an extra IF amplifier and, to improve selectivity, double IF transformers were used in two of the IF stages. With 12 tuned IF transformers, the receiver was very selective. The HRO-50-1 was produced in 1951 through 1952.

The final evolution of tube-type HRO receivers was introduced in 1953 - the HRO-60. With 18 tubes, double conversion and a current regulator on the oscillators, it was the final evolution of the tube-type HRO. Double conversion was utilized above 7MC when using coil sets B and A or any of the A prefix bandspread coils available. Bandspreaded 40M required the C coil set which wasn't double conversion. The HRO-60 accessories included coil sets J, H, G, E and F, providing general coverage from 50kc to 430kc and 480kc up to 2.0mc. Additionally, an AC coil set provided bandspreaded 15 meter coverage. AD covered 6 meters. There were two other "A prefix" coil sets, the AA and the AB, both of which bandspreaded in the 27mc up to 35mc ranges. The HRO-60 was available from 1953 up to about 1964. At the end of production, the selling price had escalated to an unimaginable $745.00!

As with all of the HRO receivers, rack mount versions were available that featured a 30" tall table rack that allowed mounting of a coil storage unit and a rack mounted speaker. These rack mounted speakers with coil storage were available as early as with the HRO Senior receivers and, with the introduction of the HRO-50, the type SC-2 coil storage and speaker combination became available. Special "dust covers" were installed on these later rack-mount receivers that fit over the side panels and are held in place with two knurled thumb-screws. When using one of the rack mounted speaker set-ups one will notice somewhat of a reduction in the bass response due to the lack of any true enclosure for the speaker. This is typical of most rack mounted speaker set-ups.   >>>

photo left: 1955 HRO-60R  sn: 393 0255, rack mount HRO-60 receiver in MRR-2 table rack with SC-2  8"speaker & coil storage unit

>>>  The HRO-60 first conversion oscillator frequency was changed during production. Early versions of the receiver used 2010kc while later versions use 1990kc. At least two versions of the manual were published with early manuals referencing 2010kc as the first conversion frequency and later manuals showing 1990kc. In fact, I have an original HRO-60 manual that has a hand written note saying "1995kc - per National", so certainly there was much confusion over what frequency was used in any particular receiver. While aligning any HRO-60, it's easy to input a specific amplitude signal at 2010kc and adjust the two first conversion transformers for maximum output. Then measure the output of the receiver exactly - use an audio output meter connected to the 500Z ohm output. Next, input the same exact amplitude signal at 1990kc and readjust the first conversion transformers for maximum. Measure the output of the receiver exactly. Whichever alignment frequency results in the highest output level is the correct frequency for that receiver. The correct frequency should be marked on the two first conversion transformers for future alignment reference.

With the direct readout dial on the HRO-60 it is possible to achieve very accurate tracking. You'll find that it is tedious work since each LO low-end adjustment will require coil set removal, then making a slight adjustment followed by reinserting the coil set to adjust the high-end trimmer and then checking the tracking. With patience it's possible to have the HRO-60 dial achieve a very accurate readout considering that its resolution is somewhat limited.

The HRO-60 is often times berated as less of a receiver than its predecessor, the HRO-50 but this is mainly from hams who are looking for maximum bandwidth for AM signals. The early HRO-50 used two rather conventional IF amplifiers that provide a typical "bell curve" passband which many AM ops find pleasing to listen to. However, the HRO-60 and the HRO-50-1 were trying to cope with the crowded band conditions of the fifties and sixties where successful communication was the goal. The additional IF transformers and IF amplifier stage provided a narrow passband with steep skirts resulting in very selective tuning. An excellent Crystal Filter circuit also helped with difficult QRM. Operating a rebuilt and correctly aligned HRO-50-1 or HRO-60 is a pleasure - QRM is rarely (if ever) a problem and the receiver is still a competitive performer at any frequency.


photo above: The HRO-60R chassis. The smaller IF transformers are for the first conversion set-up. Note the dual IF transformers. The rear of the chassis has the Calibration Oscillator installed but the NBFM module is not installed.

 

HRO Serial Numbering System

The Charles Fisher Survey - At first glance, National's serial numbering system seems confusing. There was a rumor that the serial number system was designed to hide the actual production quantity of receivers being built but this seems unlikely. Several years ago, Charles Fisher performed a survey of about 70 early HRO receivers to determine the serial numbering method that National used. His article was published in the AWA OTB in 1989 (it is also currently on the web - link provided below.) The Fisher article contains a wealth of information for the HRO enthusiast.

The Production Runs - Fisher's survey determined that National used a combination letter prefix and number suffix for their serial numbers on the early HRO line, e.g. D-12 or G-115. Each production run of receivers were given a new letter prefix and the numbers are then sequentially assigned. The first HRO run begins with prefix "D" and was probably only about 100 units. Run "E" and most subsequent runs are made up of about 250 units. Sometimes runs overlapped each other. Letters "O" and "I" may not have been used due to their similarity to numerals. By about mid-1938, the letter prefix had gotten to the end of the alphabet and the serial numbers were changed to number prefix with a letter suffix but numbers up to 500 were used, e.g. 123-F or 434-H. By the beginning of WWII, this type of serializing had reached the "L" suffix and was halted. Fisher wrote that during WWII, the numbering again changed - this time back to the letter prefix but using number suffixes higher than 250 with the exception of prefixes A, B and C which, since they were not part of the original serial numbering sequence, used numbers beginning with 1. Also, during WWII the letters "O","Q" and "I" were used in the serial number combinations. There are many exceptions to the WWII serial numbering, though.

Total production quantity from 1935 up to about the beginning of 1941 was around 8000 to 9000 receivers. Around 5500 to 6000 receivers comprise the letter prefix serial numbers with the remaining 3000 or so using the letter suffix serial numbers.

Western Historic Radio Museum Serial Number Log - WHRM started an HRO serial number log for ALL tube-type HRO receivers in 2007. Our serial number logs for other communications receivers have been very successful in determining production levels and other interesting manufacturing data. One of the first discoveries for the HRO receivers was the change of format for serial numbers on the HRO-5A1 receivers that continued on through the HRO-60 receivers. Additional serial number reporting has revealed the dual prefix letters used during WWII. Please keep reporting your HRO receiver serial numbers. As seen, important information can be found when enough numbers are gathered. Use the e-mail link below in "Dating Early HRO Receivers by Serial Numbers."

More on WWII Serial Numbers - After receiving several reported HRO serial numbers from enthusiasts, it has become apparent that several systems of serial numbering were used during WWII. Much of what Charles Fisher found is true but there are many exceptions turning up as more serial numbers are reported.

The HRO-5/W receivers seem to re-use older serial number sequences, note that the reported HRO-W receivers are serialized as K-127 and K-184. The serial numbers used for the HRO-W "K" run were certainly duplicates of earlier issues from the 1935-36 run. The same is true with HRO-5 sn J-39. Apparently, National believed that the HRO-5/W was a different receiver, or at least different enough that it couldn't possibly be confused with the earlier HRO Senior - after all, the 5/Ws used octal tubes.

Additionally, there have been several HRO-M receivers reported that were serialized with a two-letter prefix followed by up to three numerals,... AP-461, PC-67 and PP-988, for example (there are more.) It's possible that this double prefix was to avoid confusion with early HRO receivers built with the same number but with a single prefix, however PP-988 is much higher of a number than the earlier single P prefixes used so there might be another meaning to the double prefix. The two-letter prefix AP-461 is an exception since the single A prefixes were issued during WWII and had no special limitations since there weren't earlier "A" prefixes issued. One note is that nearly all two-letter prefix HROs are located outside the USA. Possibly, the two-letter prefix identifies production runs that were specifically for Lend-Lease sales.

Certainly, WWII was a hectic time for National HRO production and variations in their serialization of receivers should be expected. Keeping the serial numbering format and sequences in order was probably very low on the "list of priorities" at National during WWII. Even post-war serializing was not consistent and variations should be expected until around mid-1946.

 Post-WWII HRO Serial Numbering System - With the mid-1946 HRO-5A1, a completely different method of numbering was used consisting of two groups of numbers with no letters. The format is usually three numbers, a space followed by four numbers, e.g. 184 1054. This serial numbering format was used from the HRO-5A1 up through the HRO-60. It appears that the first three digits identify a production run and the four digits identify the particular receiver in that run. Note the reported HRO-5A1, HRO-50 and HRO-60 receivers and that the production run digits progress higher from the HRO-5A1 (184) to the HRO-60 receivers (345, 366, 393.) Also, note that all of the later receiver identification numbers consist of a zero followed by three digits (indicating that production run levels are seldom more than a few hundred receivers.)

Dating HRO Receivers by Serial Numbers

Charles Fisher's Survey

Charles Fisher compiled a list of serial numbers from 70 early HRO receivers during his survey for the AWA's OTB article. By using the serial numbers compared to the evolution of the example it was possible to extrapolate a logical sequence of serial numbers. To the right is Fisher's table of serial number prefixes and suffixes, which identify the specific production runs, tied to probable dates of manufacture. Since the survey was only concerned with early HRO receivers the cut-off for entry into the survey was that the receiver had to use the plug-in crystal on top of the crystal filter unit. The dates only go up to the beginning of production of the HRO-M receiver which had crystal installed inside the crystal filter unit.

Charles Fisher didn't publish the complete serial numbers of the 70 HRO receivers reported in his survey. He reports quantity per production run letters only. Here is a link to the complete Charles Fisher article: FISHER: Dating the National HRO 

WHRM's National HRO Serial Number Log   

I have started a serial number log here for ALL tube-type National HRO receivers. The log will be divided up for each model of HRO and will include all tube-type models from the first HRO receivers up to the HRO-60. This log will eventually be an on-line source for dating any of the HRO receivers. The more serial numbers sent in, the more accurate the data will be. If you have several HRO receivers, be sure to designate which serial number goes with which model receiver.

To Send HRO serial numbers:  WHRM HRO SERIAL NUMBER LOG     

Serial Number Locations

On HRO receivers from 1935 to early WWII versions, the serial number is stamped next to the Antenna-Ground input terminals insulator block, between the insulator block and the left edge of the chassis. This allowed the serial number to be easily viewed by looking through the square hole for the antenna input located on the left side of the receiver. Late in WWII, the HRO-5 and HRO-W had the serial numbers stamped to the left side of the audio output tube between the tube socket, the meter zero pot and the left edge of the chassis. This required lifting the receiver's lid to see the serial number. Early post-WWII receivers use the same location next to the audio output tube. With the introduction of the HRO-5A1 and the new serial numbering system, the location was moved to the right side top edge of the chassis mid-way back. This location was used up through the HRO-60 production.


Pre-WWII HRO Production Runs versus Build Dates
 

 SN / Letter Prefix                     Probable Dates                                     

             D, E                            January-March 1935                                          

             F, G                           April-July 1935                     

             H                              August-September 1935

             J                              October-November 1935

             K                           December 1935-January 1936

              L                              February-March 1936

             M                                  April-May 1936

              N                                     June-July 1936

           P, Q, R                       August 1936 - February 1937

           S, T, U                         March-September 1937

           V, W, X                       October 1937-April 1938

             Y, Z                              May -October 1938

       Letter Suffix  A                     Nov-Dec 1938

       Letter Suffix  B - L           Jan 1939 - early 1941

 

 HRO Serial Number Log (Pre-war and Wartime)

HRO, HRO Senior:  D-27, D-34, D-38(R), D-49 (R), D-65 (R), E-50, E-55, E-76, E-178, F-09,
F-51, F-58, G-113(Nat'lMod), G-217, G-235, H-103, H-121, J-123(R), K-83(R), L-36, L-73(R), L-114, L-175(R), L-178(mod'd), L-184, M-167(R), N-25, N-29, N-79, N-146, R-54, S-208, T-239,
V-35(scrapUSN), V-77, V-214, X-204, Y-53, 129-A, 233-C, 164-G, 170-H, 41-K (RCAF rcvr),
463-K, 434-L(R)

HRO Junior: L-58(R), P-116, O-250(R-WWII Mil.), 326-K, 103-L,

HRO-M, HRO-MX, Canadian HRO:  A-69, A-181, AP-195, AP-461, B-72, C-262(MX), C-392(MX), C-638, D-656(MX), D-706(MX), H-420, J-942(R-106), M-243, P-377, P-554, PC-67, PP-128, PP-569, PP-808, PP-988, 0605 (no letter prefix, RCCS version), 32-F,

HRO-5, HRO-W: A-67, J-39, J-431, J-607, J-879, K-127(W), K-184(W), K-650(W),

U.S.N, - RAS, RAW, RBJ, etc:           

(R) = Rack Mount, otherwise receiver is a table version     (Nat'lMod) = Receiver modified at National 
(R-106) = British designation                                             (scrap) = Receiver parted out
(W) = Signal Corps version, receiver is same as HRO-5    (USN) = Receiver used by Navy  
(mod'd) = Receiver severely ham modified                            

HRO Serial Number Log (Post-war)


HRO-5A, HRO-5A1:
K-557(TA), K-611(TA), L-80(TA1), 184 0005(TA1),
184 0180(TA1), 184 0328(TA1), 184 1054(TA1),

HRO-6: 184 0697

HRO-7:  196 0397, 196 0546

HRO-50, HRO-50-1:  280 0455(T), 293 0036(T), 293 0213(T), 311 0101(T-1),
311 0136(R), 311 0506(T-1), 327 0183(T-1),

HRO-60: 345 0127(T), 345 0165(R), 366 0800(T), 393 0177(T), 393 0255(R),
425 0234(T)

(R) = Rack Mount    (T)= Table Version    (T-1)= HRO-50-1    (TA1)= HRO-5TA1

NOTE: From late HRO-5A1 to HRO-60 all SNs are seven digits. First three are production run followed by a space with the last four digits being the serial number of the specific receiver.

 

HRO Production and Engineering Changes from 1935 to 1941

1935 - Runs D, E, F, G, H & J

White "pearlized" push-button S-meter switch (D,E)   -  S-meter switch changed to black push-pull switch (F-on) - The push button switch was noisy in operation and "popped" through the audio when released. Additionally, the push button required the operator to hold the button down with one hand and tune in the station with the other hand - inconvenient at best.

White coil graphs (D,E,early F)   -    Coil graphs changed to black (mid F-on) - black graphs did not show moisture stains or possibly glue stains. Coil shield boxes on 1RF, 2RF and Mixer have only one access hole for alignment. LO shield box has two holes. This is for runs D, E and early F only. Mid-F-on two holes are found on all Coil shield boxes. Additionally, runs D, E and early F will have very small contact buttons on each of the coil insulators. These buttons are the connections from the coil set to the receiver. Mid-F-on these contact buttons become a two-piece assembly that is larger and taller. Also, runs D, E and early F have coil insulators with no embossed nomenclature. Coil insulators on runs mid-F-on have "National Co." and pin numbers embossed into insulator.

Metal case non-illuminated S-meter with 0-5 scale (D-J) - 0-5 scale conformed to early QSA signal reporting method. 5 is at half-scale with nothing indicated on the arc of the scale above 5 (except as noted below.) -  All E production run receivers seem to have the meter cases with a smooth "rounded" flange. The earlier D-runs and post-E runs have meters that have a slight indentation along the rim of the flange. The difference is subtle. Possibly a component cost resulted in the use of a different style meters during the early production runs.     METER SCALES - D-run S-meters have a pinkish-gray colored NC diamond logo with white "NC", "PLUS" is printed above 5. This same scale was also used on the E-run but late E-run meters may have the later 0-5 scale. Later, by F-run meters, the NC diamond is reddish-orange with black "NC" and the "PLUS" is no longer printed above 5. For examples of each meter scale see photo C in "Features Found on Earliest HRO Receivers" in a section above. For F-run to J-run type of scale see photo of HRO SN: F-09 in "Collector's Gallery" in a section below.

Round IF cans (D-P) - hex adjustments of air trimmers required special tool (insulated .25" hex screw head driver) - these are actually "slotted" screw heads that have been soldered over so only a hex adjustment tool can be used. An early Millen article on the HRO in "Short Wave Craft" issue of March 1935 shows the HRO chassis. Interestingly, chassis photo also shows the BFO can with an adjustment knob on the top. Fisher reports this BFO knob is also seen in a rear page photo in first instruction manual. Doubtful this knob was on any production units.

Black chassis (D-L) - conformed with the SW-3 and FB-7 chassis paint use

Tuning condenser insulators are stamped out of sheet insulating material leaving rough edges used on D and E runs only. With run F, the tuning condenser insulators are molded bakelite with smooth edges and have a embossed spacer where it meets the metal spacers (F-on) -  the bakelite insulators were less hygroscopic than former material which tended to warp in excessively humid areas

"NC" in red background on the dial pointer mounting screw (D, E-partial, F-partial)   -  Dial pointer mounting screw changed to plain nickel-plated "raised diamond" (G-on) - cost reduction. Fisher indicates that the "raised diamond" first appears on the E runs but is intermixed with earlier "NC" pointer until G run. The "NC" pointer emblem mounting screw was also used earlier on the SW-5 escutcheon. Many D and E run HRO receivers no longer have the original "NC" mounting screw because this part was very fragile and tended to break after a few times of removing the upper rail (which was necessary for coil set alignment.) The replacement was naturally the later version part, the "raised diamond" mounting screw-nut assembly.

No pilot lamp (D,E)   -   Pilot lamp incorporated (F-on) - there was no visual "power on" indicator until the pilot lamp was installed

No external standby switch terminals (D-E)   -   Stand-by terminals marked "BSW" (B+ SWitch) added to rear of chassis (F-on) - allowed for remote control of B+, typically with a T-R switch that controlled the transmitter and receiver operation. Initially, the "BSW" terminals allowed a parallel connection with the B+ "ON-OFF" panel switch. This required the receiver be operated with B+ "OFF" on the panel switch and then allowed the remote switch to control receiver operation. The parallel connection is shown in the 1936 manual. By 1937, the "BSW" terminals were then connected in series with the B+ "ON-OFF" switch. This allowed the B+ panel switch to remain in the "ON" position and still allow remote switching. Also, the panel B+ switch was functional as far as being able to remove (only) the B+ independent of the remote switch in the "receive" position. This function would be necessary when changing coils. If the 1937 or later HRO receiver was going to be operated without a remote T-R switch, as in a "receive only" installation, then a jumper had to be installed across the "BSW" terminals to allow B+ to be routed to the B+ "ON-OFF" panel switch and allow receiver operation using the panel B+ switch.

White ceramic resistors with hand written values in blue ink (D-F) - Black ink-stamped values  (G-~R)  - both types of resistors were built at National. By run R resistors are purchased from Centralab.     

Tubular paper dielectric capacitors used in the first few production runs appear to be Micamold manufacture. This is a departure from the Sprague capacitors that National used in the AGS and FB-7 receivers. It has been reported that HRO J-123 (from early 1936) has all original Aerovox capacitors installed. Also, it's been reported that HRO N-146 was equipped with paper capacitors that only had the values, a run number and a foil indicator band on each capacitor. By production run-V (as seen in V-35,) National had returned to using all Sprague manufacture capacitors (change probably happened much earlier.) A close examination of the under chassis photo used in the 1935 and 1936 manuals will show that the paper capacitors are Micamold manufacture. Note that the cathode bypass electrolytics are of Aerovox manufacture, however. It's likely that National was watching expenses due to the Depression and shopping around for the best prices from various capacitor companies.

German Silver plated PW-D micrometer dial (D-H) - the Index and Number dials are cast pot metal made by Doehler Co. Very fine castings on early dials since they were to be German Silver plated. Later castings used for Lacquer painted dial (J-on) are poorer finish casting since the paint acted as a "filler" and the final finish was smooth and even. Since white paint was used for the index lines, the engraved index lines are somewhat wider (to accommodate the paint fill) when compared to the plated dial (except for early painted dials which still have the narrow index lines - probably up to the end of 1936.) 

Antenna-Ground connector uses "push" connectors with insulated button on top, fiber insulated mount (D-~ suffix B) - buttons are always missing indicating that these pieces tended to break easily. First production run units (D-run) will have the A-G terminal insulator strip mounted further into the chassis leaving about a 0.5" space for the serial number. With run-E, the A-G terminal insulator is mounted much closer to the chassis edge leaving only about 0.25" for the serial number. Also, D-run receivers will have the ground lug and wire mounted with a screw and nut centrally in the blank area adjacent to the 1RF coil terminal block. Run E units and later have the ground lug and wire attached to the mounting screw for the audio output tube shield because holes for optional audio output transformer mounting. From run-E on, five 0.25" holes and four 0.125" holes are stamped into the chassis in this formerly "blank" area to allow mounting an audio output transformer, if requested. This was commonly requested by military users and some commercial users. The five larger holes are for the transformer leads and the four smaller holes were to mount the transformer.

S-meter potentiometer is located closer to the meter in the blank area of the chassis near the 1RF coil terminal insulator on first production run (D.) Pot moved to between the Ant.-Gnd. terminal insulator and the Audio Output tube shield for run-E on. Pot location moved to allow for mounting holes for optional audio output transformer.

National Co. decal added underneath the cabinet lid by G production run.

Cabinet has small 0.25" holes in the rear panel for ventilation - enlarged holes and louvers on the sides appear with J production run

Small skirted knobs have short round boss on lower part of square bakelite portion of knob grip (D-~J) - The later knob grips have a round boss that is much taller (after ~J.)  There are also changes to the skirt in thickness and depth of engraved nomenclature. Earlier skirts tend to be thinner with shallow engraving.

Early Crystal Filter shafts support bearing is screw-mounted to inside of the front panel (D-~F.) Later shaft supports are screw-mounted to Crystal Filter assembly (F-on.) 

TC wire connection from Crystal Filter housing to Tuning Condenser frame added for better ground connection of the tuning condenser shields (~G-on.) Appears sporadically on earlier production.

Early ceramic (Isolantite) sockets, National type XC, used for front-end and 2nd detector tubes (five sockets total.)  (D-~V)  XC sockets are replaced with later version National sockets (Steatite, ceramic similar to Isolantite,) type CIR, that have a metal mounting flange (by ~V, probably earlier.)

1936 - Runs K, L ,M, N, P & Q

S-meter scale changed to 0-9 (J-on) - the scale change was to follow amateur adoption of the RST method of signal reporting - scale remains white with black numbers, red NC diamond with black "NC"

S-meter case changed to a bakelite housing (J-on) - it was possible for B+ to appear on the metal zero adjusting screw, new design corrected this

German Silver plated PW-D micrometer dial changed to "black" lacquer (J-on) - cost reduction because the paint can act as a filler. This allowed Doehler to use castings that were not as "fine" of a finish as was required for the German Silver plating. The new painted PW-D also conforms with introduction of NC-100 models (although the NC-100 dials are painted gray on the Index dial and red on the Number dial.) NC-101X uses the same black dial as the HRO. Paint color varies on HRO receiver's PW-D and generally is very dark gun metal gray on early dials (thru 1939?) and dark bronze on later dials (1940 on.) There appears to be several exceptions to this general chronology however and it is possible to find the gun-metal paint used even on some post-WWII HRO dials.

Holes top rear of cabinet enlarged (J-on) - better ventilation of cabinet.

Louvers added to side of cabinet (J-on) - better ventilation of cabinet

Chassis paint changed to gray (K-on) - probable cost reduction by conforming with NC-100 and newer models

HRO Junior introduced (February) - standard HRO construction but without Crystal Filter, S-meter or S-meter Switch. Only one coil set is included with the receiver although there are four specific coil sets for the Junior. All Junior coil sets do not "band spread" (general coverage only,) have only one frequency graph and are identified with a "J" prefix. Specific Junior coil sets are JD, JC, JB and JA. After the HRO Junior is introduced the standard HRO is referred to as "HRO Senior"

Dec.1936 issue of QST, back inside cover National advertises a Gray "Leatherette" Finish HRO in a table rack MRR and speaker-coil storage panel SPC. Ad runs several times in 1937 and 1938.

1937 - Runs R, S, T, U & V

S-meter changed to a bakelite housing, illuminated unit - made by Marion Electric Instrument Co. - scale changed to amber colored plastic - "db above S-9" added to scale printed in red

IF cans changed to square units (P-on) - improvements to air tuning condenser design in square IF transformers, adjustable with insulated blade screw driver. The adjuster heads on these air condensers were round so a hex adjusting tool wouldn't have been useable, therefore National had to leave the slotted heads as the means of adjustment.

"BSW" terminals are changed from parallel connection with panel B+ switch to a series connection (~R-on.)  Requires a jumper across "BSW" terminals to operate receiver without a remote T-R switch.

Tuning condenser spacers are changed from "turned" hex stock to all round stock - cost cutting measure as the spacers didn't require machining now.

White ceramic resistors changed to standard purchased parts with BED code (resistors purchased from Centralab at that time)

Value of C7 erroneously shown in parts lists as far back as the first manual as .1uf  200wvdc was actually a .01uf 400wvdc capacitor. Close examination of the under chassis photos used in manuals in 1935 and 1936 show that C7 was a .01uf 400wvdc cap (the value can be read in the photo) and that the error was in the parts list. Later receivers will have a .01uf 600wvdc brown square bakelite (probably Aerovox) capacitor installed. WWII and post-WWII returned to tubular capacitors of .01uf 600wvdc. Additionally, C-28 is listed as a .01uf 600wvdc tubular capacitor but nearly all receivers will have a .01uf 400wvdc tubular capacitor installed.

Ceramic tube sockets replaced with newer style "CIR" sockets made by National Co. - sockets have a metal mounting flange. (~V, possibly earlier)

Hex head screws used to assembly Crystal Filter unit. (~V, possibly earlier)

1938 - Runs W, X, Y, Z & Suffix A

ID tag added to upper right part of front panel - the uninformed would now know what kind of receiver they were looking at

Serial numbering system changed to numeral prefix with letter suffix - the "Z" prefix was used ~10/38  

1939 - Runs Suffix B to Suffix F

Antenna/ground terminal changed to polystyrene insulator and screw binding posts - the older "push" connector plastic top cap broke easily - new style connector by SN: 233-C

Model 697 power supply for 6.3vac tube HRO Sr. introduced ~3/39

2.5vac tube HRO not offered - only 6.3vac tubes used after ~4/39 - Millen recanted his opinion that the 6.3vac tubes caused modulated hum at 15MC in the receiver - improved tube quality sited

James Millen resigns from National ~ 5/39  - Various reasons sited, officially the story published was to form James Millen Manufacturing Company. There's an old rumor that says internal conflict over National going public and selling stock was the reason. Millen's personal friends relate a different story, however. While attending a meeting in Washington D.C. to discuss production of the HRO for the military, Millen represented himself at the meeting as the president of National Company. When this got back to Wm.Ready, the actual president, he was furious and demanded Millen's resignation. Needless to say, Millen's departure, for whatever reason, was the beginning of National's slow downhill spiral as far as technically innovative designs. Of course, James Millen Manufacturing Company wasn't particularly innovative either and was still selling 1940s parts and gadgets in the 1960s. Millen actually made his company successful by being an electronics contractor and building oscilloscopes for RCA and two-way radios for GE. The ham gear and old style parts were Millen's way of staying in the ham radio business.

1940-1941 - Runs Suffix G to L

Tuning condenser insulators redesigned, shield thickness increased, improved grounding contacts for rotor (Suffix G-on)  - better performance with cost reduction

Crystal filter changed to internal mounted crystal (after L) - better performance with cost reduction - This change may have taken place just after WWII began. National changed designation to HRO-M

HRO Production and Engineering Changes from 1942 to 1953

1942-1945 - General Information - Various Production Runs during WWII

Bandspread coil production is significantly reduced (eliminated?) as the military production requires "J" series coil sets for A, B ,C and D frequencies. Most of WWII coil sets are JA, JB, JC and JD coil sets along with E, F, G, H and J coil sets. Coil set C, LO inductance adjustment is changed to a moveable "loop" as coil sets A and B.

Many HRO receivers will have audio output transformers installed to remove B+ from the speaker terminals. Sometimes seen pre-WWII military units but becomes common during the war.

RBJ, RAW and RAS introduced in 1941 for USN. These are rack mount HRO Junior receivers with speaker, power supply and coil storage box included in rack. RAS has 175kc IF for uninterrupted coverage from 190kc up to 30mc using seven coil sets.

Plug-in coil sets - front panel changed from 3/16" thick aluminum to 1/8" thick steel

Many of the slotted head machine screws used in assembly of various parts are replaced with hex head screws with .25" hex heads.

The gear box is changed from a casting to a sheet metal assembly with square edges and corners. "NC" is stamped-embossed on the sheet metal cap. This incorporates a mounting change from four fillister head screws for mounting the molded cap to only two hex head screws to mount the new style cap. Additionally, the sheet metal housing requires that a bronze bearing be installed at the rear to act as a thrust bearing for the worm gear. Also, since the sheet metal housing isn't as thick as the old cast gearbox, the condenser supports are now integral to the assembly which limits the clearance for the thrust ball bearing assembly. This forced the assembly of the ball bearing thrust assembly on the worm gear shaft to now have the ball bearing at the front of the shaft rather than next to the worm gear flange as originally designed.

During WWII the wrinkle finish paint formula was changed and the "wrinkle" tends to be much finer than early model HROs.

During WWII there were many variations of the HRO, the coil sets, the power supplies, etc., built for the Signal Corps, the Navy and for Allied use - not all variations are covered in this article.

Later HRO-M, MX receivers will have ID rings for AVC and B+ toggle switches. Many HRO-M, MX receivers fitted with Marion Electric 0-1mA scaled, non-illuminated, white scale S-meter. Toggle switch replaces the "pull-switch" for the S-meter function.

HRO-5 introduced in 1945 with following changes:

Change over to mostly JAN parts, specifically most resistors are changed to JAN type

All coil sets now have an aluminum, silk-screened panel that is mounted on top of the coil set panel to provide coil identification and provide a frequency graph and a logging chart

All coil forms used in all coil sets are now made out of polystyrene plastic

Serial Number location changed to near AF Output tube and meter zero adjustment

Hum-resistor eliminated from tube heater circuit and one side of the heaters then connected to chassis instead

All tubes changed to octal tubes, all metal octal tubes except 6V6GT AF Output tube

S-meter is almost always a Marion Electric 0-1mA meter with white scale and non-illuminated

Cabinet changed to have no ventilation holes in back and no louvers on the sides.

Heavy duty 697W power supply is introduced with square box type cabinet rather than the typical National "dog house" cabinet.

HRO-W produced for Signal Corps - mid-1945

Late-1945 - 1946 - General Information

Bandspread coil sets return to production. Possibly as early as September, 1945 - when civilian sales start up.

HRO-5 becomes HRO-5A when bandspread coils are supplied.

S-meter scale returns to a white scale with diamond "NC" logo installed in WWII Marion bakelite case. Some meters may be older Marion Electric types with round housing and yellow S-meter scale.

HRO-5A1 "early version" introduced around Feb or Mar 1946 - Features Noise Limiter built onto a small chassis. These receivers use many "left over" parts from WWII production and will be found with round Marion Electric S-meters from pre-WWII production, HRO-W front panels, National ID tag will be mounted above the NL control on receivers with WWII panels. When these panels run out, then the tag is returned to the upper right corner of the panel.

Meter Zero Pot location moved behind Audio Output tube socket

HRO-5A1 "late version" introduced July 1946 - Features an "improved" Noise Limiter circuit that is now built directly onto the chassis. New "standard" circuit for Crystal Filter that uses a six-position Selectivity control and a variable capacitor Phasing control - end of the dual variable capacitor type Crystal Filter. Wider spacing between Selectivity and Phasing controls on new filter.

Also, for HRO-5A1 "late version" - New serial number location on right side-top of chassis edge midway back

Also, for HRO-5A1 "late version" - New serial number format using three digits to identify the production run, a space and then four digits to identify the receiver

Also, for HRO-5A1 "late version" - Same cabinet as late WWII used with no ventilation holes in back or louvers on sides but "NC" decal now applied under lid (was discontinued during WWII)

Also, for HRO-5A1 "late version" - New S-meter with square shaped housing made by Marion Electric. Conformed with the S-meter used on the NC-240D

Also, for HRO-5A1 "late version" -  Cadmium-plated chassis replaces the gray-painted chassis

Late 1946 - HRO-6 introduced with a supposedly "improved" Noise Limiter circuit - external appearance of receiver remains unchanged. It's known that HRO-6s and HRO-5TA1s were built with-in the same production run - run 184. See "HRO Serial Number Log" (above) -  the only reported SN for HRO-6 is production run 184 0697 and one of the HRO-5TA1 serial numbers is also production run 184 1054. It's also interesting that the HRO-5TA1 apparently has a higher serial number than the HRO-6. It's possible that National considered designating the "late version" of the HRO-5A1 as an HRO-6 since there were so many changes to that receiver from the proceeding versions. Perhaps some of the "late version" HRO-5A1 receivers were tagged as HRO-6 since there appears to be no physical difference between the examples other than the ID tag. National probably stopped using the HRO-6 designation because of the developing design work for the HRO-7.

1947-1953 - General Information

HRO-7 introduced in mid-1947 - major redesign of external appearance, coil graphs, coil removal levers (new,) voltage regulator added

HRO-50 introduced in 1950, followed by HRO-50-1 - major redesign of receiver with HRO-50 having built-in power supply, direct readout dial, push-pull audio - HRO-50-1 adds third IF amplifier and uses two dual-IF transformers

With HRO-50 or 60, coil mounting insulator blocks are changed to ceramic material. Switches are added to eliminate the need to move the four screws when changing to general coverage or bandspread

HRO-60 introduced in 1953, adds double conversion above 7mc. The 7mc coverage with double conversion requires that the user employ the B coil set in general coverage. If 40 meter bandspread coverage is desired then the C coil must be used and this coil operates in single conversion. Additional coil set offered  - 15M (BS only,) 10M (BS only) and 6M coil sets available

 

HRO Accessories

Most of the following items are actually necessities for operation of the receiver and some, like the four coil sets A, B, C & D, were included with the purchase. However, power supplies, speakers, coil boxes and extra frequency coverage coil sets were optional and added to the total cost of a receiver. I've also included National's Servicing and Upgrading as this was an option that many owners took advantage of.

HRO Coil Sets - Detailed Information


photo above: Top coil set is from HRO sn: F-13 and shows the white background graphs. Bottom coil set is from a later HRO Senior showing the black background graphs

General Information - Each of the HRO coil sets consist of four shielded coil assemblies that function in the receiver as the 1RF, 2RF, Mixer and Local Oscillator tuned circuits in conjunction with the HRO receiver's tuning condenser. The early coil sets were identified by the users by looking at the graphs and seeing what frequencies were covered, however, National identified each coil set with a letter. Coil sets A, B, C and D covered 30mc down to 1.7mc and could also be set to bandspread the 10M, 20M, 40M and 80M ham bands. Very early on, the E and F coil sets were added. These two coil sets allowed coverage of the AM BC band and provided the user with coil set E that would cover 160M and also allow leaving the D coil set covering 80M bandspread. Eventually, coil sets covering the long waves were provided with G, H and J coil sets allowing coverage from 430kc down to 50kc with a small section of the spectrum around the receiver's IF not covered (430kc - the upper frequency of the G coil to 480kc the lower frequency on the F coil.) During WWII, almost all A, B, C and D coil sets produced were of the "J" series, or HRO Junior coil sets that didn't provide the bandspread function. Many WWII coils sets had small aluminum tags installed between the frequency graph and the logging chart for coil set identification. Very late in WWII, a silk-screened panel was introduced that showed the tuning range graph with logging chart and the coil set identification by letter. These remained on the post-WWII HRO-5A and HRO-5A1 receivers with a bandspread graph added in place of the logging chart on sets A, B, C and D. The HRO-7 changed the metal tag to a rather long frequency graph for general coverage and bandspread with coil set identification. The HRO-50 and 60 use only a letter identification on the coil set panel. The HRO-50 letter ID is engraved while the HRO-60 uses a medium-sized rectangular metal silk-screened tag. Early coil panels are made out of 3/16" aluminum. During WWII, the panel material was changed to 1/16" thick steel.

Early Production - The coils sets that were provided with the HRO receivers in the first three production runs are somewhat different from later production. The photo above shows the early white background frequency charts used on early coil sets compared to the black background charts used on most of pre-WWII production. To the right shows the alignment access holes used in early coils (photo A) versus the access holes in later coil sets (photo B.)

A

B

In the photos to the right, Photo A shows the contact insulators used on early coil sets from the first three production runs. Note the small contact buttons and the lack of any nomenclature except for the "B3" engraved coil identification. Photo B shows the type of contact insulator used on coil sets up to the change to ceramic insulators for the HRO-50 series. Note the larger, two piece contact buttons and the embossed nomenclature "NATIONAL CO." along with the "NC" diamond logo and contact button number identification, which is difficult to see but consists of a number below the contact.

A

B

Coil Assembly Construction and Alignment - The two holes on the top of the coil shield allow access to the trimmer capacitors for general coverage or bandspread alignment. Coil sets E, F, G, H and J provide a padder adjustment on the LO coil that is accessible from the top of the coil set since the bandspread function is not provided on these coil sets. The holes in the rear of the coil shield allow access to the padding capacitor that is used for bandspread only or the LO inductance adjustment for general coverage. The LO inductance adjustments on coil sets C and D consist of the brass metal disk inside the coil form that rides on a threaded rod. The metal disk is spring-loaded to retain its adjustment and can be changed by rotating the disk on the threaded rod using the hex nut provided to move the disk further into or out of the coil form. Since the disk is made out of brass, the further into the coil the disk is moved the lower the inductance is adjusted. Note in the photo to the right the various adjustments provided on the Local Oscillator coil. It can be seen in the photo that both coil assemblies use the brass disk for general coverage inductance adjustment. Differences are that the coil assembly to the left is from a very early coil set (D-run) and has a hex head adjustment on the bandspread padder condenser. The newer coil assembly on the right is from a 1936 coil set (J-run) and it can be seen that by this time the bandspread padder is slotted for screwdriver adjustment. The upper air variable capacitors are general coverage and bandspread trimmer capacitors. It can be seen that the type of air variable was changed to a more "fixed" assembly by replacing the hex nut and threaded end on the rotor with a press-fit rotor plates into the brass rotor shaft assembly. This applies to the bandspread padder also.
Since coil sets B and A are operating at a much higher frequency, a different LO inductance adjustment is utilized. This inductance adjustment consists of a loop of wire at the end of the coil wrap that is located inside the coil form. By moving this loop's position, the inductance of the coil is changed. The loop from an A coil can be seen in photo A to the right. During WWII, coil set C LO was changed to "loop" adjusted L.

In photo B to the right, note the compensation condenser behind the bandspread padder condenser. Note also that this padder adjustment is a compression capacitor. This is a Mixer coil assembly showing the difference in construction.

Nowadays when aligning the HRO coil sets, just adjusting the top trimmers will usually align the coil sets fairly well. See "Restoring and Aligning HRO Coil Sets" in the section below for more details on alignment. Also, all HRO manuals have the detailed procedure for full receiver alignment and owners should use that as a reference as well.

A

B

Individual Coil Assembly Identification - Looking closer at each individual HRO coil assembly, it will be noted that a number and a letter are engraved into the insulator block of each of the coil assemblies that make up the coil set. The letter will designate the frequency range and the number identifies the function of the particular coil. Thus coil assembly "C3" is the Mixer coil for the 3.5-7mc tuning range or coil assembly "A4" is the LO coil for 14-30mc. During WWII, the RAS version of the HRO Junior was built for the USN. To have this receiver tune uninterrupted from 180kc up to 30mc, it was necessary to move the IF down to 175kc. This will require slightly different coil assemblies and these were given a different identification code. A coil set for the RAS that has a coil assembly marked "G6" will indicate that the assembly is RAS RF2 for 180kc to 430kc. The various codes are shown below. During WWII, the engraved coil ID was changed to an ink-stamped ID and remained ink-stamped through the HRO-60 production. Since it's very easy to remove the coil assemblies for cleaning and restoration, it convenient that each one can be easily identified for proper reassembly. 
 

Frequency Ranges 

A =  14mc to 30mc (BS 10M)

B =  7mc to 14mc (BS 20M)

C =  3.5mc to 7mc (BS 40M)

D =  1.7mc to 4.0mc (BS 80M)
E =   .9mc to 2.0mc

F =   .48 to .92mc (or.5mc to 1.0mc )

G =  180kc to 430kc

H =  100kc to 200kc

J =   50kc to 100kc

AA = 27.5mc to 30mc

AB = 25mc to 35mc 

AC = 21mc to 21.5mc

AD = 50mc to 54mc

ADX = 35mc to 50mc

Coil Function

Standard HRO with 456kc IF

1 = RF1      2 = RF2     3 = Mixer    4 = Oscillator
 

RAS version with 175kc IF

5 = RF1      6 = RF2     7 = Mixer     8 = Oscillator
 

Band Spread - On pre-WWII coil sets, the tuning range graphs can instantly identify a coil set as belonging to the bandspread types A, B, C and D by the fact that two different graphs are provided. The left side graph is for general coverage and the right graph is for bandspread. Switching this function is accomplished by moving a 4-40FH screw from threaded holes in each of the insulator blocks - see photos above. The screw position should correspond with the position of the proper graph to select either general coverage or bandspread, e.g., screw in left hole = general coverage, screw in right hole = bandspread. NOTE: Only the A, B, C and D coils sets can be set to bandspread the amateur bands 10M, 20M, 40M and 80M.

The bandspread option was eliminated from most of the WWII coil sets. When the bandspread option was eliminated from the A, B, C or D coils, a "J" was added as a prefix, thus a non-bandspread "C" coil would become a "JC" coil. The "J" prefix originally came from the pre-WWII HRO Junior coils that were non-bandspread coils but was later applied to many coil sets used during WWII. Some coil sets will have numerical identification with the various USN versions of the HRO Junior known as the RAS, RBJ or RAW having coil sets numbered 1 through 7. The HRO- 5 and the HRO-W (Signal Corps) were also provided with "J" prefix coil sets. Post-WWII coils return the bandspread function to the A, B, C and D coil sets.

Coils with "J" Prefix

"J" prefix on Frequency Range on A, B, C & D coil sets = Non-band spread version (HRO Jr. and most WWII coil sets)

 

Coil Set Serial Number Location - Since the coil set of four A, B, C & D were sold with the receiver, they are serialized with the same letter-number combination as the receiver. The original four coil sets were aligned to their specific receiver, thus the importance of having the "matched" original coil sets. If optional coil sets were ordered post-purchase of the receiver then coil sets were not serialized. If optional coil sets were ordered with the receiver then generally they were serialized since National aligned them before the complete order was shipped. Any of the coil sets, regardless of the serial number, can be used with any HRO but they must be aligned to the receiver that they are intended to be used with for best performance.

The coil serial numbers are usually stamped on the LO coil shield. Sometimes the serial number will be stamped on the Mixer coil shield or the upper rail of the coil support on the right side of the coil set. Any sort of cleaning, past or present, tends to remove the ink-stamped serial number so it's common to find coil sets that appear to be un-serialized. Depending on the cleaning, sometimes a "shadow" of the serial number can be discerned in certain light. By observing the various acceptance stamps and marks on the coil sets, it's possible to see a similar markings within a receiver's four coil sets that would tend to confirm that a complete set is matched to the receiver, even if the serial numbers have been removed by cleaning. Some of the acceptance markings were in wax crayon and other more resistant material and these usually survive moderate cleaning efforts.
Medium & Low Frequency Coil Sets - Coil sets E, F, G, H and J cover the frequency range from 2000kc down to 50kc with a 50kc gap around the 456kc IF of the HRO. All of these coil sets had only one graph provided since band spread was not available. E and F provided a logging chart for favorite AM BC stations. MF and LF coil sets G, H, and J also provided logging charts for coastal station settings and other types of stations that were operating in that region of the spectrum. The plastic cover that was installed over the logging chart was a matte finish that allowed writing (in pencil) on its surface, thus stations could be logged without removing the frame and plastic cover to write on the paper logging chart. The "pencil on plastic" writing could be erased if station logging changed. Buying the E coil provided a ham with a way to have 160M coverage and then to have the D coil set to bandspread the 80M band. E and F coil sets were fairly popular sellers with the hams and it's not too unusual to find an HRO that has six original coil sets A through F.

photo right: G coil set showing the logging chart with matte finish plastic cover that could be written on with pencil

The G, H and J coil sets were more for commercial use at shore stations, airports and with other users that needed coverage of the lower medium frequency range and low frequency range. During WWII, most HRO receivers were equipped with full sets of coils, so it's not uncommon to find the military versions of the G, H and J coil sets. During the 1930s, 40s and 50s, most (if not all) ship to shore traffic, ship navigation, air navigation, weather forecasts for aviation and sea, time signals and the military (especially the Navy) were on the lower frequencies and the amount of interesting signals found there was incredible. Listening on the lower frequencies was extremely popular. Of course, the use of the "long waves" has changed over the decades since then and nowadays almost all of the signals in that region of the spectrum are now using digital or data transmissions. If you listen below 530kc without the receiver's BFO turned on, you'll hear very little. This has led many casual listeners to believe that nothing can be found below the AM BC band. Additionally, this part of the radio spectrum is subjected to a lot of man-made noise along with considerable noise from solar activity. Special antennas are required for successful reception of signals below 530kc.

photo right: This is the "H2" coil (RF2) from an H coil that covers 100kc to 200kc showing the much larger inductances required to tune in the LF range. Since the H coil is a general coverage coil only one trimmer capacitor is required. This is a WWII vintage coil assembly showing the polystyrene coil forms.

Post-WWII Coil Sets - During WWII, probably with the HRO-5/W, all of the coil forms used in all of the coil sets were changed from the brown phenolic material to polystyrene. Polystyrene plastic was used up through the HRO-60 production. With the introduction of the HRO-7, levers were installed on the receiver to allow extracting the coil set and the coil set panels had guide pins that interfaced with the levers. This eliminated the grab handles on the coil set panels. This type of coil panel with guide pins was in use through the HRO-60 production. These later coils had ceramic insulator blocks that integrated switches that easily allowed selecting bandspread or general coverage. These switches eliminated the nuisance of moving the 4-40FH screws to go from general coverage to bandspread.    >>>

photo right: Post-WWII coil sets showing the HRO-5A1 coil set style with silk-screened panel that incorporates the graphs and coil set ID. The HRO-5A1 coil sets were the last type to use the grab handles.

>>>  With the HRO-60, double conversion was added to the receiver. The actuation of the switch to accomplish the conversion is provided by the particular coil set installed in the receiver. Since double conversion only affected coil sets A and B, along with the other "A" prefix coils sets, AA, AB, AC, AD and ADX, only these coils sets had a hole located in the LO coil shield between the two rear alignment access holes. This additional hole is a clearance hole for the double conversion switch. When coils sets are installed that don't have the clearance hole, the switch is kept from actuating, keeping the receiver in single conversion. When any A prefix, A or B coils were installed into the receiver, the double conversion switch was actuated by allowing a switch lever to protrude into the clearance hole.
photo left: Close-up of the LO coil assembly from an HRO-60 C coil set. Note that the insulator base in now made out of ceramic. Note also that the coil form is polystyrene which had been in use since late WWII.

photo right: HRO-60 C coil set showing the metal ID tag, the locating pins, ceramic insulator blocks and the switches that allow selecting either general coverage or bandspread. The earlier HRO-50 coil sets will have the identification information engraved into the front panel of the coil set rather than the aluminum ID plate. Note that only the last four digits of the entire receiver serial number is stamped on the coil sets. In this case, located on the mixer coil shield is "0255" which is from HRO-60R sn 393 0255.

Restoring and Aligning HRO Coil Sets

Nearly all of the HRO receivers that are found today are missing most (if not all) of their original coil sets. This means that most HRO enthusiasts are always on the lookout for "orphan" coil sets, that is, coil sets that, for some reason, have become separated from their original receiver. These coils sets generally require a little work to have them "match" the intended HRO receiver. Additionally, every coil set that is going to actually be used in a receiver should be disassembled and thoroughly inspected, cleaned and then reassembled. After this procedure, the coil set will have to be aligned to the HRO receiver that it is going to be used with. This procedure assures that peak performance will result with the new coil set operating in its new "parent" HRO receiver.

Disassembly and Cleaning - All four of the coil assemblies are mounted inside aluminum coil shields that are mounted to the coil set front panel by four screws that "clamp" the coil shield to the rear panel. By loosening these screws (just loosen them) the entire coil assembly is pulled off of the rear coil panel. Now, remove the three screws on top of the coil shield that mount the coil insulator and slide the coil assembly out of the coil shield. You'll usually find spider webs, dead insects, dust and dirt, all depending on how and where the coil set was stored. Inside the aluminum coil shield will also have a lot of residue that should be removed. Clean by "dusting" - that is - use a soft bristle paint brush to dust off the dirt and insect debris. Be careful of the fine wires that connect the coils to the contacts. Carefully inspect all of the solder joints for cracks or poor soldering. Check the condition of the coil and dust off any residue. Check the trimmer capacitors for bent plates. If everything looks good, spray a very small amount of De-Oxit on the rotor contact and the adjuster screw plate and rotate the trimmer capacitor several turns to clean the contacts. Always use a 1/4" Nut Driver to do this operation as this will prevent damage to the slots on the trimmer adjustments. Be sure to return the trimmers approximately to their original position. Return the coil insulator assembly back into the coil shield and install the three mounting screws. Install the entire coil assembly back onto the coil panel. Repeat this procedure for each of the four coil assemblies. When reassembly is complete, the coil set is now ready for alignment in the intended "parent" HRO receiver.

Slotted Adjustment Screw Heads - These "domed shaped" adjustments for the trimmers on the coil sets were actually made at National. The original adjustment heads were hex shaped brass. National added the dome using solder and then slotted it so the trimmers could be adjusted by either a hex tool or with a blade screwdriver. Sometimes you'll find the domed heads so "gnarled" that the screwdriver blade won't move the trimmer and just slips out of the slot. You can resolder a new domed head on the bad trimmer and then slot it with a hack saw blade. Remove any residue afterwards. Also, shape the slot carefully so maximum blade contact is assured. This way the new slot will work fine. Be sure to re-lube the associated trimmer since the heat may have vaporized the old lube.

Cosmetic Restoration

Many times, the "orphan" coil sets we find are not specifically the type for the intended "parent" receiver. Maybe the coil set might be a military version with a steel panel but we want a pre-war coil set with an aluminum panel. Luckily, the coil assemblies themselves are interchangeable and can be interchanged from steel panels to aluminum panels with ease. All of the WWII production of A, B, C and D coil sets are non-bandspreading J-type coils. So, while electronically these coils certainly can be used, they wouldn't be proper in a pre-war HRO Senior. If everything is correct and only a coil assembly swap and chart-graph transfer is required the entire procedure can be accomplished in a few minutes.

Sometimes, the coil panels we have to use are painted incorrectly for the intended receiver. Sometimes, the charts-graphs coverings are in bad condition or missing. More than once, I've found coil panels where the grab handles were sawn off so the coil set could be used in an HRO-7 or later receiver. All of these conditions will require repainting the coil panel.

Strip the coil panel of coil assemblies and the charts-graphs. Use stripper to remove the old paint. Mask the rear of the coil panel. I use VHT Black Wrinkle Finish (BWF) paint which has a good pattern that is similar to the original. Apply a very heavy coat of BWF and apply heat, either using lamps or a hand-held heat gun. Let the finished paint set for several hours before moving the panel. Let the panel cure overnight before remounting the coil assemblies and charts.  

Where the handles have been sawn off you'll have to find a set of WWII HRO screw-on handles. The original handles used on the aluminum panels were swedge-mounted, which is why they were sawn off. You'll have to remove the rear panel by drilling out the rivets. What you end up with looks like the photo to the upper left. Note there is a heavy paper gasket between the two panels. Mount the new handles using countersunk flat head screws. Then repaint the panel. Now, remount the rear panel using "fake rivets" made out of 6-32 FH screws. The screw heads will have to be turned down to fit into the small countersunk holes in the panel.   Be sure that the screw doesn't protrude beyond the nut on the backside. If it does, it will interfere with the coil assembly insulator when mounting the coil assemblies. Mount the coil assemblies and then mount the charts and frames.  >>>

photo top: The three pieces that make up the coil panel. The handles are already mounted and the panel painted with VHT Black Wrinkle Finish paint. Note the center piece is the heavy paper gasket.

photo bottom: Completed A coil set that utilized the panel that had the handles sawn off.

>>>  If you need to repaint the metal chart-graph frames they should be semi-gloss black. I've found that many times the frames can be cleaned and polished using Wenol's chrome polish. This is the British version of Semi-chrome. The Wenol's polish will result in the frames looking in reasonably good original condition. When trying to match the coil set with an original finish HRO you don't want the coil set to look perfect - it wouldn't match.

Chart-Graph Plastic Covers - Nearly always these plastic covers have wrinkled and yellowed. I use them as original unless they are so bad that you can't read the graphs. If I have to replace a poor condition plastic cover I use the plastic material from the really cheap picture frames you find at the "Dollar Store." The plastic in these picture frames is just the right thickness and pliability of the original covers. Use the metal chart-graph frame as a template to cut the plastic to the same exterior size as the frame. Use a hot soldering iron with a "pencil tip" to make the mounting holes in the plastic. Just hold the plastic to the frame and push the hot soldering iron through the frame hole and you'll end up with proper size and location mounting holes. For the plastic cover on the logging charts, I use a slightly thicker and harder plastic found on other "not-so-cheap" picture frames. It's still very thin plastic but a little more rigid. This plastic can be sanded with 400 grit alu-oxide paper (go in just one direction when sanding) to make the surface look matte or semi-transparent. The mounting holes are done the same way with a soldering iron. When mounted, the logging chart is easy to read and the plastic cover can be written on with a pencil. If the "cheap-o" picture frames can't be located, check stationary stores like Office Depot for plastic dividers or plastic envelope document protectors. Also, some vacuum molded containers for pastry items can be used. There are lots of sources for thin flexible plastic material.

Chart-Graph Copies - If copies of original charts or graphs must be made to replace missing or damaged charts there are two methods. If you have original graphs to copy for the coil set you're restoring then be sure to copy them on to Manila-color or light tan color paper. Copy in black only - not color. This way the charts will look "aged" and original. Some HRO charts are available as scans on the Internet and if these are your source for the graphs then still print out on Manila or light tan color paper for best results. Also, don't buy an entire ream of colored paper. Stores like Office Depot will sell you individual sheets of colored paper (two cents a sheet last time I bought some.) The individual pieces of colored paper are available at the "self-serve copy" section of the store.

Coil Sets - Basic Alignment

Once the coil set has been thoroughly inspected, cleaned (if necessary) and has had the adjustments lubed, it can then be aligned to a specific receiver. Each of the top holes are assigned numbers 1 through 8 with 1 being the hole on the left. All General Coverage adjustments are made with even number holes and all Band Spread adjustments are made with odd number holes. The adjustments are as follows:

2 = 1RF      4 = 2RF     6 = Mixer     8 = LO    -   General Coverage Adjustments

1 = 1RF      3 = 2RF     5 = Mixer     7 = LO    -    Band Spread Adjustments

On the back of the shield/can of the LO coil there are two holes. The hole directly behind 8 is the inductance adjustment for General Coverage. The hole that is directly behind 7 is the padder for the low end of the Band Spread coverage. There are padders for the Mixer and the two RF stages for Band Spread adjustment and coil loops for General Coverage inductance adjustment but these very seldom require adjustment. If, after completing the full alignment of the coil set, it doesn't have consistent gain across the entire tuning range, in either General Coverage or Band spread (exclusive of antenna resonance changes) then possibly these adjustments have been tampered with. They are adjusted at the low end of the tuning range for maximum response. Again, these seldom need any adjustment. To check the low end alignment on the Mixer and two RF stages you can use a pencil's eraser end and push the outer plate of the tuning condenser section for the stage being tested. If the gain drops by moving the condenser plate in either direction - that is, either away or towards the adjacent plate, then that stage is "in tune." If you get an increase in gain as you move the outer plate as described then that stage needs to be adjusted on the low end inductance (or padder.)

For J Series Coils and the other non-band spreading coils (E, F, G, H and J) these are the adjustments:

2 = 1RF     4 = 2RF     6 = Mixer    7 = LO padder    8 = LO trimmer

8 is adjusted at the high end of the range while 7 is adjusted for the low end of the range. 2, 4 and 6 are adjusted at the high end of the range.

Coils sets must be adjusted in the General Coverage range first. So, install the 4-40FH screws in each of the coil insulator blocks into the left side screw hole (this selects General Coverage.) Install the coil into the receiver. Let the receiver warm up at least 15 minutes along with the test gear. You can do so quick trim adjustments but the final settings should have the coil in operation in the receiver for about 15 minutes minimum. 

Tune the receiver to the high end of the range and check the graph for the proper PW-D setting versus the frequency tuned. Most of the coil sets tune to 490 on the PW-D at the highest marked frequency on the graph. Not all coils sets are the same so check each graph to be sure of the correct frequency. Input that frequency into the Antenna input of the receiver. Be sure the signal generator is accurately set by using a digital frequency counter to check the actual frequency input. Keep the amplitude of the signal generator as low as possible and yet still have a signal that can be heard in the receiver and measures about mid-scale on the S-meter. You'll have to use a 400Hz modulated signal from the generator and then have the receiver in AVC with the BFO off for the S-meter to function. Now, adjust 8, 6, 4 and 2 for peak. Then tune the receiver to the low end of the graph frequency. This will be around 20 or so on the PW-D. Again, not all of the ranges are the same so check the graph to be sure. Adjust the inductance adjustment that is directly behind 8 for the low end. This will require removing the coil set from the receiver, making a slight change in the adjustment and then returning the coil set to test. You will have to turn off the B+ each time you remove the coil set and turn on the B+ after the coil set is reinstalled in the receiver. Remember that this inductance adjustment will affect the trimmer adjustment (8) so you will have to go back and forth a few times to get the coil to track correctly. Once the tracking is correct, then re-peak 6 and 4 at the high end of the dial. Now with the receiver connected to a tuned antenna for the high end ham band of this specific coil set, adjust 2 for the highest reading on the S-meter. Return the signal generator to the receiver Antenna Input for the next set of adjustments.  >>>

>>>  Once the General Coverage is tracking correct remove the coil set and move the 4-40FH screws to the right side hole for Band Spread coverage. Return the coil set to the receiver and tune to 450 on the PW-D. On coils sets D, C, B and A the ham band spread coverage is 450 to 50 on the PW-D. Check the graph for the proper high frequency input. Remember, some of the ham bands are different today then they were pre-WWII. For example, 20 meters' high end was 14.4mc then instead of the 14.350mc that it is today. Set the receiver to correspond to the graph. Adjust 7, 5, 3 and 1 to peak. Now tune the PW-D to 50 and input the correct low frequency and adjust the padder that is directly behind 7 for peak. Again, you'll have to remove the coil set each time to make the adjustment and then return the coil set to the receiver to check. Again, the adjustments do interact so both trimmer 7 and the padder adjustment will require a back and forth procedure until correct tracking is achieved. Once correct tracking is set now adjust the PW-D to 450 and peak trimmer 6 and 4. Now connect the receiver to a antenna that is tuned for the amateur band that is the Band Spread coverage and adjust trimmer 1 for a peak reading on the S-meter.

This completes the alignment of this coil set. Remember, since there isn't an Antenna Trim control on the HRO you must set the 1RF for peak S-meter reading on the ambient noise using the intended antenna for that band. You have to do this in both General Coverage and in Band Spread.

Images While Aligning Coil Set A - The A coil set is the most prone to misalignment due to peaking the adjustment response on an image. About half of the A coils I've tested were found aligned to an image. To avoid this always use the lowest possible signal level input from the signal generator. The image response is usually at a lower amplitude level than the proper signal response. On the A coil set only, the image will be below the proper signal by 912kc. This is one way to check to make sure that the LO section is set correctly. Also, when checking the tracking, if you're aligned to a image, the tuning will not correspond to graphs. When checking General Coverage, check 20mc WWV and 15mc WWV to be sure they are tuned at the proper PW-D reading (~ 230 for 20mc WWV and ~ 60 for 15mc WWV.) When checking the Band Spread (which can also be adjusted to an image independently from the General Coverage) test the tuning at 29mc and 28mc to see that those check points are correct.

When the tracking is correct on both GC and BS test the output response of the receiver across the tuning range in both GC and BS. If in GC it is consistent and maintains good sensitivity across the entire range then the Mixer and RF stages are aligned properly. If you find that the sensitivity falls off in the center of the tuning range or dramatically changes from one end to the other then it's likely that the Mixer is aligned to an image. Try aligning the Mixer to the other peak and recheck the coil tracking. If the improvement is noticeable then you were aligned to an image.

Without a doubt, the A coil set is the most difficult to get aligned correctly. It will take time and a lot of rechecking of the settings. When it is aligned correctly, the HRO performance is incredible on 14mc to 30mc. Since so many A coils are aligned to the image rather than the proper settings the performance of the HRO on the higher frequencies is sometimes thought to be rather poor. This isn't the case. With a properly tuned antenna (not an un-tuned random length wire) 10M performance will be surprisingly good. Also, the stability of the frequency is very good when the A coil set is cleaned and aligned properly.

Poor performing A coils should be checked thoroughly, cleaned carefully and then lubricate the trimmer contacts. Be sure to clean the contact buttons with De-Oxit. Follow this with a careful alignment and your HRO should perform very well up to 30mc. Be sure to consider though, you are using a mid-1930s designed receiver, so don't expect 21st century performance. However, you should be very pleased at the high frequency ability of the HRO.

 

The Various National Co., Inc. Power Supplies

Initially, there was only one of the smaller power packs ( the "Dog House" style) that could be used with the early 2.5vac HRO - the 5897. The 5897 was originally designed for the SW-5 receivers in 1930 but was later redesigned for the FB-7 receiver in 1933 and designated 5897AB. It had enough current carrying ability to operate the HRO. In fact, many early HRO receivers were paired with 5897AB supplies that have printed on the top ID label "Especially Designed for the FB-7." Under full load, the 5897AB will provide 2.5vac heater voltage at the receiver and +230vdc B+. All of the National power supplies have an increased tube heater voltage at the transformer to compensate for the IR drop across the power cable to the HRO (or other receiver.) There were also rack mounted power supplies, the GRSPU to operate a single receiver and the GRDPU for operating two receivers. Either rack supply could be ordered for either 2.5vac, 6.3vac (or a combination of the two) heater voltages. All of the power supplies used an 80 type rectifier and were a pi-type filtering system consisting of two filter capacitors (generally 8ufd) and a filter choke. A bleeder resistor wasn't used.

Be aware that there are a few of other "dog house" power supplies that look just like the HRO types but do not have the current carrying ability to operate the receiver properly. For instance, the 5887 was another FB-7 power supply but its B+ rating is for +180vdc. The 5886 was designed to operate the National SRR and later the 1-10 receiver. Although it will seem to operate a 6.3vac HRO Senior, actual measurement of the B+ will show that the B+ is around +170vdc. National did publish that the HRO would operate on B+ as low as +135vdc but added "at some sacrifice in performance" (which was mostly audio output related.) There is also the 5880 power supply that was advertised for the SW-3 or FB-7 receivers, though similar in appearance, it also has low B+ voltage of around +175vdc. Many HRO enthusiasts do use the 5880, 5886 or 5887 to operate their HRO receivers because they feel that the stability and sensitivity is improved at the lower B+ voltages. Even James Millen recommended +180vdc B+ at one time in one of his QST letters. I've tried operating the HRO receivers from +175vdc up to +240vdc and haven't noted any significant changes in performance within that range of voltages. Only the audio output is slightly less at the lower B+ voltages. Stability and sensitivity changes would require actual measurements to for comparison but I would expect that lower B+ voltages would benefit these two areas of performance.

photo above:  Power Supply Model 697 on the left and the earlier "Velvet AB" 5897AB Power Supply on the right.

The later (1939) Model 697 "Dog House" was designed to operate the 6.3vac HRO Senior receivers and provides the correct voltages under load. During WWII, a heavy-duty 697 was produced, the 697W. It is has a square housing, plug-in electrolytic can capacitor and is usually fungus proofed. The later 697 power supplies will have a switch to select the primary voltage, 115vac or 230vac. There were also two DC versions, the 686 for 6vdc operation and the 1286 for 12vdc operation. Both were vibrator-type power supplies with the 686 powering a standard six-volt HRO while the 1286 powered a special HRO, the HRO-12S that used 12 volt tubes. The 686 used a 6X5 rectifier and the 1286 used an 0Z4 rectifier.

The internal construction of the "Dog House" power supplies are all similar in that two sheet metal boxes contain the power transformer in one box and the filter choke in the other box. Both boxes are filled with a black tar-like wax to keep moisture out and to prevent noisy operation. Typically, a dual or triple filter capacitor is mounted to the inside base using a clamp-type holder. Triple capacitors had two sections connected in parallel for the output capacitance of the pi-filter.


photo above: The 5897AB "FB-7" power supply showing the construction of the chassis and how the choke and power transformer are encased in tar. Note the date written in orange crayon, Dec 1, 1933.


photo above: This shows the 697 power supply from 1946. Note that the construction of the chassis hasn't changed much, only it is now painted gray. The filter capacitor is now a three capacitor unit (8+8+8) but the output cap is actually tied in parallel (see jump on cap terminals.)

The power switch and the four-pin output socket are mounted to the angled panel. The power cord is routed through a grommet that is mounted to the chassis. The black wrinkle finished housing slips over the chassis and is mounted with four sheet metal screws.

Generally, to be able to safely use any of the "Dog House" power supplies it will be necessary to replace the filter capacitors. The original dual or triple capacitor can be left in place and modern tubular electrolytic capacitors connected to the wire lugs after they have been disconnected from the original capacitor's terminals. For a more original appearance, it's also possible to rebuild the old filter capacitor by installing new electrolytic capacitors inside.

I've tried re-forming the originals and have had fairly good luck doing so but make sure that the current drawn by the individual capacitors doesn't exceed 100uA at full voltage after reforming. Most original capacitors will end up with about 10 to 100 times that current draw at full voltage and while they may work okay for awhile, they should be replaced as a safety measure.

National Co., Inc. Power Supply Data

  PS TYPE                     INPUT/OUTPUT VOLTAGES                          RECEIVER

5897, 5897AB         115vac in, 2.5vac 10A, 230vdc 70mA                   SW-5, FB-7, HRO
GRSPU                     115vac in, 2.5vac or 6vac, 230vdc 70mA                    HRO, HRO SR.
GRDPU                     115vac in, 2.5vac or 6vac, 230vdc 70mA               (2) HRO, HRO SR.
697 (early)               115vac in, 6.3vac 3.5A, 230vdc 70mA                          HRO SR.
697 (late)                115/230vac in, 6.3vac 3.5A, 230vdc 70mA              HRO-5, HRO-5A, A1
697W (MIL)            115/230vac in, 6.3vac 3.5A, 230vdc 70mA                 HRO-5, HRO-W
686                           Battery, 6vdc in, vibrator type, 230vdc 70mA             HRO SR., HRO-5
1286                        Battery, 12vdc in, vibrator type, 230vdc 70mA             HRO-12S only
5886                        115vac in, 6vac 3.5A, 180vdc ~ 70mA                       SRR, 1-10, AGS
5887                        115vac in, 2.5vac 10A, 180vdc 70mA                         alternate FB-7 ps
5880                        115vac in, 2.5vac ~5A, 180vdc ~ 40mA                              SW-3

If the choke is open or if there is a power transformer problem then the black wax will have to be melted out. This is messy and difficult to accomplish requiring some way to heat up the entire chassis. Heat guns and propane torches don't apply enough heat evenly to remove all of the wax. Larger toaster ovens may work. I usually buy them at the "thrift stores" for a few bucks and then throw them away after they have served their purpose. Low heat, 200º to 250º F, is sufficient to melt out the wax since the heat is even and continuous but it will take an hour or so for all the of wax to run out. Replacement chokes aren't too much of a problem but a defective power transformer might be difficult to replace since the heater voltage is somewhat higher than standard to compensate for the IR drop across the power cable of the HRO receiver. Sometimes vintage power transformers from late-twenties or early-thirties console radios could work. These transformers were originally designed for 110vac input and at today's higher line voltages (115-120vac) the 2.5vac filament would be about right and the older B+ of around +180vdc used on this vintage tubes is about right for the HRO (probably around +200vdc with today's line voltages. A custom-built transformer would be an expensive solution since it would probably cost more than just buying another "Dog House" power supply. Fortunately, the choke and the power transformer seem to be pretty robust and are seldom a problem.

Home Brew Power Supplies - Since the HRO was usually offered with the power supply as an optional purchase, there are some home brew power supplies around that range from crude efforts to nicely designed and built units. It isn't difficult to build a better power supply than the one that National offered. After all, it was being built during the Depression, so the use of pi-filtering provided adequate hum reduction and kept the cost low. It's easy to install a second choke and another filter capacitor for a dual section filter that would provide a very smooth B+ and no noticeable hum. When considering building an HRO power supply the main drawback is the IR drop across the power cable on the 2.5 volt heater receivers. To compensate for the drop, National increased the heater voltage about 20 percent. Nowadays most power transformers available don't have a 2.5vac 10A winding, let alone one that has 20% higher voltage output on the filament winding. This requires using a filament transformer with 2.5vac 10A ratings (they are available) and actually the 2.5vac will measure slightly higher in voltage, usually around 2.6vac. This will work fine with the 2.5 volt HROs, although the heater voltage at the receiver may be just slightly low (probably around 5% low which is within the tube specs.)

As mentioned above, late-twenties console radios may supply a transformer with a 2.5vac winding designed for 110vac line which today would produce somewhat higher, probably 2.7vac or 2.8vac with 120vac line input. Console radios are mentioned because most of that style cabinets had nine or ten tube radios that had lots of current requirements and the 2.5vac tubes were used up to around 1932-33. Check the radio schematics though as it was popular to use separate windings with one 2.5vac winding for the RF/IF/DET requirements and separate 2.5vac winding for the P-P audio tubes. You want a power transformer that has all of the tubes running on a single 2.5vac winding.

With the 6 volt heater receivers, the IR drop is minimal and a standard 6.3vac heater winding will function fine. In fact, if you measure most of the 6.3vac heater windings you'll find they are up around 6.7vac or even a bit higher which will work great with the 6 volt HROs. Certainly voltage regulation can be added to the B+ which will help with receiver stability. Other improvements could be to add a B+ voltage meter or a power ON pilot lamp, etc. I home brewed a dual supply built into one box that allowed operating either a 6 volt HRO or a 2.5 volt HRO. A switch on the front panel allowed selecting either the standard 6.3vac power transformer winding or a 2.5vac 10A filament transformer winding. With a little thought, it's pretty easy to design and build a better power supply than the National "Dog House."  If you want to build your new power supply into a housing that resembles the original "Dog House" cabinet, here's a rough sketch that gives the overall dimensions of the outer cabinet. DOG HOUSE SKETCH

 



photo above
: The 10" speaker for the NC-100 series on the left and the 8" speaker, MCS, for the HRO on the right. Note: the hole plug installed in the upper right corner of the MCS speaker box is not original.

National Co. Speakers

National was a little vague about speakers when the HRO finally became available in March 1935, saying only that one could be supplied if desired. The rack mounted speaker was the first to be shown in an advertising photograph. It looks very much like the rack speaker that was supplied with the AGS receivers. These panel mounted speakers are designated as RFS or sometimes RFSH. The standard National black box speaker with the cloisonné "NC diamond" emblem was available by 1936 and is listed in the 1936 HRO Instruction Manual as MCS, selling for $23.50. The MCS photos were not used in National advertising until much later. The MCS National box supplied for the HRO contains an 8" Jensen PM speaker with audio output transformer mounted inside the box. Today, most the the grille clothes will have faded to a gold color but originally the cloth was very dark, almost black. National also built large "NC" black box speaker that used a 10" diameter Rola Electrodynamic speaker. This speaker was designed for the NC-100, NC-101X family of receivers that had push-pull audio and provisions to excite the field coil of the speaker (the HRO didn't have that capability.) After WWII (and probably before,) empty speaker cabinets were available from National as NDC-8 and NDC-10, so it might be possible to find an original large National speaker cabinet with a post-WWII 10" PM speaker installed. In fact, that is what I use with my HRO-W. A proper audio output transformer must be used, e.g., the Triad S-53X or similar. The bass response from the 10" speaker box is quite superior to the 8" speaker box.  NOTE: There was also an optional large wooden speaker enclosure sold by National for the NC-100 family that contained a 12" electrodynamic speaker. Also, the was an earlier 10" metal version that doesn't have the vertical "bars" and has a large "NC diamond" embossed on the top of the enclosure.

HRO Coil Boxes

National offered at least two types of wooden coil boxes for storage of unused coils. The smaller coil box allowed for storage of three coils and assumed that HRO owner had the four coils that came standard with each receiver. One coil would be installed in the receiver and the remaining three would be in the small coil box. Also offered was the large coil box that would allow storage of five coils. It was for the HRO owner who had opted for the .5-1.0MC coil (F) and the .9-2.0MC coil (E.) These two coils provided coverage of the Standard AM BC band and also allowed the user to bandspread coil D (80M) and still cover 160M with coil E.

The wooden boxes are assembled with finger joints and the rear strips are mounted with nails as are the small blocks that hold and space the coils. The wood used is boxwood or poplar and the finish is just a light stain, though sometimes they are found with a black stain. Boxes have also turned up painted gray and some have metal front covers, though these variations are probably for the later HRO receivers. Very simple construction - but the boxes really made storage of the unused coils convenient. There was a rumor that National utilized the wood from their "used" shipping crates to build their coil boxes. It's probably more accurate to say that National used the same wood to build the coil boxes that they used to build their shipping crates.

National's Receiver Upgrades and Services

More than any other radio manufacturer, National advertised their willingness and ability to do rework, upgrades and general service to their receivers. It was well known that National would upgrade their HRO Junior to an HRO Senior for about $20 - probably the cost of the parts. However, National would also upgrade earlier receivers to the latest specs (if practical.) Shown in the photos below is the HRO receiver originally owned by F.E. Handy, W1BMI, the well known Communications Manager for the ARRL in the twenties, thirties and forties. Copious notes in Handy's original HRO manual reveal that he sent the receiver to National in March of 1946 for a new S-meter and alignment. In November 1946, he sent it in for upgrades that included a Noise Limiter circuit, polystyrene antenna terminals, new style IF transformers and changing the cabinet to the later style with improved ventilation. The workmanship is first class, including the small chassis that was added to house the noise limiter circuitry. Serial Number is G-113. The F.E.Handy HRO is owned by: Dale Sanford

The F.E.Handy HRO Receiver G-113 - Handy was Communications Manager for the ARRL in the 1920s,30s&40s

 

Vacuum Tube Types Used in the HRO Receivers

HRO, HRO Senior 2.5vac - 1RF=58, 2RF=58, Mixer=57, LO=57, 1IF=58, 2IF=58, Det/AVC=2B7, BFO=57, AF Output=2A5, Rectifier (in power supply)=80

HRO Senior 6.3vac version, HRO-M - 1RF=6D6, 2RF=6D6, Mixer=6C6, LO=6C6, 1IF=6D6, 2IF=6D6, Det/AVC=6B7, BFO=6C6, AF Output=42, Rectifier (in power supply)=80

HRO Junior - Uses same tube line up as HRO or HRO Senior depending on whether the receiver is the 2.5vac or 6.3vac version

HRO-5/W - 1RF=6K7, 2RF=6K7, Mixer=6J7, LO=6J7, 1IF=6K7, 2IF=6K7, Det/AVC=6SQ7, BFO=6J7, AF Output=6V6, Rectifier (in power supply)=80

HRO-5A-1 - Same as HRO-5/W except add Noise Limiter Amp=6J5, Noise Limiter Rectifier= 6H6

HRO-6 - Same as HRO-5A1

HRO-7 - 1RF=6K7, 2RF=6K7, Mixer=6J7, LO=6C4, 1IF=6K7, 2IF=6K7, Det/AVC=6H6, BFO=6J7, NL=6H6, 1AF=6SJ7, AF Output=6V6, Voltage Regulator=0A2, Rect (in PS)=80

HRO-50 - 1,2RF=6BA6, Mixer=6BE6, LO=6C4, 1,2IF=6K7, Det/AVC=6H6, NL=6H6, 1AF=6SJ7, Phase Inv/S-meter Amp=6SN7, BFO=6J7, P-P AF Output=6V6(2), Rect=5V4, VR=0B2

HRO-50-1 - Same as HRO-50 except change 2IF to 6SG7 and add 3IF=6SG7

HRO-60 - 1,2RF=6BA6, Mixer=6BE6, LO=6C4, 2ndConverter=6BE6, 1,2,3IF=6SG7, Det/AVC=6H6, NL=6H6, Phase Inv/S-meter Amp=6SN7, BFO=6SJ7, 1AF=6SJ7, P-P AF=6V6(2), Rect=5V4, VR=0B2, Filament Current Regulator=4H4

A Note on Vacuum Tubes - Although any receiver will function fairly well with marginal tubes, best performance will be obtained by installing tested-good NOS tubes or tubes that test "as new" on a quality mutual conductance tube tester.

 

Guide to Restoring the HRO Receiver

Though much of the restoration process is similar to any other vintage receiver's restoration, the HRO does have some specific parts the require special knowledge and techniques to restore or to duplicate. The PW-D Micrometer Dial section below is very detailed since it is quite easy to reassemble this device incorrectly. The PW-Gear Drive below is also detailed for the same reasons. In addition to the restoration and rebuilding information, three specific HRO restorations are profiled. These sections show the various restoration problems that are encountered during the process and each restoration is shown from beginning to end with lots of photographs.

 

PW-Gear Drive and PW-D Micrometer Dial - Detailed Information

The heart of the HRO is its fabulous PW-Gear Drive and its PW-D Micrometer Dial. These incredibly engineered components working with the precision tuning condenser and the innovative coil sets account for the HRO's top performance, both electronically and mechanically. Unfortunately, almost every HRO receiver encountered will have problems with the PW-Gear Drive or the PW-D Micrometer Dial, or both. Most problems found will be traced to incorrect reassembly by former owners. The following section gives the HRO owner detailed information on how to correctly disassemble, service and reassemble the PW-Gear Drive and the PW-D Micrometer Dial. "PW" was National's designation based on the gear drive's use of a "precision worm."

Lubrication and Assembly of the PW Gear Drive  

The Cast Metal Gear Box - Every HRO I've worked on has needed servicing of the PW Drive Gear Box. Usually, when the top cover is removed, it will be apparent that the old grease is very dry and not coating where the gears mesh. Also, you might find that the split-ring anti-backlash gears are stuck from old dried grease that prevents the anti-backlash from working. You might even find that the worm gear shaft is assembled incorrectly. Down where the worm gear is located can't be seen from the top. To fully service the PW Gear Drive requires disassembly. To check the worm gear assembly the elliptic bearing hub has to be removed. Be careful when removing the last screw that holds the elliptic bearing hub in place because the worm gear shaft thrust is spring-loaded. Hold the split-ring gear together with your fingers as you remove the worm gear for inspection and lubrication because the split-ring gear is also spring-loaded for anti-backlash. As you remove the worm gear, note how many teeth backlash is set on the split-gear - it shouldn't be more than two teeth. Clean all of the old grease off using a stiff tooth brush with WD-40 or some kind of light oil. Then apply a light-weight grease like Lubriplate to the gears. A small amount of light oil should be worked into the split-ring gears so they can easily move. Be careful of the three springs that load the split-gear. They sometimes come off their mounting pegs but they are easy to reinstall.

Note how the worm gear shaft is assembled. It's not uncommon to find the HRO gear drives assembled incorrectly. Look at the National artwork to the right. This shows the correct assembly of the worm gear shaft, washers, spring and the ball bearing. Note from the worm gear flange there is a ball bearing then a washer, a spring and another washer. The assembly is held in place by the elliptic bearing hub. The ball bearing appears to have no support but the design is for the ball bearing to apply thrust force against the shaft flange without an excessive amount of friction. The worm gear thrust bearing can be assembled in reverse order and the drive works okay but National's design provided minimum friction from the thrust force.  >>>


photo above: The Early HRO PW Gear Drive (Cast Metal) - enlarged for increased detail  - provided by: Charles Hentsch

>>>  It's also possible to assemble the worm gear thrust bearing with the ball bearing reversed. This will not allow the ball bearing to rotate properly. Usually the side of the ball bearing with the stamped identification should be facing the flange. However, it's best to test whether or not the ball bearing will rotate correctly. Do this by assembling the thrust bearing on to the shaft. Then hold the pointed-end thrust bearing against a piece of wood by pressing against the spring with your fingers. Then rotate the worm gear shaft with the fingers of the other hand. If the thrust ball bearing is mounted correctly, the outer housing of the ball bearing will not rotate but the inner race will. If the bearing is incorrectly reverse mounted then the outer housing of the ball bearing will rotate with the shaft.

To reassemble the gear box first apply some light-weight grease (I use Lubriplate 130-A MIL grease) to the conical thrust bearing of the worm gear as this is spring loaded against a mating conical thrust bearing that is machined into the cast gear box housing. Don't over lubricate any of the individual parts by packing the gear box full of grease since the thrust ball bearing and the split-gear must be able to move easily which is difficult when either part is loaded with grease. Be sure to thoroughly clean the thrust ball bearing as it is essential to the design that this bearing moves easily. Use a very light oil and grease mixture for the ball bearing. Lube the worm gear with light-weight grease. The split-gear looses its anti-backlash position for spring loading when the worm gear is removed (the "loaded" position is held by the worm gear) - just rotate the two gears in opposite directions a couple of gear teeth while engaging the worm gear. Be sure to watch the split-gears as it is common to have the anti-back lash setting slip as the worm gear is inserted into position. Check the anti-backlash tensioning springs to be sure they are installed on their pegs. Lube the inside of the elliptic hub with light-weight grease. Reinstall the elliptic bearing hub casting which spring-loads the thrust on the worm gear. Note that one of the flanges is marked "TOP" so be sure to mount the the elliptic bearing hub housing with this flange up. This is important because it places the elliptic hub in the correct position to function with the PW-D micrometer dial. Check the gear drive for smooth operation and zero backlash.

The Cast Metal Gear Box is used from 1935 up to around 1945. The replacement Stamped Sheet Metal Gear Box was introduced with the HRO-5 and HRO-W receivers.

The Stamped Sheet Metal Gear Box - During WWII, the cast metal gear box was replaced with a stamped sheet metal housing. This was a cost-reduction that eliminated the difficult to machine cast metal gear box. Since the new sheet metal gear box was thin metal, it couldn't be machined to provide a thrust bearing at the rear. Instead a bronze bearing was pressed into the sheet metal to provide a thrust bearing. Also, since the sheet metal housing was thinner than the cast metal housing, the condenser supports were integrated into the assembly support of the gear box. This then reduced the clearance inside the sheet metal box and the thrust ball bearing assembly on the worm gear shaft had to be redesigned. With the sheet metal gear box, the thrust bearing assembly now had the ball bearing at the front of the shaft and the spring butting up against the worm gear flange. Also, the washers were eliminated and a brass fitting was installed to allow the spring force to be applied to the inner race of the ball bearing. If you try to assemble the thrust bearing like it was for the old cast metal gear box, it will not fit into the sheet metal gear box.

This reversal of the thrust ball bearing assembly on the worm gear shaft works essentially the same as the older assembly does and does provide a reduction in friction for the thrust force applied. The sheet metal gear box is introduced with the HRO-5 and HRO-W models (the new octal tube versions of the HRO for WWII production.) This dates from about 1945. All post-WWII gear box housings are similar in construction and include the HRO-5A, HRO-7, HRO-50 and HRO-60 versions.

The later version of the split-gear anti-backlash drive is heavily spring-loaded. Usually the split-gears can only be set to "one tooth" of backlash. Since the springs are very strong this "one tooth" loading is sufficient for removing any backlash. 

 

PW-D Micrometer Dial

The PW-D Micrometer Dial Design - The famous "micrometer" dial used on the HRO and several other National receivers was based on a Sperry Gyroscope design. The National "Micrometer Dial," or PW-D (PW-D is the National designation for the complete Micrometer Dial) doesn't change during many years of manufacture except for two very minor mechanical modifications made during WWII. Only two moving parts and eight stationary parts are used in this elegantly simple design. The following are the parts used:

1 - Index Dial (or Outer Dial  - moving part 1)
1 - Number Dial (or Inner Dial  - moving part 2)
2 - Retainer springs (holds Number Dial to Index Dial)
1 - Knob - HRK type - (mounted to Index Dial)
3 - 4-40 Flat head machine screws (to mount knob to Index Dial.)
1 - Set Screw (during WWII - two set screws)                                          

How the PW-D Works - The PW outer Index dial by itself will not not turn the inner Number dial - it must be used with the PW Gear Box that has the elliptic hub mounted on the front of the gear box. The elliptic hub actually provides the eccentric that always forces engagement of the inner Number dial gear teeth to the gear teeth of the outer Index dial as the PW-D is rotated. The gear teeth are located on an embossed rim inside the outer Index dial and are cast into a circular recess in the inner Number dial. All of the parts must be assembled together  correctly and the PW-D must be mounted on the elliptic hub of the PW Gear Box in order for the micrometer dial to operate and indicate properly. More details on how to disassemble, service and reassemble the micrometer dial in the "Lubrication of the PW Gear Drive and PW-D Micrometer Dial Assembly" section further below.

PW-D Evolution - The first PW-D had a flat nickel finish on the outer Index dial with black index graduations. The inner Number dial was also nickel plated with black numerals. German Silver was used on the smaller HRO knob skirts. German Silver is an alloy made of copper, nickel and zinc. Since the PW-D dial is cast metal, it can't be solid German Silver but is actually German Silver plating. Generally, the PW-D was supposed to appear "flat white-silver." By 1936, with the introduction of the NC-100, a number of different lacquer finishes were available for the PW-D. The HRO Senior and Junior PW-Ds were painted with a dark gun-metal gray lacquer finish with white index lines at first. This gun-metal gray was very dark and appeared black except under bright light. By the late-thirties, the dial paint had been changed to dark bronze (a black-brownish-olive color) lacquer. Again, this dark bronze appears black except under intense light lacquer finish. Note the photos below of both the HRO Sr and Jr PW-Ds. These colors generally appears black in normal room light but under the intense light of the camera "flash" the true color shading is seen.


Early HRO - German Silver Plated


HRO Sr/Jr- Lacquer Paint (Gun-Metal Gray)


HRO Sr/Jr (late) - Lacquer Paint (Dark Bronze)


HRO-W - Bronze Lacquer & MFP

 


HRO-60 - Matte Chrome Plating

The PW-D for the NC-100 was blue-gray on the outer dial and red with white numerals on the Number dial. The NC-100X used a gray Index dial and the HRO type black Number dial. The PW-D was also used on the National Airway Communication Receivers that were based on the NC-100 receiver. These receivers generally have gray PW-D Index dials with black Number dials with white numerals. The later Airway receivers will generally have the dark bronze PW-D dials.

When the NC-101X was introduced, it usually was fitted with the same "black lacquer" PW-D as the HRO. Some pre-WWII military HROs were fitted with light gray PW-D dials. During WWII, some HRO receivers were moisture and fungus proofed (MFP) which sometimes included coating all of the knobs and the PW-D Index dial. MFP gives the outer Index dial an olive-drab appearance. 


photo above: The gray with red number dial PW-D used on the NC-100 receivers.

After WWII, the HRO-5A series goes back to the dark bronze finish for the PW-D with some exceptions that may have resulted from National "cleaning house" and thus early dark gun-metal gray dials do show up on post-WWII HRO receivers. When the HRO-7 was introduced the PW-D was given a light gray paint job to match the changes that the HRO-7 cabinet featured. The HRK knob was also changed to an HRT style knob. Some rack mount HRO-7 receivers were finished in wrinkle black and these receivers will have the standard "black" PW-D dial installed. The HRO-50 returned to a nickel or chrome finish to the outer dial with the inner dial black with white numerals. The HRT style knob was retained. The HRO-60 returned the HRK knob to the PW-D assembly. The HRK knob was also usually installed on the HRO-50 if it was rack mounted.

Throughout this evolution only one minor mechanical change to the design was incorporated. During WWII a guide pin was added to the inner wheel which helped with alignment and made installing the PW-D onto the elliptic hub easier. Additionally, a second set screw was added, probably to meet "military specifications."

National, at times, seemed to use whatever PW-Ds they had in stock - or maybe, in "excess" stock. It's common to find late-thirties HRO receivers with gray NC-100-type PW-Ds. Gray PW-Ds will also show up on early NC-101X receivers and some National Airway Receivers. Examine such variations for authenticity but don't be surprised to find that the PW-D is original to the receiver.

 

PW-D Micrometer Dial Assembly

The PW-D Micrometer dial only has two moving parts and seven stationary parts. Elegant in its simplicity of design, it is very easy to work on if you know the "tricks." About the only thing that goes wrong with the dial is that it becomes noisy in operation and might have a rough feel when tuning. Synchronization might be a problem too - if the dial was reassembled incorrectly. The inner dial has gear teeth cast into its inner perimeter and also has the bearing that fits on the elliptic hub of the tuning shaft bearing housing. The outer dial has mating gear teeth that are cast into a rim projection and a hub that is set screw attached to the tuning shaft. By having the inner dial ride on the elliptic hub cast into the tuning shaft bearing housing, the gears of the two dials engage and as the outer dial is turned, by way of the mating gears, it turns the inner dial and since the outer dial is coupled to the tuning shaft, it also moves the tuning condenser. The elliptic hub is actually round but since the tuning condenser shaft exits the hub "off-center," the hub bearing surface appears as an ellipse to the tuning condenser shaft. This eccentric placement forces engagement of the inner and outer dial gears, thus driving the inner dial with the rotation of the outer dial.  >>> >>>  To disassemble the Micrometer dial, first tune the receiver to "250" then loosen the dial set screw. The dial should come off of the elliptic bearing hub and tuning condenser shaft with very little effort. If the PW-D dial is hopelessly stuck, read the section below "Dismounting a Stubborn PW-D" for the removal procedure if this problem is encountered. With the PW-D dismounted, look at the back of the dial and, if the PW-D was synchronized prior to removal, you will notice that the three screws that hold the knob to the dial appear through cast holes in the inner dial. Also note that the two springs that hold the inner dial to the outer dial are pointing straight up. Remove the knob. Then remove the two springs - their ends fit into slots at the end of the shaft hub of the outer dial (where the set screw is.) Now the dials can be separated. Examine the cast gear teeth for condition. Usually they are in good shape and all that is present is old dried grease. Use a tooth brush and light oil to remove all of the old grease and then apply a light coating of Lubriplate to the inner dial gear only. Position the inner dial to the "250" position with the cast holes lined up for the knob screws. Then install the two retaining springs. >>>
>>>   Before putting the PW-D back on the receiver, lightly grease the elliptic bearing hub (since the inner dial rides on this it should be clean, smooth and lightly greased.) Verify that the elliptic hub has the "TOP" embossing on the flange at the top. This places the elliptic in the correct position for the PW-D gears to engage. The PW-D should already be set to "250" and the receiver condenser should not have been moved. Place the PW-D back on the receiver by carefully noting how the elliptic hub requires the inner dial to be slightly down from center. If you have assembled the PW-D correctly, it will just slip right on to the hub. Tighten the set screw and test the operation - it should be ultra-smooth and very quite in operation. If your PW-D dial doesn't just slip onto the elliptic hub then it isn't assembled correctly. The PW-D can be assembled several ways but only one way is correct. There are no exceptions to this - if the PW-D doesn't slip right onto the hub then it is assembled incorrectly. If your PW-D doesn't just "slip onto" the bearing hub, read the section below "Correct Assembly of the PW-D Micrometer Dial - Details."

Around the beginning of WWII, a guide pin was added to the inner dial, located just above the inspection/grease hole. This pin limits the vertical movement of the inner dial and was probably installed to keep the gear alignment fairly tight and allow easier installation of the PW-D onto the elliptic hub. Later production dials will have this guide pin molded in the casting of the inner dial.

 

photo left: Two moving parts and seven stationary parts are all that are used in the PW-D. A second set screw was used during and after WWII - total of eight stationary parts then.

Correct Assembly of the PW-D Micrometer Dial - Details - When the PW-D is assembled correctly, it will just slip onto the elliptic hub. If you are trying to force the PW-D onto the hub, then the PW-D is assembled incorrectly - no exceptions. Look carefully at the photograph to the right noting the location of the smaller round hole, the two oval holes, the "NATIONAL CO., INC." embossing and the location of the screws that are for mounting the knob. If you orient the reverse side of your assembled PW-D in this position and then look at the outer dial, you should see "250" centered in the window that is directly behind the "NATIONAL CO., INC." embossing.

To check your assembly,...position PW-D as shown in photograph to the right. You should see the following:

1. "NATIONAL CO., INC" embossing should be at 12 o'clock

2. Small round hole should be at 9 o'clock

3. Two oval holes should be at 2 o'clock and 4 o'clock

4. Note that all three knob screws are directly centered in the small round hole and the two oval holes

5. Turn dial over from right to left and you'll see "250" centered in the window at 12 o'clock (the window should be directly behind the embossing "NATIONAL CO., INC" on the back of the inner dial.)

If your assembled PW-D meets this criteria, then it will slip right onto the elliptic hub. You may find that, for some reason, the PW-D just doesn't want to fit onto the elliptic hub. Don't force it. When assembled correctly the PW-D will slip right on to the hub - no exceptions. If it doesn't, remove the PW-D and look carefully at the Number Dial bearing and its relationship to the tuning shaft hole in the Index Dial and note that the "off-center" nature of the opening is correct for the elliptic nature of the hub. Also, verify that the elliptic hub is installed correctly with "TOP" in the upper position of the mounting flange. If the all of these relationships are correct, then, on the PW-D, very slightly push the Number Dial to engage the gear teeth of the Index Dial to see if a better fit can be achieved between the gear teeth of both dials. You may hear a slight "click" when the "alignment slips into place" and is correct. Verify that "250" is still centered in the window of the Index Dial. At that point the PW-D will just slip right onto the elliptic hub.



photo right
: Correctly assembled PW-D micrometer dial looking at the back side.

 

Dismounting a Stubborn PW-D - Sometimes removing the PW-D Micrometer dial proves to be a very difficult operation. Almost always, the difficulty is caused by excessive marring of the tuning shaft by the knob set screw(s.) If everything seems it indicate that the PW-D is not coming off of the shaft no matter what, this is the procedure necessary to remove the dial. First, for pre-WWII HROs, you'll need a couple of right-angle screw drivers. I usually have to make them because everything commercial is too big to fit between the back of the front panel and the tuning shaft bearing hub screws. Make at least two that have different angles for the blade engagement. Next, remove the upper rail that is mounted to the back of the front panel and forms the front top piece of the cabinet. Now, using the right-angle screw drivers, loosen and remove the four screws that mount the elliptic bearing hub screws. This bearing hub is spring loaded for thrust so be careful removing the last screw. Now, remove the entire PW-D and the bearing hub and the worm gear out through the square hole in the receiver's front panel. Now, remove the knob from the PW-D assembly. This provides access to the front of the tuning shaft. Then, using a soft metal drift, gently tap out the shaft from the PW-D dial assembly. You will have to reassemble the gear box per the instructions in the "Lubrication of the PW Gear Drive" section above. Also, be sure to dress down the marring on the tuning shaft so the next time the PW-D has to be removed, it will be an easy job.  >>> >>>   For the WWII and later HROs, you'll need an open-end 1/4" wrench. Removing the top two hex head screws that hold the bearing hub is pretty easy but the two bottom screws will take patience. Usually only one hand will fit into the space between the back of the front panel and the tuning condenser, so, to remove the lower right screw, I use a small thin rod manipulated with my left hand fingers to lift the wrench onto the hex head and then turn the screw one flat with the wrench manipulated with my right hand fingers. Reposition, and again using the thin rod to help position the wrench, loosen the screw another flat. On and on, it takes awhile but you can get both of the bottom screws out that way. You'll have to change hands depending on which of the bottom screws you are removing, right or left. I usually take the bottom ones out first and then the top ones. Reassembly is easy since you can access the hex screws through the square opening in the front panel using a 1/4" nut driver.

NOTE: When remounting the bearing hub note that "TOP" is embossed on one of the flanges. This flange obviously must be mounted up. This is important because it positions the elliptic bearing correctly for the PW-D dial.

 

HRO Restoration - Hints and Suggestions

Manuals - Fortunately, the HRO didn't change very much from 1935 up to 1945. The receiver was so well thought out and so well designed, few engineering changes were necessary. As a result of this, almost any manual will provide enough correct information for operation, repair and alignment of most HRO receivers. Be aware that a few changes in resistor values occurred when the 6.3vac tube HRO Seniors became standard (~4/39.) The screen divider (R10 & R15) was changed slightly to provide a lower screen voltage for the newer manufacture 6C6/6D6 tubes. In the 1940s, National issued manuals that were usable for all of the HROs, e.g., the HRO, Senior, Junior, M, MX, RR. Later, the HRO-5 was also included in the compilation. These manuals are available as copies from several sources. If you are working on an early 2.5vac HRO be sure to use the earlier compilation for the correct resistor values. As for original manuals, they are interesting to own but not absolutely necessary as an information source. Even the Rider's Perpetual Troubleshooters Manual Vol. VIII has enough information on the early HRO for repair and alignment. Capacitors - All paper-wax capacitors need to be replaced in any vintage receiver if you intend to operate the set more than just casually. In the HRO, all the capacitors are easy to access and replace. In fact they are so easy to get to, I usually go a step further in the restoration process and restuff the original capacitor shells with modern film caps. This is just a cosmetic issue and doesn't make any difference to the final performance but it preserves the original under-chassis appearance. The two cathode bypass electrolytic caps are riveted to the chassis with one of the rivet heads under an IF transformer shield. They can be dismounted, rebuilt and re-riveted - if you are a purest. Also they can be left in place, disconnected and the new replacement electrolytic cap mounted nearby if under chassis appearance isn't an issue. Actually, these two electrolytics were so well built that all of the originals I've ever tested are perfectly usable - they are low voltage (25vdc) and were well sealed - just reform, test and use.
Ceramic Lead-end Resistors -  National made their own resistors. They consist of a piece of ceramic round stock that has been drilled lengthwise to accept small diameter carbon rods. The rods protrude out the ends and when the lead-end was cast, contact was made with the carbon rods. Usually two rods are used on larger resistors and one rod on the small size. Over the years the contact to the carbon rod(s) may become compromised or maybe the rod(s) will crack - either will cause the resistor value to change significantly. Usually a restorer will find at least a few of these National resistors that need to be replaced. If under chassis appearance is not important than just replace the resistor with a modern equivalent. If you want to preserve the original appearance of the National resistors then a reproduction has to be made. These are actually easy to replicate using the lead-ends removed from the original resistor - they just twist off without too much effort. Select the proper value IRC resistor (International Resistance Co. - the type made in the forties are the correct diameter.) Drill 1/16" holes at the center of the lead end-caps and insert the resistor leads through the holes, sliding the end-cap on to the resistor body. Solder the leads to secure. Paint the resistor body white and you're done. Install the repro resistor. For larger types, use the closest size resistor and if necessary slightly enlarge the end-cap opening with the correct size drill bit to get the proper fit. See the "1935 HRO H-103 Restoration" section further below for photos and more details.>>> Crystal Filter Problems - Most of the time, the Crystal Filter works quite well and we hardly ever think about a problem developing inside. The "ON" switch is incorporated into the SELECTIVITY control and actuates the filter when advanced. You can usually feel a slight "thump" as the switch changes position since it is cam-actuated. Sometimes the cam wears or other mechanical problems happen requiring some adjustment to the action of the cam and switch. Geoff Wooster (in the UK) ran into a problem involving the crystals themselves. This problem showed up on two different filters. Oxidation on the plates inside the crystal housing caused the crystal activity to be much lower than normal. The filters acted more like attenuators than anything else. Geoff's solution was to disassemble the crystal and thoroughly clean everything including polishing the metal plates inside. This cured his filter problems. It is possible in humid areas that oxidation can work its way inside the crystal. It can take years but these crystals are over 70 years old now. James Millen mentioned the HRO crystal filter in one of his QST letters in the 1930s. His recommendation was to clean the crystal with denatured alcohol. He also mentioned that the crystal should be slightly loose in the holder. This would be the older plug-in style crystal used in the pre-war HRO.
 
New Power Cables- If your HRO doesn't have its original power cable, examine the one installed carefully. The amount of current required to operate the tube heaters will cause a significant voltage drop unless the proper gauge wire is used in the cable. The 2.5vac tube heaters draw a total of 9.55 AMPS, or about 23 watts! Also, there is a center tap resistor to eliminate hum that is also across the tube heater line. The 2.5vac HRO cable should have 10 gauge wire in the cable for the tube heaters. All reproduction brown cloth covered cables are inadequate due to the small gauge wire used in the cables, (there are two types of repro cables, one has six wires and one has eight wires.) Even connecting two or three wires in parallel will not lower the resistance enough, resulting in too much IR drop for proper heater voltage (along with heating up the wires in the cable.) The only solution is to build the power cable using two 10 gauge wires for the heaters and two 18 gauge wires for the B+ and B-(chassis.) The 6.3vac HRO Seniors are a different matter as they only draw 3.1 amps at 6.3vac for the tube heaters. By connecting two or three of the wires in parallel in the reproduction cables, adequate current carrying ability will result and the IR drop will be minimal.   >>> >>>  The National power supplies do have an increased voltage for the heaters at the transformer. Usually about 25% increase to compensate for the IR drop across the power cable but this was designed to compensate for cables having large diameter heater wires. The best test is to check the voltages with the HRO operating on the intended power supply and measure the heater voltage across the hum elimination resistor. Heaters should be within 5 - 10% of the specified voltage. With low heater voltage (>20% drop,) the HRO seems to work okay but you'll notice a general lack of sensitivity requiring the RF Gain to be advanced more than usual. Also, check B+ at the tie point where the cable connects. With low B+, the HRO also might seem to work okay on the lower bands but the S-meter will not work correctly and the RF Gain will have to be near maximum for CW reception. B+ should be 230vdc at about 70ma. In actual measurements the B+ will vary depending on if the HRO is receiving AM or CW signals, how loud the set is playing, etc. - it is normal for the B+ to vary from 220vdc up to about 245vdc depending on the load. National stated that the HRO would work on as little as 135vdc B+, but they did add, "at some sacrifice in performance." Usually, the B+ IR drop is never a problem because the current draw is so low.
 
Building a New Power Cable - Even if the original cable is tattered and worn, it is best to keep it since it has the correct current carrying ability for that HRO - but if the restorer wants to replace a missing old cable on a 2.5vac HRO with a new cable of the same original length, it will have to be built. The original HRO power cables were about four to five feet in length. In order to have the new cable be flexible and to have very low resistance, using modern "auto-stereo-boom-box" cable is an excellent choice. "Stinger Pro" 10 GA Power Cable is easily available and comes in various color jackets - also it is not expensive. The "Stinger Pro" cable is extremely low resistance and is made up of a huge quantity of small gauge copper wires resulting in a very flexible, very conductive cable. When the two 10 gauge wires and the two 18 gauge wires are cut to length they can be wrapped with olive drab or brown colored cloth tape to make a cable*. If carefully done, the new cable will look okay but, more importantly, it will work great. You will have to tin the 10 gauge wire ends before soldering them into the four pin plug. In fact, you may have to do a little "sizing" of the tinned wire ends with a file to get them to fit into the plug pins, but they will fit. When completed, you will notice that the voltage at the hum eliminator resistor will be very close to 2.5vac and that the HRO will perform better than ever.

*There might be some fabric braid that could be utilized as the cable cover. Sometimes certain types of round braided shoe laces can be "opened up" and used as cloth braiding. It's also possible that a special build from one of the antique automobile electrical harness builders could supply an authentic HRO power cable with the correct gauge wires and the proper cloth covering. Probably expensive, though.

2.5vac Tubes versus 6.3vac Tubes - The Myth - James Millen wrote, in his monthly QST letter of March 1937, a recommendation that when the HRO was operated on an AC power supply, the 2.5vac tubes should always be used. Millen stated that the 6.3vac tubes produced a noticeable increase in the hum level and noise that would in some cases mask weak signals. In the April 1939 QST letter, Millen recanted what the earlier letter had recommended. Millen stated that the 6.3vac tube quality had improved significantly and there was no longer any advantage to using the 2.5vac tubes. The letter elaborated that a modulated hum around 15MC had been an earlier problem with the 6.3vac tubes. Additionally, the letter mentioned the new Model 697 power supply for the HRO which provided 230vdc B+ and 6.3vac. Also mentioned was the fact that earlier HRO battery operated receivers could not be operated on the new 697 due to the B+ being too high unless the receiver was modified. Some former National employees speculate that the actual reason for continued use of the 2.5vac tubes was due to National being "over-stocked" with power transformers that had the 2.5vac winding which were essentially useless unless used in the HRO power supply.

Unfortunately, many hams and collectors only remember James Millen's original, 1937 remarks. This had led to an "Urban Ham Legend" that the 2.5vac tubes are superior and re-tubing a later HRO Senior with early style tubes will give great performance results. This isn't true. The first problem encountered might be insufficient current carrying ability of the power cable for the 9.5 amps required to operate the 2.5vac tube heaters. It depends on the particular HRO, along most do use the same power cable regardless of the heater voltage. Also, the screen divider was changed for the 6v HRO Senior to provide a slightly lower screen voltage required for the newer manufacture 6C6/6D6 tubes. A somewhat higher screen voltage was used on the old 57/58 tubes and early 6C6/6D6 tubes. The later HRO Senior, with the 6.3vac tube IDs on the chassis, will perform great "tubed" as it came from National. The early HROs that were originally 2.5vac receivers should be operated "as designed" - with the 2.5vac tubes. Although, the early 2.5 vac HRO can be re-tubed to 6.3vac and it will operate okay, you usually can't re-tube a 6.3vac HRO to 2.5vac tubes and expect it to work well without some modifications.

 
Speakers - The HRO normally does not have an audio output transformer installed in the receiver. This was common practice when the HRO was designed. Most manufacturers mounted the audio output transformer on the speaker. This practice does allow full B+ to be present the speaker pin jacks at the receiver - always a source of unnecessary worry for electrophobics. Even the Navy worried so much about the B+ on the speaker pin jacks, they wanted output transformers installed on the HROs that they ordered (all military National receivers, in fact.)  If you don't have the "NC" speaker, or you have the box with a non-original speaker and need an audio output transformer, the HRO output Z is 7000 ohms on the primary (single ended) and the speaker Z is usually 8 ohms. Most universal audio output transformers will have a range on the primary, e.g., 4K to 10K ohms Z and a selectable secondary of 4 or 8 ohms Z. These usually work fine with the HRO.    >>> >>>   Check the DC resistance of the primary, it should be around 150 to 300 ohms but this depends on the power rating of the transformer. Higher power transformers will have larger gauge wire used and therefore a lower DC resistance. You should go by the impedance first and just reference the DC resistance. There are many "universal type" audio output transformers that will work fine with the HRO. The Triad S-53X is an excellent choice (but certainly not the only one,) providing 7K ohm primary with an 8 ohm secondary with the primary DC R at 300 ohms. (Actually, the S-53X is a universal audio output transformer with the option of selecting many different combinations of primary and secondary impedances but a 7000 ohm Z primary and 8 ohm Z secondary are options that are provided.)
 

Modifications - Unfortunately, the HRO and HRO Senior receivers were built during the Depression. This means that many hams - out of economic necessity - had to keep their receivers as long as possible. The temptation to modify a great design was going to be there. Also, during WWII there was a complete absence of parts for those hams who could still operate - the only way to legally operate was as part of the Civil Defense emergency nets that were on usually once a week during WWII (transmitters had to be registered with the Navy to be legally used during that time.) This also led to some modifications out of necessity in keeping the receiver operational. After WWII, cheap HROs were around and subject to more abuse during the "modification mania" of the fifties. Very few pre-WWII HROs are found today that don't have some kind of mods installed. Whether the receiver can be returned to original will require a detailed examination. Unfortunately, many hams drilled holes and cut panels or chassis to install their so-called upgrades. Certainly, the rarity of the HRO will dictate whether the effort is justified to return the great old receiver back to its original design and appearance. Be aware that National performed their own upgrades on receivers that were returned to the factory for repairs or "updating." Usually the caliber of workmanship and the engineering thought process is far superior to that encountered from the average "hamster."   >>>

>>>   Post-WWII production, that is the HRO-5TA1, HRO-7, HRO-50 or HRO-60, are less likely to be found extensively modified. The abundance of surplus gear to modify and the fact that the receivers were new or fairly new seemed to curb "cutting and hacking" activity. Sometimes product detectors will be found installed in the NBFM accessory socket of the HRO-50/60 but these are normally homebuilt and based on an article that was published in CQ magazine in the mid-sixties (later in Electric Radio.) The problem with the HRO-50/60 product detector plug-in mod is that the BFO is normally disabled when in NBFM and the stock National switch is a special build that doesn't provide a terminal to actuate the BFO when in NBFM. A separate BFO switch is necessary although some mods have the BFO wired to be "on" all the time - okay for CW and SSB (the main concern at the time) but a problem if AM reception is desired. These mods are easy to remove and return the circuit to an original configuration. Operation today using the HRO-50/60 is normally for vintage AM nets and the need for a product detector is nil. However, if CW or SSB operation is desired using a stock HRO-50/60, just reduce the RF Gain and increase the AF Gain for the proper ratio of signal to BFO injection at the second detector - that's the way it was done before product detectors came along.
 

HRO Restorations in Detail   
The following articles detail the restoration of a few HRO receivers. Each project has various techniques involved in the rework of some type of damage that is found from time to time, not only in HRO receivers, but in others types of equipment as well. Hopefully each Restoration Article will have some helpful information for restoring HRO receivers.

1935 HRO Receiver SN H-103 - Restoration

To the right is a photo of HRO sn H-103 taken when the receiver was first obtained. It is obvious that the cosmetics were in very good condition. The yellow "R-014-8" designation may have been an identification for commercial use but nothing was documented as to its meaning or of the receiver's former use. Most of the problems with H-103 were under the chassis. Foremost was the 1.25" hole that had been punched in the front left section of the chassis. This was going to be a challenge to repair. Also, the receiver had been totally re-capped using modern-style yellow plastic capacitors. Additionally, there were some non-original resistors, a few modifications with some extra non-original parts installed.

The goal in the restoration was to bring H-103 back to full operation using the original design circuit and using parts that would have the appearance of the originals but were rebuilt new parts. The most time consuming part of this kind of restoration is restuffing the paper-wax capacitors. Since I didn't have the original caps, I ended up robbing a complete set of HRO "Sprague" paper-wax capacitors from an old HRO parts set.  NOTE: Run-G HRO receivers probably used Micamold paper caps but these are difficult to find so I used Sprague which were used in all post-1936 HRO receivers.   These caps were restuffed using modern "yellow jacket" caps installed into the original paper-wax shells. The shell ends were then sealed with colored wax. Proper orientation was maintained though this was for aesthetics only. While this is sometimes tedious work, the end results are a receiver that looks totally original and performs as new. Photo right: HRO SN H-103 before restoration

Below is a "before" photo of the underside of the chassis showing all of the non-original type capacitors along with other non-original parts and circuit modifications.

Below is an "after" photo of the completed restoration of the underside of the chassis showing how restuffing caps preserves the original appearance.

Another challenge were the white ceramic resistors with lead end-caps that National used in the HRO receivers up to about mid-1936. I had to "recreate" three of these resistors to replace modern resistors that had been installed. I checked the parts boxes and found three correct size lead end-cap resistors. Of course, they weren't the correct value but all we needed were the lead end-caps to build our replicas. Next, I needed to find correct value, IRC (International Resistance Corp.)1/2 Watt Carbon resistors from the late thirties. These resistors are the correct physical size to fit into the lead end-caps. The lead end-caps will just "twist off" of the old resistors and then, by drilling a 1/16" hole, the lead end-caps can be fit onto the new resistor (with the leads protruding through the holes.) Soldering secures good electrical contact to the lead ends. All that remains is to paint the body white and install the resistor replica. Originally, the ceramic resistors were lettered by hand or stamped with the value, however it looks better to just leave the body white as nothing looks quite "correct" or "original" for putting the value on the resistor. The photo to the right shows one of the replicas installed next to a rebuilt (restuffed) paper-wax capacitor.

This photo shows the aluminum hole plug installed but not painted. Also note that the plastic cap is missing from the push-type "ANT" post.

The 1.25" hole punched in the front-left part of the chassis was a real problem. It appears that someone wanted to install yet another mod into the receiver but fortunately stopped after they had done this damage. The hole was so large an epoxy fill would not have had enough strength. I decided to make a "custom-fit" plug and epoxy it in place. The plug was made out of .050" aluminum since this was about the thickness of the chassis steel.  The rim interface would be filled, sanded and then painted to match.

After the plug was fit and the interface filled, the paint matching was attempted. Since this was a rather large area, artist's acrylic would be too flat of a finish to match. The photo to the right shows one of the "black" paints used that was a fair match. Several "black" paints were tried but the best match came from Testor's Jet Black, plastic model paint that comes in the little glass jars at hobby stores. See the chassis photo below for the final appearance. Though not perfect by any comparisons, I think the fix is better than the gaping hole.

Most of the above chassis work involved repair of the large hole but several other minor tasks were also necessary. On most HRO receivers, the gear box has probably never been opened since the receiver left National. The grease is usually dried up, the anti-backlash split-gears are tight and the other bearings also need lubricant. I thoroughly cleaned the old grease out, then used "3 in 1" oil on the split-gears to assure that they can move freely, which eliminates backlash. I used "Lubriplate" for grease applications. The worm gear might also need to be removed and lubricated if the gear box is particularly dry but most of the time the worm gear can be lubed with oil drops to the end bearing in the housing and by working grease to the worm gear by lubing the split-gear. The rotor contacts on the tuning condenser should be sprayed lightly with a contact cleaner that has some lubricating qualities. When installing the micrometer dial, notice that the elliptic rides on the boss of the tuning shaft bearing housing. I usually find that this area is entirely devoid of lubricant which results in a "rough" feel or noisy operation of the micrometer dial. I lightly grease this boss before installing the micrometer dial.

All of the tube sockets should be cleaned. I usually spray De-Oxit on the pins of the tube before inserting it into the socket a few times. I also clean the grid caps as they are always oxidized. On the HRO, you also have to clean the flex finger contacts for the coil sets. These are usually very dirty and if not clean will compromise the all-important contact with each coil in the coil set. 

After a good set of tubes are installed, all that remained was test and alignment. There were several circuit modifications that I had to remove on this HRO and then return the circuit to original. Testing was going to reveal whether I did this correctly or not. Fortunately, everything worked pretty much as it was supposed to. During test, the receiver's gain was up and down a lot so I removed the tubes and cleaned the sockets again which corrected that problem. The S-meter seemed to be far too sensitive and the adjustment pot had little effect. The problem was the R10, the screen load resistor (which is part of the S-meter bridge,) was only about 8K and the correct value should have been 20K. So, another lead end-cap resistor replica was made to replace R10. All resistors were checked during the rebuild but I obviously missed the low value R10 - at least it showed up as a genuine problem during test. With R10 at 20K the S-meter circuit worked correctly and the S-meter responded to signals in normal manner.

The next step was alignment of the IF, which is straight forward although the IF frequency is determined by the actual crystal frequency - in this case 456.5KC. After the IF is aligned, each coil set has to be aligned for proper tracking. Most HRO receivers don't have all of their original coil sets anymore. H-103 was no exception, having only the 40M coil set with it. We usually have to rely on "orphan" coils to make up a complete set of coils for each HRO receiver. These "orphan" coils must first be disassembled, cleaned and inspected. Many times dirt, dust, bugs and spider webs, along with broken wires or bent condenser plates will be found in each of the coil boxes. In fact, the 40M coil that came with H-103 had a broken connection inside the 1st RF coil that had to be repaired. After reassembly, the coil set needs to be aligned to the specific receiver that is going to be used with. There is a significant difference between each HRO receiver, especially between the 6.3vac tube type and the earlier 2.5vac tube type. While any coil set will function okay in almost any HRO, the alignment will assure greatest sensitivity, correct tracking and best performance of the receiver. When aligning HRO coil sets, the general coverage must be aligned first and then the bandspread. Switching to bandspread requires relocating four screws on the coil contact insulator blocks. The eight adjustment trimmers are accessed from the top of the chassis through holes just in front of the tuning condenser. It is easier to access these holes if the front-top piece of the cabinet is removed. It is held by the two top screws in each corner and the dial pointer "diamond." If the lower end of the tracking needs to be adjusted, the padders and inductance disks can be accessed through the holes in the back of the coil shields on each coil set. On coil sets A and B, the LO inductance is adjusted with a "loop" of wire inside the coil form that has its relative position moved to change total inductance. Once the coil set is aligned, the final adjustment is to readjust the 1st RF coil trimmer to the station antenna.   >>>

>>>   I happened to also have correct vintage E and F coils for this HRO. The E coil provides tuning of the 160M band along with the upper part of the AM BC band. Coil F covers 480kc to 950kc for the lower part of the AM BC band. These coils do not bandspread. They also have a padder adjustment on the top of the coil set so when aligning, both high-end and low-end tracking can be adjusted from the top access holes.

The photo above is the finished result. Like all fully restored HRO receivers, H-103 is very sensitive and the bandspread capabilities make the amateur bands seem to tune on and on - 400 divisions in fact, or the equivalent of about nine and a half feet of linear dial. The feel of the micrometer dial is smooth and the accuracy of any reset is precise. Audio is very pleasing even though it is just a single-ended 2A5. I am using an early 8" National speaker box with the cloisonné "NC" emblem. Since the HRO receiver originally has no internally mounted audio output transformer, National installed the output transformer in the speaker box.

1940 HRO Senior Receiver  SN 463-K - Restoration
 
The following is an article I wrote in 2006 for use in  "Assembling an Authentic Pre-war Ham Station" that detailed the restoration of my 1940 HRO Senior SN 463-K.
Temporarily deleted - but not lost. Here it is again....

I have had HRO 463-K for over 25 years having purchased it from Fred Winkler, an old radio collector friend of mine, for $40 back in the mid-1980s. I never did much with it except acquire a complete set of coils (it did have the matching C coil with it,) replace some missing tube shields and a general clean-up. I thought back then that the performance wasn't as good as other HRO receivers that I had (and had sold) but I never investigated further. So, the HRO became just part of the pre-war equipment that was on display but not used. 20 years later, a rekindled interest in early HRO receivers prompted me to take another look at this old relic of mine. With a little coaxing the HRO did seem to function. An alignment also seemed to help but generally the receiver didn't have the sensitivity expected and also gain across the band was not consistent.

 Taking a closer look into the plug-in coil receiving bay revealed the problem - one of the flexible contact fingers for the mixer section was broken off. This was the ground return for the tuning condenser section contact. Fortunately, I did have a parts set HRO that could provide a replacement for this defective part. The problem was to find where I had stored the HRO parts set. After several hours of moving many heavvvy boxes, I found the HRO parts set stowed behind a six foot tall Navy TAJ-19 transmitter that was in the garage. Though the parts set was missing about half of its parts, it did have the needed finger contact assembly. Replacing the mixer finger contact assembly requires removal of the tuning condenser to gain access to the part. This is an easy task as the HRO was designed for ease of maintenance. Once this problem was repaired I decided to go ahead with a total rebuild of the HRO.

 

A complete re-cap job was performed on the HRO but rather than just replace capacitors with no regard for under chassis appearance, I decided to re-stuff all of the paper wax caps to preserve the original looks of the assembly. Performing this type of detailed rework turned up something unusual - a factory miss-wire that was not caught during test and alignment at National. The error was that C9, the bypass cap for the 2nd IF transformer secondary, was connected to the wrong side of R-8 which placed C9 connected directly to the AVC line. Additionally, C9 was only wrapped to the terminal strip - not soldered. Obviously, this was a "transparent" type of problem - one that would not be caught in test because the error would not have affected performance enough to be noticed. Then one also has to take into account that the receiver was used for many years and this problem never noticed - certainly not a major concern but interesting how these sort of errors are found so many years later. When finished with the re-stuffing of the capacitors all resistors were checked for correct value. I usually allow about 25% tolerance but all of the resistors were much better than that. The photo to the right shows the finished result.

As for the top side of the HRO chassis, a thorough cleaning showed that there were a couple of minor rust areas. These were touched up using artist's acrylic that I matched to the exact color of the particular chassis or cabinet. Since every painted piece of an old receiver has aged somewhat differently the colors always require custom matching. Fortunately the areas involved on the HRO were small and touch up not noticeable. The tuning condenser had to be removed for the mixer finger contact assembly repair so the entire assembly was cleaned and lubed while off the chassis. When the gearbox cover was removed for cleaning and new lubrication it was found that one of the anti-backlash springs had come off of its mounting peg. Just another example of minor problems that turn up while doing a detailed restoration. The S-meter scale was severely discolored which is typical for plastics of this time period. I happened to have a very nice original Marion Elec. Inst. Co. S-meter that had a nicely colored scale. Although the red was somewhat faded, the overall appearance was a vast improvement over the original meter scale. The cabinet and front panel were in nice shape so only minor touch up was necessary to have everything ready for reassembly.

After installing all of the tested tubes, shields, the front panel and knobs we were ready for alignment. The HRO has an IF frequency of 456kc +/- whatever the exact crystal frequency actually is. This is determined by sweeping the frequency around 456kc using a signal generator with the crystal filter on and listening for the peak that occurs at the exact crystal frequency. In my case, the crystal frequency was 456.3kc and the IF was aligned to that frequency using the S-meter as an output indicator.

 After the IF alignment, it is necessary to begin aligning each of the coil sets. You have to start with the general coverage section first then proceed to the bandspread section. Switching from general coverage to bandspread requires moving four flat head screws located on each of the molded contact pin sections on each coil. The graphs give the micrometer readout to frequency relationship and usually all that is necessary is to trim up the adjustments on top of the coils. These are accessible from the top of the chassis through the eight holes just in front of the tuning condenser mount. I use a long shaft blade screw driver with the shaft taped to prevent shorts against the cabinet or tuning condenser spacers. If the gain is not reasonably even over the entire range in the bandspread mode, the back padders need to be adjusted at the low end of the scale. Though there are access holes that make it possible to adjust the inductance disks (general coverage) or padders (bandspread) while the coil set is installed in the receiver, it isn't recommended. It's easier to just remove the coil set, make an adjustment and then install the coil set and check. After the alignment is complete, you still have to adjust the 1st RF coil in each coil set to the particular antenna that is to be used. Since there is no antenna trimmer, this has to be done for best response at the normal operating frequency with the antenna that is normally used. The photo to the left shows the completed 1940 HRO Senior, SN 463-K.

1935 HRO Receiver SN E-50 - Restoration

E-50 was donated to the Western Historic Radio Museum by Jim Larsen AL7FS of Anchorage, Alaska in September 2011. E-50 was given to Jim by Val Anderson of Seward, Alaska. In the 1940s, E-50 was owned by Anderson's brother-in-law, Carl Carlson of Anchorage, Alaska. Carlson may have gotten E-50 through his son-in-law who worked for the CAA in Alaska at the time. By the 1950s, E-50 had gone to Cordova, Alaska and when Carlson died in the 1950s, the receiver was shipped to Val Anderson. It seemed that E-50 had spent most of its existence in Alaska. The receiver was stored for many years in Anderson's basement and, due to the cold, humid coastal environment of Seward, E-50 now had some rust issues. Additionally, several years of modifications had left E-50's originality somewhat compromised. A couple of the modifications suggest that the receiver was rack mounted (possibly when used by the CAA.)

NOTE: The following article is an edited version of "1935 HRO Receiver E-50 Restoration."

If you want to read the entire, lengthy article and view many, many more photographs taken as the restoration proceeded, Click here: National HRO - Restoration of E-50 - Unedited

Pre-restoration - The photos to the right show E-50 as received. The condition is actually not too bad when considering the storage and history of E-50. Obvious problems are the rust issues which will require extensive disassembly. The modifications to the Mixer and Detector tube sockets will require obtaining original type sockets. The PW-D is incorrect and the plug-in coil is from a post-WWII HRO. The S-meter and switch are not original. Underneath E-50 is fairly original, with many of its original parts still present. Despite the "rough" look to E-50, it is a very restorable receiver.
Disassembly and Chassis Restoration - Note the photo to the right showing the severity of the rust. It was going to be necessary to completely disassembly the E-50 chassis right down to the rivets in order to fix the rust problems. With careful drilling the rolled ends of the rivets can be removed and then the rivet driven out with a small punch. This allowed complete removal of all parts on top of the chassis. Additionally, with the rivets removed the complete circuitry from under the chassis can be removed intact. This will make reassembly of the circuitry much easier than if all of the parts were removed individually.

The chassis is painted with nitrocellulose lacquer and we are going to repaint using the original type of black nitro-lacquer paint. This means the complete removal of the old paint is not entirely necessary. The new paint will not react with the old paint and the places where the new paint covers old paint will not show since the edges are sanded and feathered. The easiest way to get at all of the rust is to just use sandpaper and wire brushes to remove it. The end result can be seen in the photo below.

The chassis then needed the miniature tube socket hole filled. It can be seen in the "before" photos above and in the "sanded" photo below. Bondo was used to fill the hole. After sanding down the Bondo, it has to be primed with lacquer and then sanded lightly again. This seals the Bondo for final painting. 

Several coats of lacquer were applied to the chassis and the finished results are seen in the photo to the right. Don't worry if you get a "run" in the paint when doing the chassis. ALL originals I've seen have runs here and there, after all, it's just the chassis. Also, you don't want a "mirror" finish to the paint on the chassis. All of the originals I've seen are mildly satin finished with very little, if any, gloss.

Fake rivets were made to replace the original rivets. Fake rivets are made from 6-32 screws that the head are reshaped to look like rivets. The parts are then mounted with lock washers and nuts. The end result is original appearance and better electrical contact to the chassis.


photo above: The chassis after painting

The aluminum shields for the IF transformers, the BFO coil and the tube shields all required a chemical treatment to return the flat-satin finish that the aluminum pieces originally had. I had to disassemble the IF and BFO units so only the shield-cans would be involved in the chem-treatment. I use sodium hydroxide which is easily found in Easy Off Oven Cleaner (EOOC.) The active chemical is basically lye and it will react fairly quickly with aluminum. I spray on a heavy coat of EOOC on the aluminum piece and then wait about one minute. If there is a lot of corrosion, I'll work the EOOC around the aluminum piece with an acid brush. After about two minutes, I rinse the piece in cold water. You can't rub the aluminum after this treatment or it will slightly polish and ruin the flat finish we are trying to achieve. Let the pieces "air dry" and reassemble after that. The photo to the right shows E-50's chassis assembled with the EOOC treated aluminum pieces installed.

HRO Cabinet and Panel Painting - The original black wrinkle finish used on all early radio equipment was a two-part process that used a base coat of Nitrocellulose Lacquer followed by a spray-on catalyst that was heat-activated, usually requiring oven-baking to produce the finish. Today, all we have available is a single-part process that can work quite well if the proper conditions are provided for the wrinkling process.

First, one has to observe how the original cabinet and panel were painted. Since the base coat was smooth nitrocellulose lacquer, this will appear as gloss black. Note that all inside surfaces of the cabinet and panel are gloss while the outer surfaces are wrinkle. The outer surfaces was where the catalyst was sprayed. When baked to activate the catalyst, the result was gloss black on the inside and wrinkle finish on the exterior.

Before painting, wipe down all of the surfaces with lacquer thinner. This is to remove all grease, oil or other contaminates. To duplicate the original finish appearance, we first have to spray the gloss black lacquer on all of the inside surfaces of the cabinet, the lid support lips and the back of the front panel. On the removable front rail, spray gloss black on the inside surface and only on the lip that supports the lid. Let the gloss black lacquer dry overnight.

To do the wrinkle finish successfully you should only do one surface at a time. If you try to paint the entire external surface of the cabinet, you're sure to have a problem heating the entire paint job enough for the wrinkle to appear evenly. Before painting, I set up at least two heat lamps that are nothing more than 100W incandescent lamps with the aluminum bell reflectors as the primary heat source. I also have a handheld heat-gun to apply heat to specific zones that are the usual problem areas - like edges or corners.

I apply the wrinkle paint in at least four heavy coats with about one minute waiting time between each coat. Each coat is applied in a different direction to avoid a "striped" pattern. Once the surface is painted, it is placed it under the lamps. After about five minutes, I start using the heat-gun to generally heat the entire surface but to also heat areas that aren't receiving a lot of heat from the lamps. I usually will apply the heat-gun from underneath of the painted surface because too much heat directly onto the wrinkle paint will cause it to "gloss" and not match the texture. I always keep the heat-gun moving to keep the heat distributed and to avoid burning the paint. After a few minutes, the wrinkle pattern will begin and then, using the heat-gun, I apply heat just ahead of the pattern to move it along. The entire surface should be wrinkled in about ten minutes total time.

Now, I move on to the next surface. At the edges, I shoot the paint from over the surface being painted rather than from the opposite side. That way the edges will blend and not have an "over lap" of paint that won't match correctly. When I do the top lid, I set it up so that the lid is down on the supporting lips. That way, the gloss black will be protected from the wrinkle finish and the end result will look like the original application did - see photo below.

The wrinkle finish is very, very soft at first so it's best to avoid any contact with the wrinkle finish "surface." I have to move the pieces carefully as I set up to shoot each side. When the painting is finished, I'll let all the pieces cool down for awhile. It's best to let everything set undisturbed for about one hour. After an hour, the wrinkle has set up enough to handle and move the pieces indoors but the wrinkle finish will take about two days to develop it's hard, durable surface. The longer it ages, the harder it gets. Avoid mounting anything to the wrinkle finished surface for at least two days.

photo left: The cabinet and panel after painting with VHT Hi-Temp Black Wrinkle Finish. Note that the hole for the non-original pilot lamp has been filled. Additionally, the finger lift has been reformed. Since E-50 was once mounted in a rack, the finger lift had been "flattened" for rack clearance.

 

photo right: This is a close-up showing the detail of the lid support lips. Note that on the support section the paint is smooth gloss black while the top edge is wrinkle finish. Also note that the interior of the cabinet and the bottom of the lid are also gloss black.

Replicating the S-Meter -  Close study of a photo sent to me by K6GLH of his "E-run" HRO allowed me to see how the unique E-run S-meter looked. I found that the same meter was used on the RME-69 and I had a parts set '69 that could provide the proper meter. The next step was to replicate the scale. I had the scale from HRO H-103 to start with. This scale is the later type used by production run F but it's possible that some of the later scales made it into run-E receivers. I made copies of that scale onto photographic paper to achieve a slight gloss to the scale. The finished meter is shown in the photo to the left.

 

Replicating the Pearl Button Switch  -  The pearl button push switch used for enabling the S-meter was also fairly easy since I had a vintage push button in the junk box. These push-button switches are actually "doorbell switches" and they usually came in black, ivory or pearl. The one I had was an Ivory button, well,...ivory-colored plastic. To change the appearance required disassembling the switch and then painting the button using Revlon "Perfect Pearl" fingernail polish. This gave the button the correct color and the correct appearance of a pearlized button. These switches just push into the panel and lock in place with expanding tabs. By compressing these tabs the switch can be removed, if necessary.

photo left: The S-meter is from an RME-69 with a replica HRO scale and the Pearl Button switch is painted with fingernail polish.

Nickel Plating the PW-D - I thought I might be able to create an early style nickel plated PW-D by using a later painted PW-D. The painted dial is slightly different with thicker index lines on the outer dial and thicker numbers on the inner dial but with the correct color coming from the plating, these slight differences wouldn't be objectionable.

Nickel plating generally should be done over copper plating or brass. I had to try to nickel plate pot metal. I've plated aluminum with no problems, so I thought the pot metal would do okay. Well, the first attempt turned splotchy and dark, a sure indication that too much current was being used for the electroplating. The second attempt, at reduced current, also turned dark. The third attempt reduced the current even more and the results were okay. There was still some splotching but overall the dial looked close to original. I felt that the plating might have turned out better had I used a different kind of anode. I was using pure nickel for the anode and I'm pretty sure the original PW-D dials were German Silver plated. Maybe I'll use a nickel coin next time for the anode. I think the best solution would be to find an original plated PW-D dial for E-50.

Rebuilding the Circuitry - This is the next step in completing the E-50 restoration. Like the HRO restorations above, E-50 will have re-stuffed capacitors and replica lead-end resistors installed. I did have to replicate the shielded audio cable since the original was missing. Also the shielded antenna leads to the 1RF coil primary had to be replicated due to a modification. Using "fake rivets" allowed the components that require a chassis-ground connection to achieve an excellent ground due to force that can be exerted with these modified screws. I would like to find the few remaining parts that are needed for the restoration before going on with the project, so at the moment we are "on hold."

Doorbell Switches - I ordered one of the doorbell switches that were made by Philmore to see how they look. If it looks better than my "Revlon" version, I'll use it instead.

UPDATE: The Philmore doorbell switches are not useable because the button surface is "convex" instead of "flat" like the originals.

photo above: E-50 as of January 3, 2012. Shaping up pretty well

White Graph B Coil - I was able to build up an early version B coil thanks to the acquisition of D-65 with its matched coils. I was able to copy all of the white background graphs from D-65's coils. I had the coil assemblies for an early B coil set, all with the small contact buttons typical of the D and E run HROs. I had a thick panel with wrinkle finish left over from the F coil that came with E-50 (those F coil assemblies were mounted to the correct type steel panel to build up an F coil set for my HRO-W.) After assembling all of the early style parts, I now have a complete white graph B coil to go with the white graph C coil for E-50.

Thanks for the Parts - Thanks to Scott McDonald KA9P for the small pointer knob for the Selectivity control. Thanks to Bill Fizette W2DGB for the two National ceramic tube sockets needed. Thanks to Jim Allen NU6AM for the small red "NC" pointer mounting screw.

The Final TWO "Missing" Parts Acquisitions - I still need to find a nice condition early style PW-D micrometer dial for E-50. Also, the RF GAIN dial that came with the receiver is from a later HRO and we are still looking for the early style, short-boss RF GAIN dial.

Paper Capacitor Update - A thorough examination of HRO D-65 has lead me to believe that all early HRO receivers came with Micamold brand paper caps. D-65 has 15 Micamolds with the remaining 6 caps being mostly Sprague (one Tobe.) E-50 has 5 Micamolds remaining but E-50 has an extensive repair history. It's unlikely that so many Micamolds would turn up in two different HROs unless they were original caps. A check of the underchassis photo from the 1935 and 1936 HRO manuals is clear enough to read the "Micamold" logo on the capacitors in that receiver which seems to confirm that they are the original brand used. Restoration of the capacitors will require a method of recreating the orange paper label of the Micamold caps in order for E-50 to have the correct under the chassis appearance. 

To be continued,...

The restoration information for E-50 will be added to this article as the work progresses. We have had a serious delay in completing the restoration of E-50 due to our moving the museum to its new location ( Oct-Nov 2012.) I would estimate that we should be back to being set up and ready to complete E-50 by the winter of 2014.

 

Collector's Photo Gallery of National HRO Receivers

E-mail us a photo of your HRO receiver along with some comments about your opinion of your HRO's performance. If you are restoring your HRO, feel free to relate any rework experiences you would like to share. Photos can be of any of the HRO family of receivers. E-mail your HRO photo to:  Western Historic Radio Museum - HRO Photo

HRO  SN: F-09  -  This early third production run HRO is in excellent condition and has its original coils (six of them) along with the coil box and the power supply. Note that the "NC" pointer retainer screw is still being used at run-F along with the white graphs but the pilot lamp is installed as is the pull switch for the S-meter. A very nice original HRO owned by K4OZY.
HRO  SN: G-235 - This early HRO is owned by Greg, K6SRO. It dates from July 1935 and includes the matching coil sets in the National wooden box. The meter is the 1-5 scale early version meter with the metal housing. The Noise Limiter looks like it could be a DIY addition based on the standard National modification or it might be a National supplied kit. Greg is in the process of restoring this great example of the early HRO.

HRO Senior SN: 170-H - This HRO is from Great Britain and dates from about 1940. It is owned by John G3SLX. 170-H is in great original condition and features an unusually heavy texture wrinkle finish paint.

HRO-MX  SN: D-706 - This is the WWII version of the HRO dubbed the HRO-MX. This version still has the large six pin glass tubes but many improvements were already incorporated by the time D-706 was built (probably around late 1944 to early 1945.) Note that the serial number uses the "D" prefix which was also assigned to the first HRO production run in 1935, however the number at "706" is much higher than the numbers assigned in the initial "D" run. Also, note that this receiver has the Marion Electric 0-1mA S-meter that was installed on some of the WWII production HROs. This HRO-MX is owned by Marc, GI3YDH, of Northern Ireland.
HRO Senior - Modified by Royal Canadian Corps of Signals  -  In the late thirties or early forties, the Royal Canadian Corps of Signals ordered approximately 200 HRO Senior receivers.* These receivers were modified to what the RCCS needed for their use. The addition of a shield to cover the tuning condenser can just be seen in the photograph to the left. The antenna terminals were replaced with a more robust British version. All controls were labeled  using engraved tags mounted with screws. Additionally, a serial number tag was added to the lower part of the front panel. A Hammond audio output transformer was added to the chassis. Three phone jack were added to the front panel. The 2000 Phones was from the plate circuit of the 1st Audio Amplifier, 500 Monitor was from the cathode of the Audio Output tube and 500 Line was from the Hammond transformer. On the rear chassis a phone jack was added that allowed disconnecting the Hammond transformer and connecting a National speaker box or any other speaker that had an internal output transformer. There are other minor modifications to the back of the S-meter and the toggle switch replacement for the pull switch for S-meter disable.

This RCCS modified HRO Senior belongs to Fred Archibald VE1FA, who has seen six other examples of this version of the HRO. Several versions were seen at swap meets in Canada and a couple have shown up on eBay. Photo provided by Fred Archibald VE1FA.

* Tom Brent reports that in 1975 Fair Radio Sales offered (on the front page of their catalog) these receivers "in the wooden crate as refurbished by the Canadian government" for the price of $125 with five coil sets included but no power supply. This information explains why so many of these RCCS HRO Senior receivers are found supplied with later HRO-5 type coil sets.

Amalgamated Wireless Australasia (AWA) - AMR-100 - AWA of Australia offered this HRO knock-off during WWII. It came with five coil sets which were general coverage only. This wonderful example is owned by Peter Brickey K6DGH, who did the restoration. The front panel has been re-silk screened and the receiver is shown with its correct matching loud speaker. This AMR-100 also has its complete five coil sets. This is the second AMR-100 that Peter has owned. The first was found decades ago in a surplus store in California. That particular receiver was sold long ago. This second one was found at a ham swap meet in California about 15 years ago. Peter finds that his restored AMR-100 seems to out perform its National HRO-5 contemporaries.     photo by: Peter Brickey K6DGH

HRO  Performance Comparisons

The Early Competition - To say that there was no receiver that came close to the National HRO performance in 1935 is not an exaggeration. The competition consisted of the Hammarlund Comet Pro - a receiver that was designed before the AGS and didn't have an RF amplifier or any sort of dial calibration. It was a popular receiver that performed quite well up to the point where images and lack of sensitivity became a problem (around 10 to 12MC.) The fabulous Hammarlund Super-Pro was in its design stages and was still at least a year from release. The Patterson PR-10 was available and with an R-meter along with a 10 tube circuit it was a good receiver but with no RF amplifier, no xtal filter or calibrated dial (other than the BC band) it was limited by images at higher frequencies and QRM everywhere else. At the time, there were various after-market preselectors that could be purchased and used with any of these receivers, providing the necessary RF amplifiers to increase selectivity, sensitivity and reduce images. Radio Manufacturing Engineers had the RME-9 which did have an RF amplifier, R-meter and xtal filter along with direct frequency readout plus bandspread. It was a compact, well built, good performing receiver but suffered with an extremely poor tuning dial design. In November 1935, RME introduced the RME-69, an excellent receiver that corrected the dial problems of its predecessor. Breting had just come out with the Breting "12" and this receiver had a TRF stage in addition to a non-tuned pre-amplifier that was switched in above 7mc. The "12" also had a crystal filter, R-meter, Volume meter and an illuminated, direct readout dial. Except for the lack of bandspread, the "12" had a lot to offer for about $100. In 1935, Hallicrafters was still having their receivers built by contractors and had not offered anything significant at the time although the very successful SX-9 was just about to be released (in late 1935.) RCA was offering the ACR-136 with an RF amp but no xtal filter or bandspread. It appears to have been a typical "all-wave" receiver for the SWL. RCA's AR-60 used double preselection on its three highest frequency bands but each receiver was literally "hand-built" and so expensive no ham could afford it ($495 - not to mention that only a few hundred were produced.) The AR-60 was certainly intended only for commercial-military applications. The other receivers from Sargent, Ross or other lesser known builders were generally lacking an RF amp, xtal filter or bandspread.

The HRO Design - The HRO was the only 1935 receiver that came with two stages of tuned RF amplification provided on all frequencies. This virtually eliminated images and later became the standard method used in single conversion superheterodynes for image reduction. The use of a plug-in coil module that shielded each coil and itself was installed into a shielded compartment assured that stray losses were not encountered. Connecting the coil set to the tuning condenser via flex finger contacts and short, direct buss wire assured that tuning losses were at the lowest possible. Efficient design of each tube stage, especially the First RF Amplifier, reduced thermal noise and increased the signal to noise ratio. These design factors allow the user to experience the best in sensitivity with low internal noise. Copying very weak signals is standard with the HRO - the signals would not even be heard on other receivers of the time. The other stand-out feature was the incredible bandspread. Though only available on the ham bands 80, 40, 20 and 10 meters, it gave the user unbelievable tuning with the micrometer dial providing the equivalent of nine and a half  feet of linear dial for each ham band. Tuning across the ham bands seems to go on and on - it was literally the best bandspread system available. 

Performance Today - What about the HRO performance today with modern QRN and QRM levels? The HRO is a very sensitive receiver capable of receiving DX signals that would be lost in the noise on other receivers. Of the many types of vintage receivers I've used over a long period of time, I have always found the HRO to be the most consistent great performer when it came to receiving weak DX signals. The HRO is the only vintage receiver that I've used that would receive Russian hams on 20M CW using just a wire antenna. This was in the mid-1970s when most UA/UKs were not running much power and were always just above the noise, even in a then-modern receiver. Additionally, when it comes to QRM, the early HRO crystal filter is quite amazing. It is one of the few crystal filters that will eliminate QRM, even on AM phone operation. By tuning the AM signal "on the nose" with the crystal filter on you will notice just how narrow the bandwidth has become and a slight detuning will decrease the signal significantly. The same is true for CW but these days there is little CW activity, let alone CW QRM.

The only receivers that approach the HRO in performance actually came out after the HRO.   Here's three of them,...


The Hammarlund Super-Pro was introduced in March 1936 as a high performance, professional receiver. It is a great receiver that was built to a high quality standard and performed quite well. The Super-Pro had variable coupled IFs, high fidelity audio, accurate direct frequency readout. It also used double preselection on all bands and used a separate power supply. The only complaints heard regarding the Super-Pro is limited frequency coverage (this is for the X and LX versions though, the SX covered 160M to 10M), high front-end tube noise masking weak signals (mostly due to improper antennas) and non-calibrated bandspread dial (it was a commercial receiver, too.) In actual use, the Super-Pro is an excellent receiver with great audio but if you are looking for weak DX signals, you will have to use a matched antenna. Hammarlund expected the user to operate the receiver into a matched antenna system and did not provide an antenna trimmer adjustment. On the audio side of things, the Super-Pro has 14 watts of high fidelity audio available and it can sound incredible when the speaker is matched to the audio output Z. The Super-Pro has the power to drive the speaker to a thunderous volume. Certainly a rebuilt Super-Pro will perform as well as a rebuilt HRO in almost every type of reception. Separate power supply and speaker are the required accessories. Photo shows the 1936 SP-10 version of the Hammarlund Super-Pro.

Radio Manufacturing Engineer's RME-69 was another great performer that was a contemporary of the HRO. When used with the matching DB-20 Preselector, the RME-69 has three tuned RF amplifiers and is capable of impressive results. The bandspread is not calibrated but it has an ultra-smooth vernier effect. The built-in power supply is handy as is bandswitching the six tuning ranges. The dial nomenclature is miniscule and difficult to read but the dial illumination is great. The audio is single-ended and sounds pretty good driving a matched speaker. While you could get very good results with the RME-69/DB-20 combination, just the RME-69 alone is a rather anemic performer. The build-quality the RME-69 is certainly far below the HRO with extensive use of compression trimmers for adjustments, even using compression trimmers for the LO adjustments. Additionally, the RME doesn't align the RF and Mixer stages but provides a "Trimmer" control on the front panel for peaking the stages for the frequency tuned. Finally, the RME-69 doesn't provide a remote stand-by function. Up to 1937, the RME was priced well below the HRO but when the price increased for the RME-69/DB-20 in 1937, the cost difference compared to the HRO was minimal. The RME-69/DB-20 only required a speaker for an accessory, maybe its only advantage after 1937. Photo shows the 1937 version of the RME-69/DB-20 with matching RME speaker.
1940 Hallicrafters SX-28  - With five years of receiver development behind it, a rebuilt SX-28 will perform as well as a rebuilt HRO and it also has more features with less accessories. Unfortunately, double preselection is not provided on the SX-28 until band 3, or above about 3.0mc. Hallicrafters insisted that the only reason for two RF amplifiers was image rejection but certainly economics had a lot to do with their opinion. Sensitivity is very good with low thermal noise. While the calibrated bandspread is convenient to use on the SX-28, the "extreme" bandspread that the HRO is capable of seems to be a better system when searching for weak signals. The P-P audio on the SX-28 has about 8 watts of power that when driving a matched speaker sounds very impressive with lots of bass and the speaker is the only accessory required. Like the Super-Pro, the SX-28 audio can reach thunderous volume levels. No doubt, the SX-28 will give an HRO serious competition, but after all, it came out five years later. Photo shows the 1940 version of the SX-28.

 

Using the HRO Today as a Vintage Communications Receiver

Some hams are reluctant to use a pre-war receiver in actual "on-the-air" operations for fear that adjacent frequency QRM will limit their ability to successfully copy stations and that they will be unable to complete QSOs or Vintage Net operations. The HRO might have problems today coping with adjacent frequency interference from powerful SSB signals when the user is trying to copy an AM signal (and this will be the case for almost all vintage communications receivers.) Even narrowing the bandwidth by using the crystal filter doesn't seem to help much. However, most of the problem is caused by the user operating the receiver as a "broadcast radio" rather than as a "communications receiver." Most manuals will direct users to operate the receiver in the "standard AM mode," that is with AVC on, the RF Gain at maximum and the Audio Gain rather low. This allows the receiver's front end gain to be controlled by the AVC in response to whatever the strongest "signal" happens to be. This works fine when listening to strong broadcast signals. But many times the desired signals are weak and the stronger energy will be atmospheric RF noise. Often times it is adjacent frequency interference. The best solution is to switch off the AVC, increase the Audio Gain to near maximum and operate the receiver's front end gain manually with the RF Gain reduced to the minimum necessary to copy the desired AM signal. You will find that now the adjacent frequency interference is greatly reduced since its strength compared to the tuned signal is reduced. Also, the AVC-controlled gain is no longer responding to variations in what it "sees" as a signal - the "pulsating" and "popping" of adjacent frequency signals within the passband.  >>> >>>   Additional selectivity using the crystal filter will also help to reduce the ratio of adjacent frequency interference to tuned signal. You will have to tune the desired signal "on the nose" for good copy. A couple of losses in this method of operation will be the S-meter function but it is really not very useful and only provides a relative measurement anyway. Also, audio fidelity suffers somewhat in this mode. But, the goal is successful copy and a completed AM QSO, in other words - communications. What about in actual use? I use only vintage receivers for operations on the ham bands and my station receiver for my vintage military radio set-up is the HRO-W. Using the "communications receiver" method of operation has allowed me successful copy of all stations checking into the vintage military AM net regardless of the adjacent frequency QRM. Weak signals can be a challenge but switching in the crystal filter usually provides the extra edge necessary for successful copy. Of course, it isn't enjoyable, "arm-chair" copy but it is a successful completion of the QSO or of the net operations, which is the goal. Unfortunately, deliberate on-frequency interference is another matter and no receiver, no matter how sophisticated the design, can cope with that type of intentional QRM.

 

Suggestions for Best Performance

The Type of Antenna is Important - If the HRO is used with an untuned, random length, end-fed wire, performance results may not be up to the receiver's capabilities (this is true for almost all communications receivers.) End-fed wires without a matching device are notoriously bad antennas - they are actually equally bad on almost all frequencies, so the user thinks the antenna is performing okay since there are no obvious peaks or nulls. However, with an antenna that is tuned for the received frequency, the HRO (and almost all receivers) will perform to its design limits. The HRO doesn't have an antenna trim, so when aligning the coil sets, the 1RF amp coil should be aligned to the station antenna, tuned for the specific band, for best performance. If you are an SWL, then the general coverage set-up should be used and the receiver's 1RF peaked for maximum noise using the regular listening antenna for that coil range.

More Alignment Notes and Coping with Drift - Proper alignment also is important for best performance with any receiver. Though it is possible to find an HRO receiver that is mostly original parts and is functional, it certainly will not be working at its design limits. A full rebuild is usually required for top performance. This is also true for orphan coil sets which might seem to function okay in a rebuilt HRO but full performance capabilities require that the coil set be fully cleaned internally, trimmers should lubricated and the coil set fully aligned to the HRO receiver that it is going to be used in.  

The original coil set alignment instructions had the user remove the upper front lid support that's held in place with two screws at each end and by the dial pointer mounting screw in the center. This was to allow easy access to the coil trimmers. I've found that if you use a very long and thin blade screw driver with the metal shaft wrapped with tape (to prevent shorting to the condenser frame or the cabinet) all of the coil trimmers are accessible without removal of the upper lid support.  

The basic alignment is described in "Restoring and Aligning HRO Coils Sets" elsewhere in this article. The specific manual for the particular type of HRO will generally provide more details and should also be used when aligning the receiver. As far as the alignment of the IF and Crystal Filter, this is standard faire and as long as the IF is aligned to the exact crystal frequency, no problems will be encountered.  >>>

>>>  The IF is 456kc and the crystal will be very close to this frequency. However, sweeping the signal generator across the IF can determine the exact crystal frequency and this is the frequency that the IF should be aligned to. In fact, your Crystal Filter will function better if you perform the IF alignment with the Crystal Filter on and set for fairly narrow selectivity. This assures that the Crystal Filter is operational and can be a useful tool for combating QRM.

Although the manual will recommend that the BFO be set to zero-beat with the control at "9" this is not really necessary and actually reduces the effectiveness of the BFO. In the 30s and 40s the BFO was for CW and upper and lower sidebands don't even exist in that mode. Today we have SSB and by setting the BFO to zero-beat at 6.5 on the scale will allow you to select either upper or lower sideband when receiving SSB. As with all early receivers, the RF Gain must be reduced and the AF Gain set to near maximum with the AVC off and the BFO on to receive SSB signals. You have to set the receiver RF Gain so that the signal to BFO injection ratio is correct for non-distorted SSB reception.

Coping with Drift - Drift is just a "fact of life" for early receivers. There is a lot of discussion today for ways to eliminate drift in old receivers but it really depends on your individual method (or preference) of operation. All amateurs in the 1930s and 1940s were always tuning the receiver during a QSO. That was just the way it was - you followed the received signals by frequent adjustment of the tuning dial. It became "second nature" for hams to "tune around" for the best reception during a QSO. By the 1950s and later, a different belief was beginning to be instilled into hams - "you shouldn't have to touch the tuning dial on the receiver. The received signals shouldn't drift and neither should the receiver!" This has resulted in today's hams expecting "drift-free" operation in a receiver that was designed when the components and circuits available made this difficult to accomplish. Add in the economic factors of the Depression and it became next to impossible to build a "sellable" receiver that didn't have some drift. The HRO will drift in frequency for about 30 minutes and then settle down somewhat. In the bandspread mode the drift is much more noticeable. This is because of the resolution of the PW-D in the bandspread mode. A small drift of a couple of kilocycles appears as a drift of five to ten "index marks" on the micrometer dial (depending on the coil set used.) So, if you want to operate your HRO "as original" you'll have to get used to the drift. Otherwise, you can change the LO coupling capacitor to a NPO type and add in zener regulation on the oscillators or build a regulated power supply for the receiver. Original is easier and better.

 

Conclusions - The pre-WWII versions of the HRO receiver generally aren't found in too many vintage AM ham stations today and that's a mystery. Why is such a high-performance communications receiver virtually ignored by the vintage AM enthusiasts? Probably, it's because the HRO is rather small and basic in its appearance. It doesn't have a large illuminated dial but rather has a device that requires the tuned frequency to be indirectly determined by referencing to graphs. It has single-ended audio, no tone control and the only selectivity control is via the Crystal Filter. It requires storage for the extra coils and it does require more accessories than normal, like the power supply, extra coil sets and speaker. However, when it comes to sensitivity you can't beat the HRO. When it comes to bandspread resolution, there is no better receiver than the HRO.

The HRO's later kin - the HRO-50 and HRO-60 are fairly popular today with vintage AM enthusiasts, still giving competitive performance even on 10 meters. These later receivers do feature a built-in power supply, push-pull audio and a direct readout dial which seems to confirm our opinion as to why there's a lack of interest in using the pre-WWII HRO as the station receiver.

Today, the HRO is revered as one of the great designs of pre-WWII communications receiver production. Though collector emphasis and resulting high prices are on the rarer early versions, any of the HRO receivers will give the user superior performance, especially following a careful and thorough rebuild and alignment. With a production history spanning 30 years, certainly the HRO design concept was a long-lived one. It was a great receiver then and is still a fabulous performer some seventy-plus years later.


photo above: 1935 HRO sn: D-65 (first production run) rack mount receiver

References:

1. "The Wonderful HRO Receiver" by William Orr W6SAI,  published in CQ magazine, May 1975 - Orr's article describes the origins of the HRO receiver

2. "The HRO Report: Dating the Early HRO " by Charles Fisher,  published in AWA, Oldtimer's Bulletin, Vol. 4 - 1989 - Fisher deciphers National's serial number system for the HRO, production history and engineering changes from 1935 up to 1941. Fisher's article contains detailed information on early HRO production history and his observations from the results of a survey that references over 70 examples of pre-WWII HRO receivers. Fisher's article is available on the web at:  FISHER: Dating the National HRO  

3. "The Evolution of the National HRO and its Contribution to Winning WWII"  by Barry Williams KD5VC - Excellent detailed history of the entire HRO line, info on British use in WWII. Barry's article is available on the web at:  Evolution of the HRO - Barry Williams

4. QST Magazines 1934 through 1939, various letters by James Millen, ads, etc. - Info on product development and upgrades, ads show evolution of the design

5. National Co., Inc. - HRO Manuals - detailed circuit description, design intent and performance expectations

6. Rider's Perpetual Troubleshooter's Manual - Vol VIII - most of the information needed for the early HRO receiver is in VOL. VIII

7. Article in Shortwave Craft  March 1935 by James Millen - Basically, this is a detailed advertisement for the HRO, circuit description and schematic

8. National Co., Inc. Bulletin #250  1936 Catalog - Information on model numbers, availability and prices

9. National Co., Inc. "The HRO" Advertising Brochure - Information on circuit design and performance

10. "Discovering Vintage Radio" - Chapter 14, "National's Legendary HRO" - This is a British book with some interesting history and information on the HRO

11. "Communications Receivers - The Vacuum Tube Era" by Raymond Moore - Excellent reference book on HROs and all other models of tube-type communications receivers

12. Thanks to all of the National Co., Inc. and HRO enthusiasts that have sent in photos, serial numbers and other information. Your input has helped to expand this article and add detailed information on the production and history of these great receivers.
 

Henry Rogers © April 2007, new info added Jan 2008, Mar 2008, Sept 2008, June 2010

Entirely Re-edited, re-styled and expanded with new material to match our other communications receiver articles, corrections made, new photos - Henry Rogers © September 2011

More information and photos added: November 2011, January 2012, new photographs February 2012

March 2012 - Corrections to HRO-5A1, HRO-6 and HRO-7 introduction dates   June 2012 - Minor corrections, Royal Canadian Corps of Signals HRO Senior - Fair Radio Sales info added,

July 2012 - Additions to details on HRO-5A1 "late versions" and relationship to HRO-6

Jan 2013 - HRO Junior information expanded with new photos

May 2013 - Correction to AWA reference - now Amalgamated Wireless Australasia

August 2013 - More details on two-letter prefix SNs from WWII production

January 2014 - More details on alignments of Coil Sets and IF alignment. Correction to assembly of the worm gear thrust assembly.

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