Western Historic Radio Museum
 

NATIONAL  COMPANY,  INC.

 HRO  - COMMUNICATIONS RECEIVERS

"The Cream of the Crop"

  Design and Production History  ~  The HRO Models

Serial Numbering Analysis  ~  Dating by Serial Number 

   Engineering Changes  ~  HRO Accessories, Coil Details 

  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.

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:  This 1935 ad is from Leeds, a New York radio dealer, showing the high regard that was accorded the new National HRO

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. Around this time, a photograph of a "prototype" HRO was released showing a receiver with a micrometer dial but with a "Volume" control and a BFO switch on the front panel but no front panel BFO adjustment. The receiver also had AGS knobs installed. This was never a production receiver, only a prototype or a mock-up for the photograph. 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.

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

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, all 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  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 and the early-style cabinet.

Compare D-65 from the first production run to fifth production run receiver, G-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. G-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 has the 0-5 S-Units scaling that was used on runs D - H.  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)

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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 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 actually inquired if they could order the HRO - minus specific circuits they believed they wouldn't need - at a reduced price. 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 income to get an HRO. National announced the budget 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 push-pull switch and adjustment pot. Additionally, the coil sets for the "Junior" did not bandspread and were identified with a "J" proceeding the regular coil set letter designator. 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 40 meters was a CW only band. National even offered to upgrade the budget receiver at a later date (perhaps when the owner had saved up the money) for a very reasonable price. >>>

HRO Senior

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 blackish-brownish-olive color that appears black in normal room light. 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. 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. Also, within a few months, 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.

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

The next change (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.  

photo above: Gray HRO Rack Mount from 1936 QST Advertisement

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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. In fact, some catalogs do mention that any 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, the Navy HROs will usually have an "anchor" ink stamped somewhere around the chassis or chassis mounted parts.

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. At first, Britain couldn't buy the HRO 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. After Lend-Lease though, the British were able to have a steady supply of HROs direct from National.

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 lose" crystal that plugged into the top of the filter assembly. The design, at that time, didn't change how the Crystal Filter operated. After that change, National referred to the new HRO Senior as the HRO-M. 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. The S-meter "pull-switch" was replaced with a black-finish, ball-handle toggle switch.

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 frequency 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. 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. There was also an HRO-12-S that operated on 12vdc, had series-parallel filaments and a 175kc IF. Actually, the Navy bought a lot more NC-100A (RAO series) variations than they did HRO variations. 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 receivers as R-106. There are certainly many more obscure designations that were used for the HRO receiver depending on the specific end-users, whether military or civilian.
 

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

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.

Kingsley AR-7 - 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 AR-7 receiver built by Kingsley Radio Co. of Melbourne, Australia. Though the AR-7 has a micrometer dial and uses plug-in coil sets, that's about as far as the Australian HRO copying went. Rather than using a separate Mixer and Local Oscillator, the AR-7 uses a Converter Stage. The receiver uses a stainless steel overlay on the front panel. Interestingly, the S-meter on the AR-7 works "backwards." Full scale is "0" and mechanical zero is "9." Additionally, the micrometer dial also works "backwards" with 0 being the highest frequency tuned and 500 being the lowest. All AR-7 receivers were rack mounted with a rack mounted power supply and a rack mounted speaker. The audio output impedance was approximately 2000 ohms and 600 ohms and the panel jacks provided both audio outputs. Probably around 2000 to 3000 AR-7s were produced. They were used primarily in airports around Australia after WWII.        
                                                                                                                                 photo right: Kingsley AR-7 receiver owned by NU6AM.

The Post-WWII HRO Receivers

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 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 came with nine coil sets, this was also offered with the civilian HRO-5. It's also likely that some of the civilian HRO-5 power supplies were the heavy duty 697W.

Updates came almost immediately and National quickly upgraded the A,B,C and D coils to have the bandspread function, designating this receiver the HRO-5A. National replaced the white scale 0-1mA scaled meter used in the HRO-5/W with a white S meter scale with the "NC" diamond logo. By early 1946, a new Noise Limiter circuit was incorporated, adding two tubes to the receiver bringing the total to 12 (counting the PS rectifier.) 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.     

photo right: The 1946 HRO-5TA1 sn: 184 1054

photo left: Inside a 1946 HRO-5TA1 showing that the chassis is now cadmium plated (later units,) two more tubes are added next to the left side of the tuning condenser for the Noise Limiter and the S-meter adjust is moved next to the BFO coil housing. Also, note the NATIONAL decal installed under the lid. These decals began to be installed on cabinets very early in production - by run-G (1935.)

HRO-7

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 - 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 each IF stage. 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 (above 7MC) and a current regulator on the oscillators, it was the final evolution of the tube-type HRO. The HRO-60 was produced 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. With the HRO-50, the SC-2 coils storage and speaker combination became available. Special "dust covers" were installed on these later 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 a distinct loss of bass response due to the lack of any enclosure for the speaker. This is typical of most rack mounted speaker set-ups.

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 HRO-60 and the HRO-50-1 were trying to cope with the crowded band conditions of the fifties and sixties and successful communication was the goal. The additional IF transformers and amplifier stage provided narrow bandwidth with steep skirts resulting in fabulous selectivity. Operating a rebuilt and correctly aligned HRO-50-1 or HRO-60 is a pleasure - QRM is rarely (if ever) a problem and it is still a competitive performer at any frequency.

photo left: 1956 HRO-60R  sn: 393 0255, rack mount HRO-60 receiver in MRR-2 table rack with SC-2  8"speaker & coil storage unit

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 the one reported HRO-W is serialized as K-127, there certainly must have been two of those numbers issued. 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.

Additionally, there have been two HRO-M receivers reported that were serialized with a double prefix followed by up to three numerals,... PC-67 and PP-988. 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. One note is that both double prefix HROs are located outside the USA. Possibly, the double prefix identifies production runs specifically for Lend-Lease sales.

 Post-WWII HRO Serial Numbering System - With the 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 are only three digits in quantity even though the format allows four digits.

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:  Western Historic Radio Museum     

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.

 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-38(R), D-49 (R), D-65 (R), E-50, F-51, F-58, G-113, G-235, H-103, K-83(R), L-73(R), L-184, M-167(R), N-25, N-29, N-79, R-54, S-208, V-77, V-214, Y-53, 233-C, 170-H, 463-K, 434-L(R)

HRO Junior: L-58(R), O-250(R-WWII Mil.),

HRO-M, Canadian HRO: B-72, C-638, D-706, P-377, P-554, O-605

HRO-5, HRO-W: A-67, PC-67, PP-988, J-39, J-879, J-942(R-106), K-127(W)

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

(R) = Rack Mount, otherwise receiver is a table version 
(R-106) = British designation
(W) = Signal Corps designation, otherwise receiver is HRO-5      

HRO Serial Number Log (Post-war)

HRO-5A, HRO-5A-1: K-611(TA), 184 1054(TA1),

HRO-6: 184 0697,

HRO-7:

HRO-50, HRO-50-1: 293 0036(T)

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

(R) = Rack Mount    (T)= Table Version

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-K) - 0-5 scale conformed to early QSA signal reporting method -  All E production run receivers seem to have the meters 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 supplier problem resulted in the use of a different style meters during the early production runs.

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 changed to plain nickel-plated "raised diamond" (G-on) - cost reduction. Fisher indicates that the "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 added to rear of chassis (F-on) - allowed for remote control of B+

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

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. 

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 terminal insulator strip mounted further into the chassis leaving about a 0.5" space for the serial number. With run-E, the terminal insulator is mounted much closer to the chassis edge leaving only about 0.25" for the serial number.

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. 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.

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

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.

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), afterwards 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 (R-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.

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)

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 to form James Millen Mfg Co. - but some say internal conflict over National going public and selling stock.

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

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.

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 cap on top of the gear box is changed from a casting to a sheet metal piece with square edges and corners ("NC" is stamped-embossed.) 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.

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

Later HRO-M receivers will have ID rings for AVC and B+ toggle switches. Many HRO-M 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

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

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

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 scaled, non-illuminated meter. Scale has "NC" diamond logo.

HRO-5A1 introduced late-1945 - Features an added Noise Limiter circuit (increases tube count by two) and new circuit for Crystal Filter.

Also, for HRO-5A1 - Meter Zero Pot location moved behind Audio Output tube socket

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

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

New S-meter with square shaped housing made by Marion Electric introduced about 1946, replaces the non-illuminated, white scaled S-meter.

Later in 1946, the gray painted chassis is replaced with a cadmium plated chassis

Summer 1946, HRO-6 introduced with improved Noise Limiter circuit - external appearance of receiver remains unchanged. It's possible that HRO-6s and HRO-5TA-1s were built with-in the same production run. See "HRO Serial Number Log" (above) -  the only reported SN for HRO-6 is production run 184 0697 and one of the HRO-5TA-1 serial numbers is also production run 184 1054. It's also interesting that the HRO-5TA-1 apparently has a higher serial number than the HRO-6.

HRO-7 introduced in Fall 1946 as 1947 model - 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 dual-IF transformers

With HRO-50 or 60, coil forms used in the coil sets are changed to ceramic - may correspond with the change to ceramic material for the coil assembly insulators

HRO-60 introduced in 1953, adds double conversion above 7mc - 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

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 480kc down to 50kc with a small section of the spectrum around the receiver's IF not covered (430kc to 480kc.) 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. 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 (left photo) versus the access holes in later coil sets (right photo.)

The left photo 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. The right photo shows the contact insulator used on coil sets up to 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.

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 the nearest photo to the right. In the photo furthest 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 quite well. The inductance adjustments are very stable over time and retain their proper settings very well. However, all HRO manuals have the detailed procedure for full alignment and owners should use that as a reference for alignment.

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 coil sets were aligned to their specific receiver, thus the importance of having the "matched" original coil sets. Non-matched coil sets can be used with any HRO but they should 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. National also sold individual coil sets and these type of coil sets were not serialized. 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.   

Other Charts and Logs - All E, F, G, H and J coil sets will have only one graph provided since band spread is not available. E and F provided a logging chart for favorite stations since these two coil sets are needed for AM BC coverage. Longwave coil sets G, H, and J also provide logging charts for coastal stations and other types of stations that were operating in that region of the spectrum. 

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 to around the HRO-50 or HRO-60 when the coil forms were changed to ceramic. 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 was in use through the HRO-60 production. These later coils had ceramic insulator blocks and switches for selecting bandspread or general coverage. 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 affects coil sets A and B, along with the other "A" prefix coils sets, AA, AB, AC and AD, only these coils sets have 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 are installed the double conversion switch actuates with the switch shaft protruding into the clearance hole.

photo left: 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.

photo right: HRO-60 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 Various National 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.

Be aware that there are a couple 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 load has dropped the B+ to ~+170vdc. National did publish that the HRO would operate on B+ as low as +135vdc but added "at some sacrifice in performance."  There is also the 5880 power supply that was advertised for the SW-3  receiver, though similar in appearance it doesn't have the current capacity to power an HRO. 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.

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

photo above: The 10" speaker for the NC-100 series on the left and the 8" speaker for the HRO on the right.

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. The standard National black box speaker with the cloisonné "NC diamond" emblem was available by about mid-1936, or so, but photos were not used in National advertising until much later. The "standard" National box supplied for the HRO contains an 8" Jensen PM speaker with audio output transformer mounted on the speaker. 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.

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 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-5A-1

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

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 "pitched worm gear."

Lubrication and Assembly of the PW Gear Drive  

Apply some grease to the conical thrust bearing of the worm gear as this is spring loaded against a mating conical surface machined into the cast gear box housing. Don't over lubricate by packing the gear box full of grease since the excess might eventually run out the seams of the gear box casting and create a dust trap mess. 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. Reinstall the elliptic bearing hub casting which spring-loads 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. After the gear box is done, then look at the Micrometer dial.
 

* The fact that many worm gear shafts are incorrectly assembled might be blamed on a cut-away drawing of the gear box that was used in National advertising for several years. This cut-away drawing shows an internal ball bearing support cast into the housing. The worm gear shaft is shown with the ball bearing next to the flange and ball bearing supported by the internal casting. The worm gear shaft then had the washers and spring assembly and, finally, the elliptic bearing hub. Unfortunately for those who referenced this cut-away drawing, the internal ball bearing support was never included in the production gear boxes and the ball bearing actually had to be supported by the front of the casting thus requiring the worm gear shaft assembly shown in the above photograph.    

PW-D Micrometer Dial

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

PW-D Evolution - The first PW-D had a flat nickel finish on the outer dial with black index graduations. The inner 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 finishes were available for the PW-D. The HRO Senior PW-D was changed to a black-brownish-olive (that looks black except under intense light) lacquer finish. Note the photo below of the HRO Senior PW-D. This color generally appears black in normal room light but under a camera "flash" it shows its true color shading. The PW-D for the NC-100 was blue-gray on the outer dial and red with white numerals on the inner dial. The NC-100X used a gray outer dial and the HRO type black inner dial. 

Early HRO - German Silver Plated

HRO Senior - Lacquer Paint

HRO-W - Lacquer & MFP

HRO-60 - Chrome Plating

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."

photo right: This isn't an HRO dial but it is the PW-D used on the NC-100 receiver.

PW-D Micrometer Dial Assembly

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. There are two ways that the outer index dial can interface with the inner number dial. One is correct and one isn't. 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.

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

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

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 will be the insufficient current carrying ability of the power cable for the 9.5 amps required to operate the 2.5vac tube heaters. 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 can't re-tube a 6.3vac HRO to 2.5vac tubes and expect it to work without serious modifications.

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."   >>>

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. 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.

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.

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.

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. 

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 set A, the 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 40M coil with it,) replace some missing tube shields and 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.

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 mis-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.

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

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. 

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.  >>>

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.

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 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.

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 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.

To be continued,...

The restoration of E-50 will be added to this article as the work progresses. Check back often. - H. Rogers, Latest update - Jan. 29, 2012

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 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.

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.

HRO  Performance Comparisons

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.

Using the HRO Today as a Vintage Communications Receiver

Suggestions for Best Performance

Alignment Improves Performance - Proper alignment also is important for best performance with any receiver. The HRO is complicated to align because each coil set has to be aligned twice - once for general coverage and then for bandspread. There are eight trimmer adjustments, one inductance disk for general coverage and four bandspread padders on each coil set. On the A and B coil sets, the inductance adjustment for LO general coverage are actually loops inside the coil that have be physically moved to change the inductance. The amount of physical change necessary in the loop's position is very small since this was a very high frequency coil (14-30MC.)

Most HROs today do not have all of their original coil sets and users have collected "orphan" coil sets to make up a complete set of coils for their receiver. Originally, the coil sets were aligned to the specific receiver they were sold with but even if you have the complete original set of coils, they probably still need to be checked for alignment as components age changing the original alignment requirements. There are no differences in coil sets, e.g. an "A" set is identical to another "A" set - all that changed was the alignment to a specific receiver. Consequently, "orphan" coils are fine to use to make up a complete set of coils for an HRO that has "lost" its original set - as long as the coils are aligned to that 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.

By far the best performance will be enjoyed when the HRO receiver is fully rebuilt and aligned. This should result in a receiver that is performing equal to or better than what the receiver was capable of when new.

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 those 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.
 

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