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"The Incredible Pre-war 'Super-Pro' Receivers"

 Production History ~ Comet & Comet Pro ~ Super-Pro Model Designations
The SP-10 ~ The SP-100 ~ The SP-200 ~ The Military SP-200
  Power Supplies & Cables ~  Serial Numbers and Production Estimates 
Serial Number Log  ~  Expected Performance 
Guide to Restoring the Super-Pro Receivers
 Restoring the WMI SP-10 Receiver ~  Restoring the SP-100 Series Competition Performance Comparisons
 Photo Gallery of Collector's Super-Pro Receivers

COMET-PRO DETAILS in Appendix A at the end of this article

by: Henry Rogers WA7YBS/WHRM

photo: QST ad 1-38, Stan Wolff of the NY Herald Tribune copying Press with the 'Super-Pro' - this is an interesting ad in that it shows both the SP-10X (right) and the SP-100X (left) models.

Was the Hammarlund Super-Pro the ultimate pre-WWII communications receiver? The advertising of 1937 stated it was "The Last Word" in communications receivers. Other advertising called it "Tomorrow's Receiver - Today."  Its build quality was second-to-none. As Hammarlund said about designing the Super-Pro, " no time was cost considered a limiting factor."  Hammarlund wanted to build a "standard" by which other communications receivers were measured. The Super-Pro was advertised as an "amateur-professional receiver" and it had several unique features that did set it apart from its contemporaries. These included a virtually sealed precision tuning assembly with custom designed variable condensers and a cam-operated knife-switch type 360 degree rotation bandswitch, variable-coupled air-tuned IF transformers that allowed a continuously adjustable IF bandwidth and a powerhouse, high fidelity audio output. In the hands of an experienced operator, the Super-Pro could out perform any other receiver. Listing at $400 and selling at discount dealers for about $250, not many Depression-era hams could afford the Super-Pro, so not much was ever written about the receiver in ham magazines. As a result, among hams, opinions are highly diverse when it comes to the Super-Pro's performance capabilities. The best pre-war receiver? Let's see,...    H.Rogers, Feb 2008

The Incredible Pre-war 'Super-Pro' Receivers


Hammarlund's Company History up to 1931

Oscar Hammarlund came to the United States from Sweden in 1882 to work for the Elgin Watch Company. By 1886, he had gone to work for Western Electric in Chicago. He later worked for the Gray National Company on the Teleautograph machine and finally, in 1910, formed his own company, Hammarlund Manufacturing Co., Inc. At first the company built gadgets but they soon became involved building Western Union call boxes. An interest in wireless led to the Hammarlund designed variable condenser which became an industry standard for quality. In the mid-twenties, Hammarlund formed a temporary partnership called Hammarlund-Roberts specifically to supply Hammarlund-built parts for broadcast radio kits. As home radio technology improved, the popularity of radio kits declined and Hammarlund-Roberts Co. folded. That certainly didn't affect the operation of Hammarlund Manufacturing Co., Inc., in fact, they were just coming out with their newest creation, a shortwave radio that was going to push the few communications receiver manufacturers of the time into the superheterodyne building business.


The Comet and the Comet-Pro  -  1931 to 1935

Hammarlund's entry into the shortwave and ham receiver market began in 1931 with the introduction of the Comet All Wave Receiver. The early Comet circuit used eight tubes, plug-in coils (without shields) and operated with a built-in AC power supply. However, what was important was that it was a superheterodyne. In fact, the Comet is generally credited as the first successful commercially-built shortwave superhet, introduced at a time when almost all shortwave receivers in use were TRF with regenerative detectors. The Pilot AC Super Wasp was typical of type of short wave receivers in use at the time and the Comet was superior in all comparisons. Of course, the Pilot Super Wasp had been a kit and was considerably cheaper than the Hammarlund Comet. Also, most of the hams at the time built their own receivers that were usually simple and inexpensive. The Hammarlund Comet was beyond what most hams at the time could "homebrew."

By late-1932, the Comet had evolved into the Comet-Pro and this version of the receiver changed the way in which commercial users, the military and the amateurs listened to shortwave signals. Hammarlund described the Comet Pro in their advertising as "The World's Finest Shortwave Receiver." It really wasn't an exaggeration, the Comet Pro was a superbly built, great performing receiver - at least when compared to the other receivers of the early thirties. Up to the introduction of the Comet-Pro, the Regenerative Detector with a couple of TRF stages ahead of it was thought to be the most sensitive receiver on shortwaves. The superheterodyne, while fine for AM Broadcasts on medium wave, was too noisy and that lowered its usable sensitivity - at least that was what most shortwave radio enthusiasts thought. Hammarlund took the challenge to build a shortwave superhet that had low internal noise and was sensitive. By careful design work using quality parts and construction, the Comet-Pro did achieve what was thought impossible - a usable shortwave superheterodyne receiver. As the Comet evolved to the Comet-Pro, it was fine-tuned into an excellent performing communications receiver that became quite popular, ultimately being found in many ham shacks of the thirties and also used by many professionals, including the military. Some Comet Pro receivers even made it to the Arctic and Antarctic with various expeditions.

By 1934, the Comet Pro was beginning to show its age. Its limitations were due to the lack of an RF amplifier, making the Comet-Pro susceptible to images as the received frequency was increased. Some users would add an after market pre-selector to eliminate the image problem. Other issues included lack of a calibrated dial readout, a BFO control that was a "swing-arm" lever accessed under the lid and the fact that changing bands required changing a set of two plug-in coils. The competition was beginning to displace the Comet Pro from its number one status. National had come out with their AGS and FB-7 receivers and was busy designing their fabulous HRO. Patterson had the PR-10 with a matching preselector available and Radio Manufacturing Engineers had their RME-9 receiver with a TRF amplifier built-in, direct frequency readout on the tuning dial and an "R" meter. No doubt, Hammarlund wanted to return to the days when they offered the only "high performance" receiver available. The Hammarlund engineers had been at work on the Comet Pro's successor since 1933.....  More information on the Comet Pro receiver at the bottom of this webpage in Appendix A.

photo left: Comet Pro ad from Radio News February 1933

The 'Super-Pro' Production History

Designing the 'Super-Pro' began in 1933 when Hammarlund announced it was working on a new shortwave receiver dubbed the "Comet Super-Pro." At that time there were only a few superheterodyne shortwave communications receivers on the market - National had their AGS and FB-7 receivers and Patterson may have just introduced their "All Wave" predecessor to their PR-10. However by 1935, several manufacturers such as  Breting, Patterson, RME and RCA had all come out with fine quality superhets, not to mention that National had released their fabulous HRO receiver. As the competition continued to build better and better receivers, Hammarlund, still in the design stages, continued to improve the concept of what the 'Super-Pro' would be. That Hammarlund wanted to produce the best communications receiver available cannot be denied. A receiver that would appeal to engineers, professionals and well-to-do hams. Quality of construction would be of the highest caliber and performance would be to the limits of the design possibilities of the day. By March 1935, Hammarlund dropped a hint that the Comet Super-Pro would be released soon but the year went to conclusion with no formal announcement. Finally, in the March 1936 issue of QST, a full two page ad announced the "Super-Pro" receiver. A personal letter from Lloyd Hammarlund accompanied the detailed introduction advertisement. By the thirties, Lloyd Hammarlund was generally running things at the company although his father, Oscar Hammarlund was still company president (Oscar H. died in 1945.) The introduction told about the great Super-Pro features and showed what the receiver looked like, inside and out. Touted as an "Amateur-Professional Receiver" no where in any advertising is the price ever mentioned. Listing at $400, the Super-Pro was one of the most expensive of the generally available communications receivers in the mid to late thirties.

photos: Page 1 & 2 of the Super-Pro Introductory Advertisement in March 1936 QST

The Super-Pro Features - With the availability of the new Super-Pro receiver hams and commercial users now had a communications receiver that boasted several innovative features. The precision that was designed and built into the RF tuning unit was incredible. 25 coils mounted on 20 Isolantite (a type of ceramic material with low loss characteristics) bases working with a Hammarlund-built 4 gang tuning condenser and 12 gang bandspread condenser along with a custom designed cam-operated knife-switch type bandswitch made up the heart of the RF tuning unit. The main tuning dial featured band-in-use masking. The antenna input coils have a Faraday shield between the primary and secondary to keep the input impedance constant regardless of tuning. The bandspread condenser has three different sections per gang that optimizes the variable capacitance needed for 90% span on the 80, 40 and 20 meter ham bands. The RF box is an entirely shielded, nearly sealed metal box and only connects to the IF/AF chassis with eight wires (and the four grid leads.) The receiver tuned from .54 up to 20MC in five bands. Bandspread was provided for the upper three tuning ranges. Another innovation was the variable-coupled air-tuned 465KC IF transformers that were part of the continuously variable Selectivity control. With this control, IF bandwidth and audio fidelity were adjustable from 3 KC out to 16 KC. Of course to take advantage of this, a full fidelity, high power audio amplifier was provided with a transformer coupled, triode-connected, push-pull audio output delivering about 14 watts of power. An Amplified AVC system was used for maximum control of the RF/IF gain when listening to SW-BC stations or local AM stations. A "Tuning Meter" measured the total IF amplifier plate current and, as strong carriers increased the AVC bias, the IF gain was reduced and the meter would read less current. The operator tuned for the lowest meter reading while tuning in an AM signal. There was a procedure for logging the "no signal" noise level current and then measuring the tuned signal current and then calculating the "db over the noise" as a signal report - Hammarlund thought all hams and engineers carried a slide rule in their pocket! Also included was a front panel controlled Beat Oscillator (BFO,) a Tone control along with separate RF, IF and AF gain controls. 14 tubes were used in the receiver. The separate power supply used two tubes, a 5Z3 B+ rectifier and an 1-V rectifier for the bias supply.

photo left: Super-Pro ad from QST July 1936 - CE William Greer, WNEL, where the Super-Pro was used to rebroadcast WABV

Super-Pro Model Designations - When the Super-Pro was first introduced it was just referred to as "The Super-Pro" as there was no need to distinguish it from any other models. This situation changed quickly as different frequency ranges, crystal filter options and speaker size options became available.  For example, an "X" suffix designated a crystal filter option while an "S" suffix referenced the ham version of the receiver. By the time the 100 Series was introduced, several options were available. They were designated as follows:

SP-100 - .54 to 20mc, no crystal filter
SP-100X - same, with crystal filter
SP-100L - 100-400kc, 2.5-20mc, no crystal filter
SP-100LX - same, with crystal filter
SP-100S - 1.25 to 40mc, no crystal filter
SP-100SX - same, with crystal filter

When the receiver left the factory it had a speaker included with it. Two choices were available with the 100 Series, a 10" or a 12" speaker. These speakers were supplied without an enclosure. To designate which size speaker was included the number indicator was modified as follows:

SP-110X - .54 to 20mc, crystal filter and 10" speaker
SP-120X - .54 to 20mc, crystal filter and 12" speaker

The other suffix designators for the other receiver versions follow this same pattern. Note that the two example receivers would be identical but with different size speakers supplied.  >>>

>>>  Nowadays, Super Pro receivers are rarely (if ever) found with their original speakers so designating the original speaker size becomes superfluous. It might be a bit of curious information that the receiver originally came with a 10" speaker but since that speaker is no longer with the receiver, it isn't important. It's likely that the lack of any original Hammarlund speaker enclosure on the early Super Pro receivers led many hams to build their own or incorporate the speaker into a wall baffle. This undoubtedly has resulted in the original speaker being "left behind" while the receiver (and, hopefully, the power supply) were sold or otherwise "relocated." This article references all Super-Pro models to the "Series" that they belong to as this eliminates the confusion of the two speaker size options changing the model designation of identical receivers. Thus an SP-110X is identified in this article as an SP-100X, belonging to the "Super-Pro 100 Series" which is how Hammarlund designated the receivers in their advertising.

Manuals - If you are in need of a manual or are looking for the schematics in Rider's Perpetual Troubleshooting Manuals, you will find that the speaker size designator is used to identify the receiver. Thus, an SP-100X is listed as an SP-110X or SP-120X. Again, it makes no sense today to reference the speaker size that was included with the receiver but that is how many Super-Pro receivers are listed in various manual suppliers and in Rider's manuals. When ordering a manual, be sure to make allowances for the model and speaker designators versus how the Super-Pro receivers are referenced in this article.

SP-200 Series - The SP-200 Series receivers follow the same designations as the SP-100 Series.


The Super-Pro Models  -  1936 to 1945


The "Super-Pro" -  SP-10   1936

Introduced in March 1936, initially, the SP-10 was just called the "Super-Pro" - there was no need to distinguish it specifically since there were no other models. The SP-10 designation comes into use around the time the "100 Series" was introduced. The first production Super-Pro used all glass tubes with vented tube shields on all tubes except the audio section tubes. The tube lineup was RF - (2)6D6, Mixer - 6A7, LO - 6C6, IF - (3)6D6, DET - 6B7, AVC - 6B7, BFO - 6C6, 1st AF Amp - 76, AF Driver, P-P AF Output - (3)42. The power supply used a 5Z3 for B+ and a 1-V for the bias.

The push-pull amplifier was capable of 14 watts of high fidelity audio power. The AF output transformer was specified in some sources as a 600 ohm Z line audio (Rider's, for instance.) However, the SP-10 manuals specify that the output Z is 8 ohms and the schematic contained in the manual marks the output as "Speaker Voice Coil."  But, there is also another published SP-10 schematic that shows an added phone jack output along with the phones and speaker terminals with resistors added in series in the audio output line. Unfortunately, there are no values shown for the components on the schematic. One could assume since the audio output line is marked "600 Ohm Line" that the resistors comprise somewhat of a match when used with the standard 8 ohm Z output transformer secondary. The speculation is that the audio output transformer was always an 8 ohm Z secondary and no real change ever took place. It's the same transformer with the resistors added for 600 Ohm Z and the resistors not installed for 8 ohm Z, (so far, all SP-10 receivers encountered have an audio output Z of 8 ohms.) Connections were on the rear apron of the chassis and were heavy-duty fiber-mount pin jack sockets for earphones and the phono input and screw terminals for speaker. The phono input allowed the user to access the high fidelity, high power audio amplifier. Additionally, there are separate RF, IF and AF gain controls along with a Tone control. None of the controls had any sort of calibration or reference scales.

photo above: The original Super-Pro from 1936, later designated as the SP-10

The Tuning Meter was more than just a signal strength indicator. Since it measured total IF amplifier plate current, it could also indicate when the IF amplifiers were being overloaded by too much RF signal. It was able to cope with a variety of RF and IF levels and still maintain a useable indication. Additionally, it also would function on CW which meters that functioned off of the AVC line couldn't do. The meter wasn't illuminated. The tuning dial featured a rotating dial mask that was gear driven by the bandswitch and provided a "band-in-use" tuning scales. Three variable coupled 465KC IF transformers were used in the adjustable Selectivity control. The spring-loaded plunger from each IF transformer is cam adjusted by levers mounted under the chassis. The Selectivity shaft is also spring loaded to provide a "positive feel" to the control. In addition to the front panel adjustable IF Selectivity, the Input and Output IF transformers to the detector and the Output transformer for the amplified AVC had adjustable coupling. The adjusters were knurled nuts on threaded shafts the protruded out the tops of the IF cans. The user could set the detector bandwidth for optimum selectivity for the receiver's particular service. The use of Input and Output IF transformers along with a 6B7 duplex diode-pentode tube for a second detector-IF amplifier resulted in the SP-10 actually having four IF amplifier stages and ten tuned IF circuits.

Within a few months the "S" version was announced in QST. This receiver tuned from 1.2 to 40MC, so hams had available to them a Super-Pro the would tune in all of the amateur bands from 160M to 10M. Additionally, the bandspread was changed to allow its use on all tuning ranges. This version was advertised extensively in the ham magazines during the last half of 1936.  There was also a Crystal Filter now available. The Crystal Filter mounted in the upper left part of the panel with the 465KC crystal assembly and the phasing condenser mounted to a small panel with an on-off switch and a scale for the phasing condenser. The front panel had to have a cut out to allow the Crystal Filter panel and assembly to be mounted. >>>

>>>  It is interesting that none of the advertising artwork ever shows the early Super-Pro with the Crystal Filter option installed. The power supply provided terminals for connection of the speaker field coil. The SP-10 power supply provides slightly lower B+ voltages than the later supplies but the difference is less than ten percent. An electrodynamic speaker was included in the purchase price but it was just a speaker - no housing was provided. The standard speaker was 8" in diameter but for an extra $25 a deluxe 12" speaker could be purchased.

It is apparent that engineering changes were incorporated into the SP-10 receivers through out its short nine month production. Perhaps the earliest change was to the biasing of the 2nd Detector Input amplifier grid (6B7.) This change added a resistor to the bias series resistor string by changing the 600 ohm resistor to two 300 ohm resistors which allowed the bias voltage to be slightly increased by moving the connection to the junction of the two 300 resistors. This arrangement is also found in the later SP-100 receivers. The BFO circuit was also changed to have a parallel grid capacitance rather than capacitance to ground connection. Additionally, the BFO plate load resistor was changed from a 5K resistor to a 50K resistor, though this may have been a parts list error that was corrected in the SP-100 series. The most visible change was the elimination of the Tone control, which was incorporated towards the end of the SP-10 production. Hammarlund probably believed that very few of the Super-Pro owners were using the Phono input for phonograph record reproduction and therefore the Tone control was unnecessary since for radio reproduction the Selectivity control would do about the same thing - limit the higher audio frequencies. The SP-10 series was in production for about nine months and the total number produced was probably around 500 receivers. Though the list price was $400, most discount dealers sold the SP-10 for around $250.

The SP-10 and Front-End OverloadingHammarlund stated in the SP-10 manual that it was possible to overload the front-end of the receiver by tuning in very strong, local signals and advancing the RF gain too much while in AVC.  The manual even gives an example of what to expect if the RF Gain was advanced further than necessary when receiving local signals. The overloading was especially likely if an efficient antenna system was used on the two lowest frequency bands. The fact that the RF Gain can be advanced sufficiently when receiving strong signals to cause overloading and the fact that Hammarlund does cover this in the manual seems to suggest that it was a deliberate part of the SP-10 design. Hammarlund's engineers did re-vamp the RF-IF Gain control circuit and replace the adjustable Detector and AVC transformers for the new SP-100 Series, making it seem likely that they were aware of the ease at which users could misadjust the SP-10. The engineering changes to the SP-100 eliminated the overloading while in AVC issue. Today, there is no evidence that overloading was a common "problem" experienced by SP-10 users when it was new. Over a period of time, component degradation, modifications, changes in wiring layout and mechanical assembly due to repairs will certainly affect how well an SP-10 functions today. By loosening the screws on the bottom cover of the RF Tuning Unit, one can easily induce RF overload and distortion with just a slight advance of the RF Gain control. Also, poor solder joints in the RF AVC line will accomplish the same thing due to improper AVC control of the RF amplifiers. There is no evidence that the SP-10 had overloading problems when new and to accomplish overloading required substantial misadjustment or modification of the receiver.  >>>

>>>  As an example, the SP-10 shown in the photo below once was in use at WMI. It was modified and repaired over the years and after its tenure at WMI it was further modified and repaired in a "creative" manner that changed the much original wiring layout and component placement. This SP-10 did easily overload on strong signals in AVC, so much so that the RF gain bias source was changed by a former owner to help solve the problem. However, after a complete "rebuild to original" of the receiver that included returning to the original wiring layout and the original component placement along with complete (correct) mechanical assembly, this SP-10 now performs flawlessly, exactly as the manual describes it should operate. That is, when tuned to a strong local AM BC station (10KW 15 miles away,) using an efficient BC band antenna (outside 50ft end fed wire,) with the RF Gain at maximum in AVC, some distortion is apparent. However, by reducing the RF Gain to about 50% the overloading is eliminated and so is the distortion (just as the manual states.) When the SP-10 is connected to my 135' tuned dipole, I can tune in the same AM BC station and have the RF Gain at maximum with no distortion at all. This is due to the inefficiency of  the tuned dipole at AM BC frequencies reducing the level of the input signal, thus the RF Gain can be substantially higher. This is exactly how the manual describes the operation of the SP-10. This should establish that the SP-10 was designed to be a highly adjustable receiver, able to cope with many different types and levels of signals. That it might be misadjusted was assumed by Hammarlund engineers and those conditions were described in detail in the SP-10 manual. 


Shown above is a 1936 SP-10 sn: 576. This Super-Pro was used at WMI, a ship-to-shore station located in Lorain, Ohio on the shores of Lake Erie. This receiver has been completely rebuilt and restored to original "as delivered to WMI" in 1936. Its terrific performance has tremendous audio range and incredible sensitivity. The variable selectivity is totally rebuilt and functions great.

 Photo above: WMI operating room in 1937. Note the three SP-10 'Super-Pro' receivers in the racks. SN:576 is certainly one of the receivers in the racks.  Photo from: Inland Marine Radio History Archive  -  here is their URL:


The 100 Series "Super-Pro"   1937-1939

photo above: Introduction advertising for the new SP-100 Series - QST, January 1937

The new "100 Series" Super-Pro was introduced in January 1937. It upgraded the receiver's front-end by changing the large glass tubes there to metal octal tubes. The two RF amplifiers were now 6K7 types. The mixer was a 6L7 and the local oscillator was a 6J7. Also, the audio section was changed over to metal octals with a 6C5 1st AF amplifier with a triode connected 6F6 driver and push-push, triode connected 6F6s for the output. In all, eight tubes were changed over to metal octals while the IF amplifiers remained 6D6s, the 2nd Detector and AVC amplifier remained 6B7s and the BFO remained a 6C6. The power supply B+ rectifier remained a 5Z3 and the bias rectifier was changed to a type 80.

Other changes were elimination of the separate RF and IF gain controls. The RF and IF stages' bias lines were combined to use a single manual control labeled "Sensitivity."  The Sensitivity control could also adjust the AVC bias line more negative than the AVC control bias and therefore reduce the overall gain while still maintaining AVC control above that point. The variable coupled Detector and AVC transformers were replaced with fixed-coupling units. Since there were now no moving parts in these transformers, the component boards that had been mounted under the chassis of the SP-10 were relocated inside the Detector Output transformer can and the Amplified AVC Output transformer can. The elimination of the separate RF and IF gain controls and the variable-coupled transformers for the Detector and AVC were probably due to the ease at which the SP-10 could be misadjusted, resulting in overloading and distortion. The Selectivity control, as it was called on the SP-10, was renamed as a Band Width control. The Tone control was also dropped because the user could limit the audio highs by reducing the IF bandwidth. Actually, it is likely that the Tone control was eliminated on the later production of the SP-10.

The BFO and Bandwidth controls were given front panel calibrated scales. Even the Sensitivity control and the Audio Gain were given reference scales. Internal changes also included different AF transformers that were vertical mount, frame-type (non-potted) units with the AF Output transformer having a single 8 ohm output Z. These were new style audio transformers and not the same style potted 8 ohm Z transformers that were installed in the late SP-10 production. Since a switch was provided for "Speaker" or "Phones" the earphone Z is rather low. A 600 ohm line audio was not provided in the SP-100 series except for perhaps the very late versions of the SP-100LX version. The standard speaker size was changed to 10" with the introduction of the SP-100. The AVC also could be used for CW as a large time constant capacitor (.25uf) is switched into the AVC line when the receiver is switched to CW.  Of course, the receiver could also be used in "Manual" control for maximum sensitivity in the CW mode.

The SP-100 series still used the "Tuning Meter" that measured total IF amplifier plate current resulting in a "backwards" operation of the meter. The 0 to 5 scale on the meter actually has ten divisions for each numeral - 50 divisions total. The push-pull audio remained at the same power capabilities of 14 watts and a Phono input was still provided as a rear connection. A "Remote Relay" operation of the B+ was added to the rear chassis. The input is via pin terminals and the function parallels the operation of the "Send-Receive" switch. When the Crystal Filter option was installed in the SP-100, the first IF transformer was changed from a variable-coupled unit to a fixed coupled transformer with leads brought out the front for connection to the crystal holder mounting and the phasing condenser. Whether this was the case with the earlier SP-10 is not known as most examples of that receiver don't have the Crystal Filter installed.

The Super-Pro's antenna input Z is around 115 ohms. This was approximately the impedance of the twisted pair antenna feed lines that were popular for dipole antennas. Since the Super-Pro doesn't have an antenna trimmer control, Hammarlund expected the user to provide a matched antenna. Most professional users and most knowledgeable hams provided matched antennas but shortwave listeners sometimes used end fed wires or inverted "L" antennas with no matching device. The normally high impedance of these antennas didn't match the Super-Pro input Z and the common result was higher noise level along with weaker signal strength.

The cabinet for the early Super-Pro and the SP-100 is really nothing more than a dust cover that is secured with eight knurled thumb nuts on the front and three thumb screws on the rear. There is a metal identification tag mounted on the back of the dust cover with places for the receiver type and serial number to be stamped however most of the time these are stamped with some kind of factory identification - usually two letters. Serial numbers usually do match on the dust cover and the chassis. The bottom cover has small metal-cup, felt-center feet mounted in each corner. This gave the user a table top cabinet.  The panel is 18" across on the table top models. A 19" rack-mount model was available and, with the Crystal Filter option included, the list price was nearly $450. The front panel on early, pre-WWII Super-Pro receivers is made of  .187" aluminum. This was black wrinkle finished first, then engraved. This resulted in the nomenclature looking bright silver.

The model designators used with the SP-100 Series were as follows: SP-100 covered .54 to 20mc and had no crystal filter, SP-100L covered 100kc to 400kc and 2.5 to 20mc, SP-100S covered 1.25 to 40mc. If the crystal filter was added to any version then an "X" suffix was added. Hammarlund's ordering department needed to specify which size speaker was to be included with each receiver package ordered.

Photo above: Article-ad from POPULAR MECHANICS  12-37 promoting the Super-Pro SP-100SX for 10 meter coverage. This article shows that the speaker included was just that - a speaker - no cabinet was included. Note also that this article never refers to the receiver by its model number - it is only a "Short-Wave Super De Luxe."

To this end, when ordering a Super Pro, "SP-110" indicated that the 10" speaker was to be included and "SP-120" indicated that the 12" was to be included. Even though the speaker size that was included as part of the entire receiver package was referenced by the numeral modification, that model number is never stamped into the metal tag on the back of the receiver dust cover. Hammarlund advertising always referred to the receiver as the "Super-Pro 100 Series" or the "SP-100 Series."

Rider's Perpetual Troubleshooter's Manual mentions the SP-100LX version and indicates that it substituted the 2.5 to 5.0mc band with a 150 to 300kc band. This is in direct conflict with the Hammarlund SP-110LX manual (that is available on the BAMA site.) That manual indicates the coverage is 100-200KC and 200-400KC and 2.5-20MC in the remaining three bands. This is the standard "L" coverage, even for the later SP-200 Series. However, even some of the SP-200 advertising also mentions the 150KC-300KC tuning range. There may have been some special orders with that frequency coverage but standard production provided 100kc to 400kc.

Another interesting "L" feature, that is not found on most "LX" receivers but is shown in the BAMA site manual, is dual secondary windings of the output transformer and a front panel "Phone" jack for the phones. This became the standard for the later SP-200 models but perhaps is only found on the late version SP-100LX models or maybe "special order" receivers. From actual examination of SP-100LX SN:2730, this receiver has no front panel phone jack but has the standard Phones-Speaker switch with separate outputs on the rear chassis. Phones out is for pin jack connection while the Speaker out is screw terminals. The nominal output impedance appears to be around 8 Z ohms for both Phones and Speaker, which is standard for the SP-100X receivers. (See photo of SP-100LX serial number 2730 in the Collector's Gallery further down this page.)

Until more SP-100LX information becomes available one can only speculate on what was "standard" for the SP-100LX receivers. This is what appears to be standard. The SP-100LX tunes from 100kc to 400kc in the first two bands and from 2.5mc to 20mc in the highest three bands. Audio output impedance is 8.0 Z ohms for both Phones and Speaker. A toggle switch selects either Phones or Speaker for the audio output. There is no Phone jack on the front panel.

Photo above: 1937 SP-100X Super-Pro (SN: 3387) - This receiver originally belonged to Pacific Gas & Electric Company in California. Since it was owned and maintained by PG & E, it was never modified and was only repaired as necessary. See the section further down this page about our authentic restoration of this great example of the SP-100X.

There is another interesting SP-100 variation that has been reported by SMØAOM. This receiver is known as the MRM-5, or Markradiokottagare 5, and it was sold only to government customers in Sweden. The receiver is an SP-100 but the frequency coverage is 200kc to 400kc and .54mc to 10mc. The receivers were available in either rack mount or table top configurations. SMØAOM has two examples, sn 1352 and a partial sn 16xx. The Swedish translation manual is dated October 1937.

The SP-100 series was in production for about two years and nine months. Though the list price was over $400, most dealers sold the SP-110X (ten inch speaker included) for around $250. Total number of SP-100 Super-Pros produced was probably around 1000 receivers.


The SP-150   1938


Beginning in July 1938, Hammarlund offered a customized SP-100 receiver in a floor standing console cabinet. The cabinet included doors that closed over the receiver panel and a 15" speaker with a bass reflex port. The receiver panel was faux finished walnut and had gold tone nomenclature along with brown knobs. The meter housing and dial escutcheons were trimmed gold and the dial masks were amber color instead of black. To assure that the receiver would appeal more to the shortwave listeners and home radio users, Hammarlund eliminated the front-panel control of the BFO, making it an internally set adjustment. The Crystal Filter option was not installed either. Otherwise, behind the panel the SP-150 receiver was a standard SP-100 series set.

What must have sounded like a great idea to Hammarlund, considering the success of Scott and McMurdo Silver high performance receivers, turned out to be a sales flop. The SP-150 was a superb receiver that was really never appreciated by very many people in the radio buying public. It just wouldn't do for hams - it had no BFO on the front and it was too big. It didn't have the Scott or McMurdo chrome chassis - how could you impress your friends? The rather plain cabinet couldn't compare to the flamboyant creations from Scott or McMurdo. As a high priced console, the SP-150 couldn't find many admirers even though it performed as well as any Scott or McMurdo and had a more accurate dial readout. Only about 70 were ever built, making it one of the most difficult Super-Pro receivers to find.

Shown in the color photos is the SP-150 owned by AA6S, Bill Jungswirth. The cabinet has been refinished and the replacement grille cloth is not the original style or color. The receiver has been totally rebuilt and performs very well. The close-up of the front panel shows the faux walnut paint job of the front panel along with the gold nomenclature and the amber dial masks. A spectacular console that performs as well as any Scott or McMurdo.




The 200 Series "Super-Pro"   1939-1945

Introduced in October 1939, the "200 Series" thoroughly modernized the Super-Pro to 1940 receiver design concepts along with a reduction in the build-cost of the receiver. The SP-200 is a more conventional approach to superheterodyne design, especially in the IF section of the receiver. The SP-200 used three IF amplifiers with standard 465KC IF transformers (except for the two variable coupled units for Band Width control) and the 6B7 duplex diode-pentode detector-IF amplifier of the proceeding Super-Pro receivers was replaced with a standard duplex diode 6H6 tube. The AVC circuit was also  simplified using capacitive coupling rather than an inductive pickup and reducing the transformers to one unit. All of these changes were a cost-to-performance decision resulting in a receiver than performed as well as its predecessors but was much less expensive to build. The list price reflected these changes by a reduction to $315.

The circuit now used all octal tubes replacing the glass tubes of its predecessor with metal tubes. Tube lineup was RF-(2)6K7, MIX-6L7, LO-6J7, 1IF-6K7, IF-(2)6SK7, DET-6H6, NL-6N7, AVC AMP-6SK7, AVC REC-6H6, BFO-6SJ7, 1AF-6C5, 1AF&P-PAF-(3)6F6, B+ REC-5Z3 and Bias REC-80 ... 18 tubes in all. Additionally, the "200 Series" did away with the IF current "Tuning Meter." Now the S-meter was illuminated and had a 0 to 9 scale. The meter operated from the AVC line so it worked normally instead of "backwards." Undoubtedly, the old tuning meter's complicated method using the "delta" of signal versus noise calculation to give the "db over the noise level" had almost everyone confused. Complaints must have been common and ultimately prompted the change to a standard S-meter. A new Noise Limiter was incorporated into the circuit. It was a "clipper" type noise limiter. The Crystal Filter assembly was now entirely installed inside the first IF transformer. Also, instead of only a Phasing adjustment, a five-step Selectivity control was added. Now, for the first time, the Super-Pro was installed inside a real metal cabinet with hinged lid and carrying handles and the front panels were standard 19" rack width on all models . The cabinet will have a metal ID tag mounted on the back and sometimes a model type will be stamped, or perhaps a serial number, but many times these tags were never stamped at all. If the model type is stamped in, it usually is "200" and a letter suffix to further identify the receiver, e.g., "200X."

The "200 Series" evolved slowly. The first evidence of change is in the SP-100 front panels no longer using the large square cut-out for the Crystal Filter panel. Instead, the Crystal Filter assembly is mounted directly to the front panel with just holes for the controls.  Next, were the four toggle switches for AVC/Manual, Speaker/Phones, Send/Receive and Mod/CW. Some very early "200 Series" may have all four switches, but quickly the Send/Receive and Mod/CW switches are changed to rotary switches with knobs and the position of the Send/Receive switch is changed. Finally, the Speaker/Phones switch is replaced with an earphone jack. With the earphone jack change, the audio output transformer also changed to a dual secondary outputs and sometime later both audio transformers became potted units (as the original SP-10 was.) The dual audio outputs provided 600 ohm Z line audio to drive a matched speaker and a separate Hi-Z winding to drive earphones or other Hi-Z device. The dual secondary windings and the front panel "Phone" jack features were also found on the SP-100L versions but, for some reason, not on the first SP-200 receivers. Though the layout of the receiver is very similar to the SP-100, the underside of the chassis uses different component mountings and the shield between the RF and IF sections was removed.

The three versions of the SP-200 are the SP-200X, tuning from .54 to 20.0MC. The SP-200SX, tuning from 1.2 to 40MC. The SP-200LX, tuning from 100 to 400KC in two bands and 2.5 to 20.0MC in three bands. Like the SP-100 series, the Hammarlund sales literature uses SP-210 if the version ordered came with the standard 10" speaker and SP-220 if the version came with the deluxe 12" speaker. Special "universal" power supplies could be ordered for operation of 115vac or 230vac. Sales literature also sometimes indicates that the LX version covered 150 to 300KC substituted for the 2.5 to 5.0MC band. Interestingly, Rider's also lists this coverage for the SP-100L version. Most likely these are both errors. The earliest known LX (at present,) SN:8423, tunes 100-400KC and 2.5-20.0MC or the standard coverage for the LX.  An error in the 1942 sales literature indicates that all versions had bandspread on all five tuning ranges. Actually, this only applied to the SX version, the X and LX bandspread is only on the three highest tuning ranges. Earlier advertising had phrased the statement as "bandspread throughout the entire high frequency range of the receiver" which is technically accurate.

photo above: The Series 200 Super-Pro as pictured in the 1941 Hammarlund Catalog. This artwork shows the Speaker-Phones switch found on early versions.


photo above: An artwork image of the SP-200 chassis from the 1941 Hammarlund Catalog showing the early version frame type audio transformers. These were later replaced with potted transformers.


The photo to the left-top shows the 1939 Series 200 Super-Pro SP-200SX sn: 6230, covering 1.2 to 40MC. This is the early version with frame-type audio transformers and using the toggle switch for selecting either "phones" or "speaker." The front panel is .187" thick aluminum that is black wrinkle finished and then engraved to have the nomenclature appear bright silver. Note on the S-meter - though this meter appears to be similar to the S-meter shown in the artwork images of the early SP-200, there is considerable doubt that this is an original meter. It has obviously been rebuilt in the past using parts from two different meters. It is more likely that the full glass front style was standard for production until the later military contracts. Interestingly, the metal tag on the back of this receiver cabinet is stamped "200SX" as the model number. 

Just before WWII started, the aluminum front panel was changed to a .125" thick steel front panel. When the front panel became a steel fabrication, the paint used changed to a smooth-finish, semi-gloss black. Additionally, the nomenclature was stamped into the panel and then filled with white paint. Most of the steel panels are copper-nickel plated on the front side only - to prevent corrosion and subsequent problems with the paint. All Super-Pro panels, from SP-10 to SP-200, are painted on the front side only. The back of the panel is left unpainted.

The photo left-lower shows the later version of the 200 Series (SP-200X sn: 9419) with the smooth finish, semi-gloss black steel panel. Also, the standard S-meter housing with full glass front is shown on this receiver. Note that the toggle switch for "Speaker-Phones" is replaced with a single .25" phone jack indicating that the dual output audio transformer is installed. Since the new steel panels were .125" thick steel, spacers were installed on the various mountings to compensate for the different thickness of the panel. It seems likely that all of the civilian Series 200 Super-Pro receivers probably had black panels. The tag on the back of this receiver's cabinet was left un-stamped which is common with many Super-Pros.

During WWII, government agencies like the FBI used the 200 Series Super-Pro at their listening posts and most likely these receivers were the civilian models. When the demand increased during WWII, some variations in the front panel paint color appear, especially those receivers that were destined for the Signal Corps. Grays, Green-Gray and Blue-Gray are the most commonly seen colors. Civilian Super-Pro production is difficult to estimate because found examples indicate that sales of these versions continued though the early part of WWII - at least to 1943. SN:16135 is a civilian X model built in mid-1943. It is likely that only a very small percentage of the wartime production were destined for non-military use and then it is likely that certain government agencies were the only users. SP-200 non-military production is probably around 1500 to 2500 receivers. Confusing this production number is the fact that the military did purchase communications receivers early in WWII for use in the war effort. These receivers are sometimes re-tagged and will have Signal Corps stamps. Hammarlund serial numbers will identify when the particular SP-200 was built and if the tags or stamps post-date the serial number then the receiver was probably "drafted" into military service.

It is very common to find some variations in the SP-200 Series receivers but whether this is due to production stock variations, special orders or subsequent repairs using later stock parts can't be verified accurately without a physical inspection of the receiver to determine the originality of the parts or assembly in question. 


The Military "Super-Pro"   1942-1945

The Super-Pro had been selling to the military on a small scale but with the start of WWII, the volume of Super-Pros going to the military increased significantly. The military versions are normally identified by their US Army Signal Corps designations. The "X" was known as the BC-1004, while the "LX" was the BC-779. The least seen version is the "SX" which was known as the BC-794. There are other military designations that usually pertain to specific changes made to these standard models - usually frequency coverage differed from the standard models. However, sometimes the end user also would require special designations. In particular, Allied users may have required special designations. There are usually suffixes that designate which power supply came with the receiver. The WWII Super-Pros were sometimes built by contractor Howard Radio Company, though the only non-Hammarlund Super-Pro versions seen are the BC-779 and power supply. Under the chassis the changes to the circuit mostly involve component types. Seventeen of the individual paper-wax capacitors are replaced with metal container units - the so-called "bathtub" capacitors that usually contain multiple capacitors inside the housing. It is likely that the 1941 and later civilian models also had the "bathtub" caps installed. These "bathtubs" are mounted on the inside wall of the chassis in the RF/IF area. Additionally, there are a few tubular oil-filled capacitors used. The chassis may be MFP'd depending on its service.

The standard "BC" receiver was a rack mount unit with a dust cover that was secured by eight thumb nuts on the front and three thumb screws on the rear. Dust covers are painted black wrinkle finish. Though the majority of "BC" receivers are rack mounted units, sometimes the receiver will be installed in the military CH-104-A cabinet. This cabinet is similar to the SP-200 civilian cabinet except there are no decorative chrome strips installed. When the receiver is installed in the CH-104-A cabinet, the dust cover holes in the front panel are usually filled with screws and nuts. CH-104-A are found in either black or gray wrinkle finish. Shown to the right is a BC-1004 installed in a CH-104 cabinet.  >>>

photo above: ASP-1004 Receiver - 1945 version of the BC-1004

>>>  There weren't very many changes required by the military, so WWII Super-Pro receivers are basically the same as the civilian versions, under the chassis. Front panels are different on the WWII Super-Pro, especially late in the war. Many of the panels were painted various shades of gray. Some of the later 200 Series receivers will have a greenish-gray paint on the front panel of the receiver and power supply. There are numerous variations in the color, with some models tending to be more blue-gray. Certainly the highest production quantities for all of the pre-war and wartime Super-Pro receivers are the military versions with total production of several thousand units of each type. Total production was probably around 7500 to 9000 receivers. Signal Corps TM-11-866 covers the most commonly seen versions of the military Super-Pro.

Shown in the photo upper right is one of the later WWII Super Pro receivers with the greenish-gray front panel paint. This one is designated as "ASP-1004." Many of the WWII Super-Pro receivers were used by our Allies. I suspect that the ASP-1004  SN:27942 with matching power supply ASP-84B SN:28870 were destined for Allied service but never shipped. Does the prefix "ASP" = Allied Super Pro? Interestingly, under the tag, the panel is ink stamped "BC-1004" but there are no Signal Corps stamps anywhere on the receiver.

photo left: The Manila, Philippines radio communications link to Tokyo. Photo is dated August 20, 1945. Note the BC-221 Frequency Meter. This unit provided an accurate way to set the Super Pro to the proper frequency. Judging by where the dial mask is, the "TOKYO" receiver is set on the 10-20MC band. Power supplies are on top of the receivers and four different receivers are visible in the shot.  photo from C. Cusick, KØDWC collection




photo right: The intercept station at the Yakutat Air Base in Alaska during WWII. Three SP-200 receivers are in use and one SP-100 receiver (with the black panel.) Judging by where the dial masks are, the two left side receivers are set to the same band while the right side receivers are set to different bands.  Note the unusual rack mounted speaker panels at the top of the racks. photo from Fred Bryan,

Allied-Built Versions of the Super Pro SP-200


Some countries built their own military versions of the Super Pro. In particular, the Australian company, Eclipse Radio, built the AMR-200 which is nearly a perfect copy of the SP-200.

The Russians produced the KV-M which is built like a Super Pro but without the bandspread dial. The chassis layout and especially the RF box are very close copies of the SP-200 Super Pro. Shown in the photo to the right is a KV-M receiver. This one has an addition of  a small lamp that is mounted to the front panel.

See references at the bottom of this webpage for links showing photos of the AMR-200 and other photos of the Russian KV-M receivers. Thanks to Steve Bringhurst for the AMR-200 and KV-M information.


Post-WWII Use of the WWII Military Super-Pro Receivers

R-270/FRR - Component Receiver for AN/FRR-12

 Customized BC-794 by:

The R-270/FRR receiver started out as a standard WWII BC-794 receiver. It's likely that some of the military Super Pros were never shipped or, if they were, were never put into service by the end of the war. After WWII, the Signal Corps was interested in developing RTTY communications to the point where it was virtually "error-free." Diversity reception assured that the incoming signal didn't fade thus providing the RTTY TU with a fairly constant level input signal (dual diversity can reduce fading by up to 90%.) The R-270/FRR is one of the component receivers of the AN/FRR-12 Dual Diversity Receiver built by Wickes Engineering & Construction utilizing customized-rebuilt Hammarlund BC-794 receivers. While dual diversity practically assured that the received signal remained constant, it didn't help with receiver frequency drift due to LO and BFO instability (which the Super Pro was famous for.) Wickes incorporated a selectable Crystal Oscillator that could be switched in to replace the LO. Of course, the receiver had to be also manually tuned to the correct operating frequency so the RF and Mixer stages would be "in-tune." The operator could also tune in the Crystal Oscillator harmonics with the Main Tuning which would then have the correct RF and Mixer tuning for harmonic operation. To prevent drift from the BFO it's operating frequency was also crystal controlled. The output from the two R-270/FRR receivers was taken from the IF output (provided by Wickes' modifications.) This was routed to a Diversity RTTY Demodulator, the CV-31A/TRA-7 that accomplished the diversity combining via its internal circuitry. Once set-up, the AN/FFR-12 could provide virtually "error-free" RTTY.

The R-270/FRR was rebuilt and customized in 1948 by Wickes Engineering & Construction Company for the U.S. Army Signal Corps. The contract date on the Signal Corps data plate on the front panel tag is 1947 (and the manufacturer is shown as "Wickes ENGINEERING & CONSTRUCTION CO.") but the MFP coating is date-stamped "OCT 1948." The custom modifications consist of adding a three channel crystal oscillator that can be selected from the front panel via a switch that provides Crystal Oscillator OFF/ON function plus the channel selection. Also, on the small Crystal Oscillator panel, which is located above the main tuning dial escutcheon, is a vernier frequency control. The Crystal Oscillator modification is the same as Signal Corps "Improvement Kit" MC-531 which is shown in TM11-866. MC-531 is the forerunner to the JX option in the SP-600 receiver and functions in a similar manner. If the Crystal Oscillator is ON, its plate voltage is derived from the B+ that is switched from the LO plate, thus disabling the LO. When the Crystal Oscillator is OFF, then the B+ is routed to the LO plate.   >>>

>>>  You must have a crystal installed that is at +465kc from your desired receive frequency and then you also have to manually tune the receiver to the desired receive frequency, otherwise the RF stages and the Mixer will not be in tune (which is exactly how the SP-600JX works.) The actual crystal frequency used will depend on the desired receive frequency (band used) and whether or not the crystal is going to be operated on the fundamental or on an overtone. 

Additional Wickes' custom modifications were to replace the AVC rectifier tube (6H6) with a 6SN7 dual triode. This provided the AVC rectification and also provided a triode buffer output for the IF OUTPUT. Also, the AVC Amplifier tube was changed from a grid cap 6K7 to a single-ended 6SK7 and the BFO tube was changed from a grid cap 6J7 tube to a single-end tube, the 6SJ7. Additionally, the second and third IF amplifier tubes where changed from grid cap 6K7 tubes to single-ended 6SK7 tubes.

Further changes included a new front panel made out of aluminum with raised lettering (rather than the Hammarlund engraved or stamped lettering.) Also, the standard steel bottom panel was replaced with an aluminum bottom cover that had the receiver schematic applied to the inside. IF transformer cans were replaced if the associated tube was changed to a single-ended tube.

The AN/FRR-12 manual (TM11-896) indicates that the receivers could also be used for diversity CW or AM. If CW was going to be used then the BFO crystal (462.45kc) had to be removed and the BFO "re-aligned" to function as a standard BFO. The user also had to "short out" the crystal socket with a jumper wire. Like the SP-600, the R-270 was modified by Wickes to use a "twin-ax" connector for the antenna input. Additionally, a CT was provided on the audio output transformer (labeled as "600-0-600" - although this nomenclature actually indicates that the 600 Z ohm output is across the entire winding, not each side of CT.) The entire chassis and shields were giving an extra heavy coating of MFP, which is typical of later, post-WWII Signal Corps equipment.

The power supply provided was the R-74A "Heavy Duty" military style. This was the later version with the 5U4 and 5Y3 rectifiers. The interconnecting power cable was also heavy duty and rubber jacket insulated. The power supply is also heavily MFP'd and date stamped DEC. 1948. The power supply bottom cover has the schematic on the inside of the cover.

The Wickes' R-270/FRR is certainly more than just a "modified" Super Pro receiver. I would call it "customized and rebuilt" for the Signal Corps requirements for drift-free and fading-proof RTTY reception. The quality of workmanship, both mechanical and electronic, is superior and what would be expected from a professional engineering company. If one didn't know how the standard Hammarlund WWII Super Pro receivers were built, the Wickes' "Custom Super Pro" could easily be mistaken for Hammarlund factory workmanship. Performance is identical to the Hammarlund receivers with the exception that several optional outputs and frequency control circuits are available. Audio is still excellent, sensitivity quite good and the variable IF coupling provides excellent control of the bandwidth.


Super-Pro Power Supplies and the Power Cable

The power supply for the Super-Pro provides the several different voltages required to operate the receiver. There are three levels of B+, +385vdc, +270vdc and +140vdc. The tube heater supply is 6.3vac and the C bias is -50vdc. Power is connected to the receiver via a four foot cable that has a ten spade lug connector that screws to the ten pin terminal strips on both the power supply and the rear of the receiver chassis. Usually the protective covers that mount over the terminal strips are missing. The SP-10 power supply used a type 1-V rectifier tube for the bias supply. This was replaced in the SP-100 supply with a type 80 rectifier tube. Early power supplies will have a second terminal strip with two connections marked "Field" for the electrodynamic speaker used with the SP-10 and SP-100 receivers. With some late SP-100s and all of the SP-200s, a choke was included in the power supply to replace the field coil of the electrodynamic speaker and allow the use of a PM speaker. The standard power supply had a metal box cover with louvers that protected the tubes. The power supplies were usually placed on the floor near the receiver. Rack mount power supplies have a 19" panel screw mounted to the front of the chassis and a metal cover over the top of the chassis with lugs the protrude out the front panel for the cap nuts. Bottom covers were standard on all types of power supplies. Metal cup, felt center feet are usually mounted at the corners of the bottom cover for non-rack units.

SP-100 PS SN:3388 - Note the two terminals for the field coil of the electrodynamic speaker

photo above: SP-200 PS Rack Mount - Note that the terminals for the speaker field coil have been eliminated

photo above: ASP-84B PS Military Rack Mount

Most of the military Super-Pro power supplies will have an oversize potted power transformer and potted chokes with oil-filled filter capacitors. Some of the late-version military supplies went to can-type oil-filled paper capacitors. About this time the rectifiers were changed to a 5U4 and a 5Y3. All of the military supplies were normally expected to operate in continuous duty service. Some of the military power supplies will have dual primary power transformers for 115vac or 230vac operation. Military power supplies for the Signal Corps are usually identified as RA-74, RA-84 or RA-94. RA-84 is for 115vac operation only. The RA-74 is a heavy duty, mulitple primary power supply and the RA-94 is the heavy duty, dual primary power supply.

Fortunately, almost any of the Super-Pro power supplies can be used with almost any Super-Pro receiver. Considerations would have to be made for the field coil connection on early supplies but the voltage output terminals and the voltages present are the nearly the same for all power supplies and receivers. The exception is the SP-10 which requires a lower B+ because of the type 42 output tubes however the voltage difference is less than 10% and operation of the SP-10 on a later supply can be easily accomplished (for testing purposes) by moving the AC lead to the 125vac tap under the power supply chassis. This will lower the output voltage slightly and allow safe operation for testing of the SP-10 with a later power supply. If long term operation is required using a later PS with an SP-10 receiver, then a 220 ohm 5 watt resistor should be added inside the power supply between the +385vdc B+ line and the output terminal. This way the PS primary tap can be on the proper AC voltage which allows the heater voltage at the receiver to be correct and also the +385vdc will be reduced to about +365vdc at the 42s in the SP-10 receiver. The advantage of using a later power supply with either the SP-10 or SP-100 is the elimination of the electrodynamic speaker. These earlier supplies utilized the field coil of the speaker as a filter choke but later power supplies already have a choke installed so a permanent magnet speaker can be used.

Unfortunately, since just about any Super-Pro power supply can be used with just about any Super-Pro receiver, very few matched set-ups are found today - that is the sequentially serialized receiver/power supply. With the SP-10 and SP-100 receivers, the original power supply serial number is one off  from the receiver serial number. The assignment of a sequentially serialized power supply to a specific receiver ended by the time the SP-200 was introduced in 1939.

The Super-Pro power cables normally encountered are usually a nine wire, cloth covered cable with a special, spade-lug type terminal strip connector on each end. These cables were used on the later SP-200s and the military receivers. Some military power cables are shielded and have rubber jacket insulation. Later nine wire SP-200 cables have tube heater wires that are of a larger gauge wire than the remaining seven wires. The tenth lug is not used on these terminal strips. On early cables, there are usually nine wires that are all the same gauge. If a tenth wire is present, it is "switched to ground (chassis) connection" that operates with the ON-OFF switch. It is not used in the circuit as supplied but was provided as a spare function for possible user applications. The switched ground operation is on most Super-Pro receivers but the an actual wire in the cable is seldom present unless added by a former user. Many Super-Pros will be missing their original power cable but the cables are easy to make. Just observe that the tube heater connections, pins 1 and 2, use about 12 to14 gauge wires while the remaining wires can be 16 to 18 gauge. The cable length is approximately five feet. The connections to the power supply match the receiver, 1 to 1, 2 to 2, etc. It isn't necessary to have the terminal strip connectors but they do make connecting and disconnecting the receiver and power supply much easier. The correct orientation for an original power cable is to have the spade lugs pointing down and the cable to exit the protective cover on the right side. 

photo right: The power cable from a military SP-200 Series Super-Pro receiver-power supply showing the unique spade-lug type connector strip


Hammarlund Super-Pro Serial Numbers and Estimated Production

From examining many pre-WWII Super-Pro receivers along with several HQ-120X receivers and Comet-Pro receivers, it appears that Hammarlund issued serial numbers in a new sequence starting with "1" (or some low number) beginning with the first SP-10 receiver and continuing in sequence right through the military versions built during WWII. Since the earlier Comet Pro receivers all have much higher serial numbers and were all built before any Super-Pro receivers, they must have been issued numbers from a different, earlier sequence. For example, a 1933 Comet Pro receiver with the SN: 7822 and a 1936 SP-10 with SN:576. It is likely that all Hammarlund products that were going to be issued a serial number were issued numbers from a common serial number roster. Therefore all Super-Pro receivers were serialized sequentially as they came off the line. It also appears that when the numbers were issued to a Super-Pro "set," that receiver's power supply was issued the next sequence number after the receiver's serial number. This is the case in the two early matched sets that I know of. One is an SP-10, sn 720 and PS sn 721. The second receiver, an SP-100X, sn 3387 and PS sn  3388. If this was the normal Hammarlund practice, then each Super-Pro "set" accounted for will use two numbers from the sequence - one for the receiver and one for the power supply. There seems to be some SP-10 and SP-100 sets that are "matched units" but are not sequential serial numbers. The numbers are very close but not sequential. It was probably standard procedure that a Super-Pro ordered from the factory would be sent out with its sequentially numbered power supply. However, if one of the many dealers got in a shipment of Super-Pro receivers and power supplies, he might not be so careful to assure that a "matched" set be provided to the purchaser. The matching of receiver and power supply serial numbers accounting for the use of two sequential numbers from the roster would hold true up to the time that the HQ-120X was introduced in late 1938. It appears that the HQ-120X used the same serial number roster as the Super-Pro "sets." After late 1938, the serial number roster is split-up by Super-Pro receivers, SP Power Supplies and HQ-120X receivers. By the time Hammarlund introduced the SP-200, the power supply serial numbers were no longer sequentially assigned. When Hammarlund starting to supply Super-Pros for the war effort (probably to expedite production) no effort was taken to assure that certain power supplies went with specific receivers. It is probable that specific blocks of numbers from the roster were used for receivers and a sequential block used for power supplies. This seems to be confirmed by Super-Pro ASP-1004 SN: 27942 with matching power supply ASP-84B SN: 28870. A second ASP-84 serial number reported seems to be too early at 9686 but it may imply that the ASP SNs are a specific roster of numbers. More ASP data will show whether this is the case. Speakers seem to have an stamped-ink number but it appears that it may have been identification numbers as sometimes different speakers are found with the same numbers stamped inside. For now, we will assume that speakers were not part of the serial number roster.

This use of sequential serial numbers issued to all receiver products as they left the line makes estimating production quantities very difficult. One has to take into account that lower priced receivers, like the HQ-120X, sold in larger quantities than the Super-Pro. Also, that the HQ-120X didn't appear until late 1938 and therefore didn't affect the 1936 to 1938 Super-Pro serial numbers. Additionally, the power supply serial number will divide the serial number total by two for production totals for 1936 through most of 1938. After the HQ-120X appears, a percentage must be factored into the serial number total to extrapolate the Super-Pro production.

Without doubt, the SP-10 is a rare receiver. Very few are ever encountered and from that one naturally concludes that production was very low. It is likely that no more than 500 SP-10 receivers were built since it was only in production less than nine months. With the SP-100, production goes from January 1937 up to about August 1939, or about two years and eight months. Again though, the SP-100 is rarely encountered. Production should be around 1000 receivers because the HQ-120X has to be factored into the serial number use for about the last year of SP-100 production. The SP-150 is so rare with only 70 produced it is included with the SP-100 group. The pre-war SP-200 is not a common receiver either. Probably only 1800 Super-Pro SP-200 receivers were built before WWII began. With the WWII production, the now commonly seen Super-Pro receivers are found. Production here was around 10,000 receivers, perhaps more. Total Super-Pro production from 1936 up to 1945 should be around 14,000 receivers. Compare this to the SX-28A production of around 11,000 receivers (not including SX-28 production) and a correlation can be seen as to how often any of the pre-war and wartime Super-Pro receivers are seen versus how often an SX-28A is encountered. The comparison seems valid.

One note,...the serial number on the military tag of all BC Super-Pro receivers will not match the Hammarlund factory serial number stamped on the rear of the chassis. The tag number was usually assigned by the Signal Corps and has no correlation to the Hammarlund issued serial number.

The following tables are rough estimates based on serial numbers encountered weighed against quantity of receivers seen and the assumed division of the serial number pool.


      Serial Number Sequence with Estimated Production

           Model                     Serial Numbers            Quantity Built

   SP-10 Series....................1 up to ~1000........actual receivers ~ 500

   SP-100 Series..................1000 to ~4000.......actual receivers ~1000 

   SP-200 Series (pre-war)..4000 to ~10000.....actual receivers ~1800

   SP-200 Military..............~10000 to 30000+....actual receivers ~ 10000+



Year to Serial Number Table

         SN Range        Year           Models

         1 to 1000 ........... 1936.......SP-10, PS

         1000 to 3000.......1937.......SP-100, PS

         3000 to 3800.......1938.......SP-100, PS, HQ-120X

         3800 to 5000.......1939.......SP-100, SP-200, PS, HQ-120X

        5000 to 8500........1940.......SP-200, PS, HQ-120X

        8500 to 10000......1941.......SP-200, PS, HQ-120X

      10000 to 14500......1942.......SP-200, BC series, PS, Mil HQ-120

      14500 to 19000......1943.......SP-200, BC series, PS, Mil HQ-120

      19000 to 25500......1944.......SP-200, BC series, PS, Mil HQ-120

      25500 to 30000+....1945.......SP-200, BC series, PS, Mil HQ-120

Known Serial Number Log

This is a log of known and reported serial numbers for Hammarlund Super Pro receivers, their SP power supplies, the HQ-120X (including military RBH and CHC-46140) and the Comet Pro receivers. The Super Pros are broken down into the various civilian models and the three Signal Corps military versions. Likewise, the power supplies are divided into civilian and military. Be sure to observe that the Signal Corps tag on the front of the SP receiver has a Signal Corps serial number and the receiver itself has a Hammarlund assigned serial number stamped on the rear apron of the chassis. The Hammarlund assigned serial number is what we are logging.

SP-10:  576, 674***, 720*, 979***, 1023, 1110,

SP-100: 1352**, 2562***, 2564***, 2570, 2672(SX), 2714, 2730(LX), 2746***, 3356, 3387*, 3417***,
3665(LX)***, 3826, 4106(LX), 4475(LX),

SP-200X:  4121, 4227, 4239, 4833, 4912, 4955, 5580, 5658, 6005***, 6239***, 7334, 7853, 8295, 9419, 10664, 16135,

SP-200SX: 4146, 4289, 6230, 6289***, 9759, 15775***

SP-200LX: 6884, 6965, 7353, 7545, 8023, 8423, 8466, 9262

SP-200X - Special: - 5733 & 5858,

BC-779 (incl. Howard:) 6873H, 7416H, 9030H, 9828, 12421, 23688

BC-794: 19828,

BC-1004: 18320, 21009, 27942(ASP-1004),

PS CIV: 721*, 804***, 987***, 2612***, 2615***, 2785*** 3388*, 3400***, 3545(100), 3649(100)***, 4215B (200), 4287, 5164, 6056(200)***, 6215***, 6331(200)*** 9238(200), 11184, 11865, 14653***

PS MIL: 5514H (ASP-84), 9686(ASP-84), 17656(RA-94), 17783(RA-94), 21186(794), 22093(1004), 26595(RA-74), 28870(ASP-84)

HQ-120X (all):7007, 7244, 7408, 7711, 9630, 9881,

Comet All Wave:  1006

Comet Pro:  7531, 7822, 8001, 8079

 * = sequential serial numbers - colors match receiver with ps                    
** = variation of standard model SP-10 or SP-100 (SP-200 listed as Special)                                          
*** = matched set with close but not sequential numbers, asterisk colors match the receiver with ps      
(Howard) = BC-779 receivers built by Howard Radio Co. as a contractor during WWII
(Special) = SP-200 Receiver with special frequency coverage or other non-standard features                                         

Serial Numbers Needed

Since it appears that all Super-Pro receivers and their power supplies along with the HQ-120X receivers were serialized from a common serial number roster, any serial number from any of these Hammarlund products provides important information as to production quantity and date lines. Even the earlier Comet Pro serial numbers should show that it used a different set of numbers. If you have one or more pre-war Super-Pro receivers, SP power supplies, HQ-120X or Comet Pro receivers, please e-mail the serial number and what model it goes with to Western Historic Radio Museum. Be sure if your Super Pro is a military version to send the Hammarlund serial number stamped on the rear of the chassis not the serial number on the ID tag . We will add the information to the Known Serial Number Log. With each number, more knowledge is gained about these incredible receivers and that information will be added to this website. Hopefully the result will be an on-line accurate source of information about the pre-war Super-Pro receivers.


e-mail Super-Pro info to: 



   Characteristics and Engineering Changes per Model/Year


1936 - SP-10 - Uses all large-pin glass tubes, has separate RF, IF and AF Gain controls along with Tone control, no pointers on knobs
                         Variable coupling on Detector and AVC transformers
                         Both audio transformers are potted units
                         Audio output transformer was probably an 8 ohm Z output on all receivers
                         Resistors were added to create a 600 ohm output Z on some receivers
                         Some receivers may have had a standard phone jack audio output on front panel
                         Aluminum front panel .187" thickness, engraved nomenclature
                         Paper-wax capacitors are usually Aerovox brand, sometimes intermixed with Cornell-Dubilier brand                        
                         Spacing rods in RF tuning unit are brass                        
                         Non-illuminated Tuning Meter measured total IF amplifier current
                         S-version introduced in June 1936 tunes 1.2 to 40 MC
                         Bias series resistor string modified - 600 ohm resistor replaced with 2-300 ohm, allowed moving detector input amp grid connection
                         to junction which slightly increased bias voltage
                         BFO Plate Load resistor changed from 5K to 50K (may be parts list error as all known examples use 50K)
                         BFO Grid capacitor (100pf) changed from grid to ground connection to parallel with grid leak resistor connection, (possible schematic error)
                         Tone control eliminated in late production
1937 - SP-100
- Introduced January 1937, uses eight metal octal tubes and eight large-pin glass tubes, Sensitivity Control replaces separate RF and IF gain controls,
                           Variable coupling AVC and Detector transformers changed to fixed coupling, Selectivity control renamed Band Width, engraved scales added to
                           Sensitivity, Beat Oscillator, Band Width and Audio Gain controls
                           Fixed-coupled AVC and Detector transformers allowed the component boards to be moved from under the chassis (SP-10) to inside the transformer cans (SP-100)
                           New style small knobs with metal pointers                  
                           Audio transformers changed from potted units to vertical mount-frame types - output Z is 8 ohms
                           Paper-wax capacitors are usually Cornell-Dubilier "TIGER" brand but could be intermixed with Aerovox brand
                           Spacing rods in RF tuning unit are steel                           

1938 - SP-100 - SP-100L introduced as long wave version, 100KC-400KC and 2.5-20.0MC, production receivers have same audio output configurations as the SP-100X.
                           Late versions of the "L" may have had front panel "Phone" jack, dual secondary windings - 600Z speaker and Hi-Z phones perhaps
                           "LX" version added a Crystal Filter
                           SP-150 console Super-Pro introduced in July 1938

1939 - SP-100 - SP-100 Crystal Filter - separate smaller panel eliminated on last of series and CF mounted directly to front panel
                           SP-200 introduced October 1939, added Noise Limiter, Amplified AVC redesigned, Crystal Filter redesigned, IF section redesigned
                           Illuminated S-meter replaced Tuning Meter
                           Over/under toggle switches replaced with rotary switches with knobs, 18 tube circuit, larger power cable because of
                           increased diameter of tube heater wires, early SP-200 may use SP-100 frame type audio transformers

1940 - SP-200 -  Front panel Speaker/Phones switch changed to .25" phone jack, added dual secondary windings to output transformer
                           allowed separate 600 ohm Z and Hi-Z ohm earphone outputs on X and SX version. It is possible that LX version already had this configuration.
                           Audio transformers now potted units.

1941 - SP-200 - Front Panel changed to .125" thick steel with stamped engraving filled white, front panel paint changed to semi-gloss black
                          Steel front panels are nickel-copper plated on front side only as corrosion preventative
                          Spacers added to panel mountings because of thickness difference

1942 - SP-200 - Probable beginning of Signal Corps BC series - some receivers may have rubber stamped SC order numbers indicating that it was purchased from civilian source.
                           Militarization of power supplies with heavy duty parts                   

1943 - SP-200 - Painting of front panel changed to red oxide primer coat with gray paint, color of the panels is highly variable with gray, blue-gray and green-gray commonly seen.
                          Tube layout charts are added to the top of the RF box
                          Schematic added to inside of bottom cover of the power supply
                          S-meter housing changed, full glass front replaced with partial glass with zero adjust mounted in metal
                          TM-11-866 Signal Corps Manual, first version

1944 - SP-200 - Probable year for contactor built BC-779

1945 - SP-200 -  Late 1945 probable design date for SP-400 as it is introduced in early 1946

Post -WWII   -  The Signal Corps continued to support and use the SP-200 Super Pro receivers. Some of the MWOs and manual additions date well into the 1950s.
                           Rectifiers changed in the RA-74D power supply, 5U4 and 5Y3, also 12 cylindrical oil-filled paper capacitors replace the two large block capacitors (1948)
                           Improvement Kit MC-531 shown in TM11-866 (Feb 1948 printing) - this is the three channel selectable crystal oscillator upgrade
                           Wickes Engineering R-270/FRR receivers supplied for AN/FRR-12 Dual Diversity Receivers - R-270 is a modified BC-794 receiver (1948)



Expected Super -Pro Performance

The Super-Pro was one of the few double preselection receivers that was easily available to hams and commercial users in the mid to late thirties. The use of two, tuned RF amplifiers on all bands means the Super-Pro was virtually immune to images and its sensitivity and selectivity were the best available. Of course, an accessory Pre-Selector could be added to any single preselection receiver to gain the advantages of double or triple preselection but the Super-Pro already had two stages on all bands and the early versions had four IF stages.

When using a Super-Pro the first thing noticed is how easy it is to over-drive the receiver with either too much RF gain or too much audio gain. This is especially true of the SP-10 receiver. There is audio power to spare and the sensitivity can easily be increased to where nothing but noise results. Operating a Super-Pro is like driving a high horse-power automobile - you don't drive around with your foot on the floorboard of a Ferarri - the Super-Pro is the same way,...back the gain controls down and only use what is necessary.

Another important thing to remember is that the Super-Pro antenna input Z is approximately 115 ohms. Faraday shields are used in the first RF coils to keep the impedance fairly constant (only electromagnetic coupling.) Also, no antenna trim control is provided. Since this is a relatively low antenna Z, certain types of antennae work better than others. Random length, end fed wires are usually a Hi Z antenna and if not matched to the receiver will give poor results. Almost all commercial users and most hams provided a matching device for their receivers in the form of the antenna coupler (nowadays called antenna tuner.) Since the antenna was matched to the transmitter, which was normally low-Z, the results of using that same antenna for the receiver worked great. Most complaints about the Super-Pro and high front-end noise stem from using a non-matched antenna.

The high power, high fidelity audio provides fantastic sound when a good speaker is used and it is matched to the particular audio output Z of the specific Super-Pro. Most SP-10s and nearly all SP-100s are 8 ohm Z output. SP-100L versions use an audio transformer with dual secondary windings of 600 ohm Z and Hi-Z phones. A few SP-200s are dual outputs with 8 ohm Z and Hi-Z phones using frame transformers similar to the SP-100 transformers but most SP-200s are 600 ohm Z and Hi-Z phones. If the SP-200 has potted transformers with the dual secondary windings then it is 600 Ohms Z and Hi-Z phones. The correct match is important for best fidelity and power. When everything is correct, shortwave BC stations sound beautiful, especially if conditions allow for opening the bandwidth up to 16KC. AM BC also sounds great if you can find a station that is playing music instead of incessant talking.   >>>

 >>>  Using a 60 foot long end-fed wire connected to an antenna coupler, I nightly copy ZL stations (New Zealand) on 40M CW on an SP-200X receiver on loud speaker. The ZLs are Q5 and about S6. At night (during the winter,) on 80M AM, stations on the East Coast can be easily copied, providing the QRM allows for it. 20M performance during the afternoon is incredible with lots of stations from South America and the South Pacific. DX is a daily or nightly occurrence using the Super-Pro and a matched antenna. Shortwave BC stations out of South America that are wide-band sound fantastic since they are strong, rarely fading and the Super-Pro bandwidth can be opened up to 16KC.

 It is unfortunate that these incredible receivers have had to endure endless deriding from hams and SWLs over the past several decades. The low opinion of the Super-Pro probably originated from hams and SWLs who, in the 1960s, happened to obtain a well-used surplus BC-779 receiver (that like most Super-Pros was still operational) and began using it right away without doing anything to the receiver. The first thing noticed was that the highest frequency tuned was 20 MC. Then it was noticed that AM BC was not included but two long wave bands were. Also, a 600 Z ohm speaker was needed. Also, the bandspread only was provided on the top three bands. Complaints were numerous and mostly based on the surplus BC-779 version. The BC-794 or any "S" version would have ended almost all of the complaints but it was one of the most difficult models to find.

 The fact that the Super-Pro was built "like a tank" and the RF box is virtually weather-tight has resulted in many Super-Pros working in "as found" condition. But, any pre-WWII or war vintage receiver should be rebuilt and aligned before any sort of critical analysis is to be performed. What you should expect from your Super-Pro depends upon its condition and this is true for all receivers, new or old. So, the fore-going comments are in reference to rebuilt, aligned and unmodified Super-Pro receivers that are operating at their original specifications. "As-found" condition will almost always give less than desirable results. Also, make-shift antennas and non-matched speakers will also result in diminished performance. When everything is right, the Super-Pro is an unbeatable receiver, whether it be the SP-10, SP-100 or SP-200, they are all great performers.


Guide to Restoring Pre-War and Wartime 'Super-Pro' Receivers

Manuals-Schematics:  Though original manuals are nice to own, they are not really necessary for the repair or restoration of an early Super-Pro receiver. In fact, most receivers do not exactly agree with the information in the manual anyway. This is because engineering upgrades are usually incorporated into production almost immediately while documentation may take quite awhile to catch up. Most of the information that is in the various Hammarlund manuals is also found in Rider's Perpetual Troubleshooter's Manuals. Also, several sources offer reprints of the original manuals and several on-line sources offer manuals that can be down loaded. When ordering a reprinted manual be aware that most manual suppliers reference the receivers with the Hammarlund ordering model numbers, thus an SP-100X receiver would be listed as an SP-110X (the SP-200 series may have the same ordering issues.) Hammarlund did identify the manuals using the speaker size modifier but since the SP-110X and the SP-120X are identical receivers, their reasons for this are a mystery. The SP-200 Military versions are covered extensively in the Signal Corps manual TM-11-866. This manual covers the BC-779, BC-794, BC-1004 and the R-129/U plus the power supplies RA-74, RA-84 and RA-94. Even if you have the civilian version SP-200, this TM is an excellent manual to have. Regarding the SP-10 schematic - the common published schematic for the SP-10 is for the very earliest receivers. Several engineering upgrades were added during production which, of course, do not appear on these early schematics. Hammarlund's documentation apparently never caught up with the SP-10, therefore, when working on an SP-10, you should also have an SP-100 schematic handy. You will find discrepancies in the SP-10 schematic versus most SP-10 receivers that are correct if you compare the circuit to the later SP-100 schematic. Also, if you are the owner of an SP-10 manual (hopefully along with the receiver) note that the manual drawing of the underside of the chassis is a "mirror image" and shows the RF/IF section where the Audio section should be and vice-versa - visually confusing.  Reworking Technique and Skills: Like many of the pre-war high-end communications receivers, the 'Super-Pro' is a challenging restoration project. Major disassembly is required to access some components that need to be replaced or at least tested. How well your restored 'Super-Pro' functions after a rebuild is dependent on your soldering technique, your ability to methodically perform mechanical and electronic work and to keep track of that work. Experienced technicians are always checking and rechecking their own work as it progresses. This ends up saving time in the long run as problems at the power up stage are minimal, if any at all. High-end receivers cannot be restored in a "rush job" manner. Take your time and recheck everything you are doing. The SP-10 and SP-100 Super-Pros have more mechanical complexity than the normal communications receiver and several of the fiber board parts can be very fragile after years of use. Don't force assembly or torque anything involving the fiber board parts or breakage is sure to occur. In a comparison to other receivers, the 'Super-Pro' is just as complicated of a restoration project as the Hallicrafters SX-28. Both require major disassembly because of difficult (or impossible) to access components that need to be replaced. Remember, the Super Pro has a rather high level of B+ in its audio section so care is necessary when doing any testing with the power on. Good troubleshooting skills are required when working on any type of vacuum tube electronics gear due to the high voltage levels required for this type of circuitry.
Disassembly: The best procedure to rebuild a Super-Pro is to start with a major disassembly. It will be necessary to remove the front panel and some of the assemblies for access to parts that need to be checked or replaced. There is no way to access the capacitors in the RF Tuning Unit except by its removal from the chassis and then removing its shielding. There is no way to remove the RF Tuning Unit (RF TU) unless the front panel is off. There are five paper wax capacitors located in the RF TU of  the early SP-10 and SP-100 versions and three in the SP-200 versions. The early versions also have an RF shield between the RF and IF section of the receiver. It is easier to work in this section of the chassis if the shield is removed. In fact, it is easier to work on the chassis with the RF TU removed also. When the capacitors are replaced and the other rework completed, the receiver can be reassembled. On early models, if there is a problem with either the "On-Off" switch or the IF gain pot (Sensitivity pot on SP-100) you have to remove the front panel and the RF TU for access to remove these components. Check their operation thoroughly while everything is apart. Capacitors: All paper wax capacitors should be replaced for best reliability and performance. Hammarlund did locate some of the paper-wax caps out-of-sight in the RF TU and in some of the AVC and Detector transformers but these are easy to access during disassembly. Whether you use "orange drops" or "yellow jackets" doesn't matter - both are much better capacitors than the originals were when they were new and, of course, now the originals have about 70 years of leakage to deal with. If you want to preserve the under chassis original appearance, then the new caps should be installed inside the original shells. This procedure is covered in the section below "Restoring the 100 Series 'Super-Pro.' Restuffing capacitors is, of course, only for aesthetic purposes. If under chassis appearance is not important, then install either modern orange drops or yellow jackets. You should use all of the same kind of caps when rebuilding a receiver. It looks professional. Mixed types of capacitors look like you were working out of your junk box! Military 'Super-Pro' receivers will have "bathtub" capacitors installed around the inside edge of the chassis. These are multiple paper capacitors installed in a metal box. Some of these are oil filled units. Also, some of the individual larger paper capacitors are also oil filled. If the oil filled units are not leaking oil, they are probably okay. They should be tested at their operating voltage for possible leakage if you plan on leaving them in the circuit. Or, you can just replace or rebuild each tub with new capacitors - your preference.

Paper Wax Capacitor Locations:  

SP-10 -  five .02uf, 400wvdc paper wax (pw) capacitors are located inside the RF TU. All other pw caps are under the chassis.

SP-100 - three .01uf 400vdc and two .02uf 400vdc pw caps are located inside the RF TU, two .05uf 400vdc and one .02uf 400vdc pw caps are located inside the Amplifier AVC Output transformer, one .05uf 400vdc pw cap is inside the Detector Output transformer. All other pw caps are under the chassis.
SP-200 - three .01uf 500vdc pw caps are located inside the RF TU, four .05uf 500vdc pw caps are located inside AVC transformer, two .05uf 500vdc pw caps      
are located inside the Detector transformer. All other pw or bathtub caps are under the chassis.

Variable Coupled IF Transformers: You should check the variable coupled IF transformers for proper action. These have spring-loaded plungers that are operated by a cam and lever system. The Band Width/Selectivity shaft is also spring-loaded for a positive feel to the control. Most 'Super-Pro' receivers require a little bit of adjustment or rework on this "shaft-cam-lever-plunger" system to have the control function correctly, feel smooth in operation and to stay in place once set. A Bandwidth shaft that won't stay in position probably is missing one of the thrust washers. Add fiber or brass washers as necessary between the shaft thrust bearing and the rear bearing support. Usually only one washer is necessary to have the shaft stay in position. The variable coupled IF transformers require a delicate rework technique to avoid breaking the fragile parts. I have encountered both broken levers and broken coil-condenser mounts. Since these are made from fiber board, they are somewhat fragile. Care has to be taken when removing any of the variable coupled IF transformers from the chassis since there is an under the chassis coupling pin between the levers and the plungers. This is a "slip-in" type of coupling but any torque on the fiber levers will break the fragile slotted portion. If you have to disassemble one of the variable coupled IF transformers (to replace a grid lead or for some other reason) first remove the associated lever from under the chassis. When removing the shield (can) watch the spring that loads the center moveable coil mount. The top coil-condenser mount is held in position when mounted inside the can but when the can is removed there is nothing except the wire connection keeping the mount on the guide rods. The spring can lift the upper coil-condenser mount right off of the guides and launch the spring into the air requiring a search for its whereabouts.  >>> >>>  Sometimes the upper coil-condenser fiber mount can become "jammed" and break the fragile fiber board holes that the guide rods pass through. During reassembly if any binding or tightness is noticed, don't force the assembly. Disassemble the IF transformer and you will certainly find that the top fiber coil-condenser mount is "jamming" on the guides and forcing the assembly would break the guide holes. If the upper and lower condenser mount screws go in without forcing, then check the plunger operation by hand from under the chassis. It should travel about one inch and should operate smoothly without binding. Mount the IF can shield to the chassis with the two nuts and washers, then check the plunger operation again. Sometimes the can has to be slightly moved as to how it mounts to the chassis for smooth plunger operation. Once the plungers all operate correctly, re-install the levers. They may require a slight offset in the angle that the lever engages the plunger slot. You can adjust this by loosening the mounting screw and adjust the lever for the proper angle and then retighten the screw. Normally, the plungers will rotate to allow further adjustment of the lever to slot engagement. On the SP-10 and SP-100 versions, the RF-IF shield has cut-out slots that the levers protrude through. Be sure that when aligning the levers to plungers that you still have the necessary clearance within the RF-IF shield slots. When everything is correct, the levers operate the plungers smoothly with no binding or sticking and with full travel. Generally, if you are careful and watch the levers, plungers and the load springs (and nobody else has been into the transformers before you to break things) everything will be fine.

Other Components: Resistors should be checked for drift. Usually 20% is okay. On the early SP-10 and SP-100 receivers there are a couple of what look like adjustable trimmer capacitors. These are actually fixed caps that are the correct capacitance when the adjustment screw is tight - no adjustment necessary.

The dial system is part of the RF TU assembly but the drive is by a "pinch-wheel" system that is mounted to the front panel. When the system is clean the tuning is super smooth. Dirt and grease somehow get in to the pinch-wheels and will cause a rough feel or even slipping. A good cleaning of the pinch-wheel and the rim of the dial will usually correct any problems and result in the super smooth tuning "feel" that the receiver had when new. >>>

>>>  On newer style pinch-wheels there is a nut on the back of the wheel that should be checked - it should be tightened when the dial edge is in between the pinch wheels. When reinstalling the RF TU, you will have to loosen or remove the pinch-wheel drives and then reinstall them after the RF TU is mounted. That way you can be sure that you don't damage the dials.

 If the Tuning Meter is open on the SP-10 or SP-100 series receivers, there will be no B+ to the IF section of the receiver. If an original meter can't be located and the defective one can't be repaired, it is possible to shunt the defective meter with a 2 ohm .5W resistor to get the receiver operational until a meter is found.

Front Panels:  The front panel of early 'Super-Pro' receiver is made from .187" thick aluminum. It was wrinkled finished in black and then engraved so the nomenclature would appear bright silver. Only the front of the panel is painted - the back is always left bare aluminum. If the front panel doesn't have its original wrinkle finish it is next to impossible to restore the panel to original. Repainting the panel with wrinkle finish will fill in the engraving and then trying to "scrape out" the paint from the engraved areas by hand just doesn't work. The nomenclature usually doesn't look correct or "professional." The wrinkle finished panels were used on the SP-10, SP-100 and early SP-200 receivers. For best results with an original finish panel, clean the panel thoroughly and then touch-up as needed. Using Krylon Black Wrinkle paint, spray some in a small cup and then paint a thick coat with a small paint brush where the panel needs to be touched up. Use a heat gun to force the wrinkle - this will take a few minutes. Don't overheat the paint or it will "gloss" and not match the original. For very small areas, artist's acrylic "Mars Black" works quite well and some texture can be imparted to the touch up with a brush application. If you have an aluminum panel that is already stripped, paint it with Krylon Black Wrinkle Finish and after it has dried over-night, try digging the paint out of the engraving. Maybe it will work okay if the engraving is in good condition. Many panels that are stripped were done so by "sand blasting," or "grinding off" the paint. This removes a lot of material from the panel surface which reduces the depth of the engraving resulting in the problem usually encountered in trying to remove paint from the engraved areas. If the panel was chemically stripped you might be able to successfully restore the wrinkle finish and engraving. If the results are not to your satisfaction about the only recourse is to paint the panel with a smooth finish paint the then fill the engraving as described in the next paragraph. I always encourage restorers to try to save the original finish if possible.  >>> >>>  The .125" steel panels, used on the SP-200 series from about 1941 on, are easy to restore. The panel front was copper-nickel plated and then a red oxide primer applied before the finish coat was painted. The back side of the panel is left unfinished. Since the nomenclature is stamped into the panel, the depression is much wider and somewhat deeper than engraving. This makes filling in the "lettering" easy after the finish coat has been applied and has dried. Original paint colors can be computer matched if some of the original paint is remaining. Professional automotive paint stores have the ability and the equipment to not only match the paint but to fill that paint into spray cans for easy application. Use a high quality acrylic enamel paint that is semi-gloss finish. Don't use bright white paint or white "lacquer-stix" to fill in the lettering. The nomenclature will look way too "bright." Instead, mix artist's acrylic white with raw sienna and just a little yellow to create a "beige" or "manila" color. This will look correctly "aged" for the fill. Apply to the lettering one control nomenclature at a time. Let it set for only a couple of minutes and then wipe with a slightly damp paper towel folded to be very flat. The "wipe" should be at a slight angle and only done once with that part of the damp towel. If more wiping is required, use a new unused section of the damp towel, otherwise you will get paint on the panel where you don't want it. When all of the lettering is finished, you can wipe down the entire panel with a clean slightly damp towel. Let the fill paint dry over-night and then apply Carnuba wax to the front panel (any non-abrasive wax will work.) Two applications will have the front panel looking great and with the patina of age imparted by the lettering not being "bright white."

Cabinets:  The early receivers just use a dust cover. It is black wrinkle finish and is secured with eight capped thumb nuts on the front and three thumb screws on the back. If the thumb nuts are missing, they are difficult to find anything like them today. They can be machined but this is expensive. You can make them from old nickel plated thumb nuts by filling the top of the hole with solder, them filing the solder flat. This usually looks okay until originals are found. The thumb screws are easier to find and usually not a problem. The SP-200 cabinets will have two chrome strips top and bottom. These can be missing or damaged. Unfortunately, they are difficult to replicate since they are thin sheet metal extrusion and chrome plated. Sometimes a cabinet will be encountered where the strips and handles were removed and the holes filled and the cabinet repainted. The military CH-104-A cabinet and the SP-400 cabinet did away with the chrome strips. The handles didn't change from the SP-200 cabinet to the early SP-600 cabinet. All are the same. The handles are easy to clean up and restore. The SP-200 and SP-400 will fit into either one's cabinet. Early rack mount SP-10 and SP-100 will also fit in any of the cabinets. Most of the SP-200 cabinets are black wrinkle finish while most military CH-104 and SP-400 cabinets are gray wrinkle finish. The SP-600 cabinet is dark brownish-gray wrinkle finish.  >>>

>>>  When cleaning these cabinets remember they are durable and the paint is tough. They can take a lot of abuse without showing it. That's why I clean the wrinkle finish with Glass Plus and a brass suede brush. The suede brushes are available at shoe stores or sometimes shoe sections of regular stores will have them. The brass bristles are not very stiff and really feel somewhat soft. Get the section you are going to clean very wet with Glass Plus and then use the Suede brush in various circular motions, scrubbing the surface. Don't be overly aggressive but also you don't have to worry about the paint either. You will notice the wet residue will turn gray-black after a while. Wipe off the wet with paper towels. The towels will show a lot of color but it is mostly dirt. Repeat the cleaning until the paper towels are wiping off fairly clean residue. Perform the cleaning procedure on all of the cabinet surfaces and then let it dry thoroughly. After drying, do any touch ups now using either Krylon Black Wrinkle applied with a brush and force wrinkled with a heat gun (for areas over .5" diameter) or artist's acrylic for smaller areas or if it is a gray cabinet you're working on. Apply Armor-All after the touch-ups have dried. You may want to do two applications. The cabinet will look practically new with this treatment.
Alignment:  Any of the Super-Pro receivers are straight forward with no odd procedures or special equipment needed. The adjustment of the crystal filter may require a sweep generator and oscilloscope if the adjustment has been misaligned, but this is seldom the case. Since the adjustable 465KC IF transformers are over-coupled in the 16KC band width, the IF should be aligned in the 3KC position at the exact crystal frequency, if the crystal filter option is installed. Due to the extreme quality that was built into the tuning unit, all Super-Pros can be aligned to exceed the dial accuracy specification of 0.5%. If you can't achieve this accuracy then something is wrong with the oscillator section for that particular band. Power Supplies and Power Cables: Early power supplies used standard can-type electrolytic filter capacitors. These are almost always dried out and need to be replaced. Even if they check good, they will certainly fail within a short time if you try to use them at full voltage. The original cans can be restuffed with new electrolytic capacitors or the new ones can be installed under the chassis. For aesthetic reasons, I place the new capacitor inside the original can. The military power supplies usually have oil filled paper capacitors for filters. Unless these have shorted or are leaking oil, they are okay to use. Some of the late version military supplies will have 12 can filter capacitors installed. These appear to be electrolytics but they are actually 4uf oil-filled paper capacitors. They seldom have any problems since they were of very high quality construction. Test them thoroughly before relying on them, though. The power supply has two large tapped wire wound resistors and it is very rare for them to have any problems. Many times the power supply will not be found with the receiver. They are fairly easy to find and almost any 'Super-Pro' power supply will work with just about any 'Super Pro' receiver. The power cable is also not usually with the receiver or power supply. The original cable is not an easy item to find so most collectors just make one. The only thing to observe is the wires #1 and #2 are a larger gauge, usually 14 or so, to provide a small IR drop across the cable for the tube heaters. The remaining wires are around 18 gauge and not critical as the current carried is low. The tenth wire, if present, is not used in the receiver circuit. It was a switched ground for optional user applications.


Modification Mayhem

The Super-Pro is a terrific receiver - whether it be the SP-10, SP-100 or the SP-200 series - they were the best of the late-thirties designs. But, they are seventy year old receivers and, as such, their performance is dated. The stock 'Super-Pro' was an incredible performer. That is confirmed by the fact that the U.S. Army Signal Corps versions of the 'Super-Pro' 200 Series are virtually identical to the civilian versions. The military felt no need to modify the SP-200 Series unless the receiver was going to be used for data reception where stability and freedom from frequency drift were of paramount importance. The military modifications were generally professionally incorporated and enhance the receiver's performance. The modifications discussed in this section are the professional "Geisler mods" and the far more invasive CQ magazine "Surplus Conversion" modifications.

As the owner of one (or more) of these great receivers, you probably want to experience what the original owner encountered when using the receiver when it was new. Something that is accomplished by a thorough rebuild and alignment of the receiver - not modifications. Understanding what the designers expected of the receiver and how they intended the receiver to be operated will help the new owner appreciate the vintage Super-Pro's capabilities.  >>>

>>>  Of course, the original owner didn't encounter SSB signals but the Super-Pro will copy SSB without a problem if operated correctly. Drift, vague dial accuracy, etc.,...all pre-war communications receivers have the same characteristics. It's all part of the vintage ham gear experience that is enjoyed by so many collector-hams today. All serious collectors and knowledgeable hams agree that major modifications to vintage radio equipment in an effort to "modernize" its performance seems to go against the whole idea of collecting, preserving and operating vintage radio gear in the first place.

Since the early SP-10 and SP-100 Super-Pro receivers were very expensive and sold during the time of economic recovery, they are normally not found in extremely modified condition. Most of the time, repairs and various component changes are all that are encountered. The unusual way that the SP-10 operates with its highly flexible front end and adjustable coupling in the Detector and AVC transformers has led to some SP-10s being modified to operate in a more conventional manner (rather than the operator "learning" how to use the receiver.) Most of the mods are to the RF Gain bias although anything might have been incorporated into SP-10 in an attempt to change its design characteristics. Most of these modifications are very easy to remove and return to the original configuration as most were wiring changes that didn't do any real damage.

L. E. Geisler Modifications - With the cheap, easy to find availability of the surplus WWII Super-Pro receivers in the mid-fifties and sixties, the "modification mania" did finally catch up with the line and the WWII Super-Pro was considered "fair game" for modifications. Most of the infamous Super-Pro modifications were derived from the first of the series, "Souping the Super Pro" by L.E. Geisler, published in the Dec.1957 issue of CQ magazine. Geisler was an engineer that worked out of Japan for a company that sold modified Super-Pro receivers. Today, Geisler's modifications are "tame" and basically replace the 6L7 mixer tube with a different octal mixer tube that is quieter, then he replaces all of the capacitors and does a full alignment - pretty much standard stuff. Geisler's earlier mods are conservative, make sense, improve performance and do no real harm to the receiver. One has to remember that Geisler's company sold these modified Super-Pro receivers so they had to perform better yet still retain the professional-commercial appearance in order to have marketability. As time went on, Geisler's modification ideas evolved. Later Geisler-modified Super-Pro receivers will have more invasive modifications.

Post-Geisler Modifications - The later modification articles go even further than the late-Geisler ideas with even more and more outrageous modifications. Included in the list of notorious "cut and hack" articles are "A Super 'Super-Pro'" and  "SSBing the Super Pro" - both published in the "Surplus Conversion Handbook," part of the CQ Technical Series. These articles advocate the wholesale modification (destruction) of the entire receiver, including replacement of the front-end tubes with miniature tubes, an on-board solid-state power supply, removal of the 14 watt P-P audio section to install an anemic 6AQ5 single ended audio section (which also then provided room for the on-board power supply,) on-board converters to cover 10 and 15 meters, product and infinite impedance detectors - on and on. It's doubtful that a receiver could ever be returned to original after being the victim of these later modifications. I have only seen a couple of Super-Pros that attempted these modifications and they were wrecks. No doubt, the end product failed to impress the owners and the receivers were afterward relegated to the junk pile. 

Initially, I thought very few Super-Pro receivers had ever been modified since it was such a great design in the first place. The Super-Pro receivers that I had found around Nevada were mostly all original. However, after talking to several collectors in different parts of the country, it seems that there are lots of modified Super-Pros out there. Many modified versions are now showing up on eBay. Certainly, the rarity of the particular receiver will dictate how far the owner is willing to go to restore the set to its original specifications. In a way, it is fortunate that the most common victims of the modification mayhem are the military receivers. They are also the most plentiful version around and that means that most of the SP-200 parts are usually easily available to complete a "restoration to original" on other SP-200 receivers that have faired better. However, a true "Geisler Modification" or a "Military Modification" receiver should be left "as-is" since these modifications made sense and enhanced the Super-Pro performance.


Using the Super-Pro as a Communications Receiver Today

Some hams are reluctant to use a pre-war receiver in actual "on-the-air" operations for fear that adjacent frequency QRM will limit their ability to successfully copy stations and that they will be unable to complete QSOs or Vintage Net operations. The Super-Pro might have problems today coping with adjacent frequency interference from powerful SSB signals when the user is trying to copy an AM signal (and this will be the case for almost all vintage communications receivers.) Even narrowing the "Bandwidth" IF passband or using the crystal filter doesn't seem to help much. However, most of the problem is caused by the user operating the receiver as a "broadcast radio" rather than as a "communications receiver." Most manuals will direct users to operate the receiver in the "standard AM mode," that is with AVC on, the "Sensitivity" at maximum and the Audio Gain rather low. This allows the receiver's front end gain to be controlled by the AVC in response to whatever the strongest "signal" happens to be. This works fine when listening to strong broadcast signals. But many times the desired signals are weak and the stronger energy will be atmospheric RF noise. Often times it is adjacent frequency interference. The best solution is to switch off the AVC, increase the Audio Gain to near maximum and operate the receiver's front end gain manually with the "Sensitivity" reduced to the minimum necessary to copy the desired AM signal. You will find that now the adjacent frequency interference is greatly reduced since its strength compared to the tuned signal is reduced. Also, the AVC-controlled gain is no longer responding to variations in what it "sees" as a signal - the "pulsating" and "popping" of adjacent frequency signals within the passband.  >>> >>>  Additional selectivity using either the variable "Bandwidth" control set at 3kc and/or the crystal filter will also help to reduce the ratio of adjacent frequency interference to tuned signal. You will have to tune the desired signal "on the nose" for good copy. A couple of losses in this method of operation will be the S-meter function but it is really not very useful and only provides a relative measurement anyway. Also, broad audio fidelity cannot be enjoyed in this mode. But, the goal is successful copy and a completed AM QSO, in other words - communications. What about in actual use? I use only vintage receivers for operations on the ham bands and many times I'll use the SP-100X version of the Super-Pro as the station receiver. Using the "communications receiver" method of operation has allowed me successful copy of all stations checking into the vintage AM nets regardless of the adjacent frequency QRM. Weak signals can be a challenge but switching in the crystal filter usually provides the extra edge necessary for successful copy. Of course, it isn't enjoyable, "arm-chair" copy but it is a successful completion of the QSO or of the net operations, which is the goal. Unfortunately, deliberate on-frequency interference is another matter and no receiver, no matter how sophisticated the design, can cope with that type of intentional QRM.


Restoring Pre-War Super-Pro Receivers


The WMI - Hammarlund 'Super-Pro' SP-10 Restoration

I found SP-10 SN: 576 listed on QTH.COM. in January 2008. Its purchase included an original SP-600 cabinet thrown-in. Unfortunately, shipping ran about 55% of what the price of the receiver and cabinet was but it was safely packed and arrived without any issues. Initial examination revealed that the condition was very good cosmetically but the under chassis was almost totally non-original. I knew the receiver had been used extensively at WMI and had seen detailed photos of the underside so this was no surprise. I knew what I was going to be getting into.

photo left: The WMI SP-10 before restoration

At first, I thought I might be satisfied with the SP-10 given its WMI provenance but closer examinations kept revealing more and more late, post-WMI components and mods. I noted that almost all of the installed capacitors were Vitamin-Q types date coded 1973 (about ten years after the receiver left WMI.) I came to the conclusion that about half of the receiver circuitry modification was not WMI's work and that only the visible front panel mods were the remains of WMI "hacking."

 Our decision was to return SP-10 SN:576 to "original - as delivered to WMI" in 1936. Since any WMI provenance had been compromised by forty years of post-WMI modifications there was no justification to the belief that the receiver was in "as used at WMI" condition. By restoring SN:576 to original, the Hammarlund historical accuracy for the SP-10 Super-Pro model would be returned to this rare receiver and, at the same time, the WMI provenance would be enhanced by having the receiver look and operate as it did when WMI took delivery.

 This was going to be a thorough, "ultimate museum quality" restoration using rebuilt capacitors in original paper-wax shells, original type resistors where needed and authentic Hammarlund parts from other Super-Pro parts sets. We even used some of the original Hammarlund wire from the parts sets. We were going to strictly adhere the original Hammarlund wiring layout and to match exactly the original circuit design - taking into account that there were two original Hammarlund engineering upgrades not on the SP-10 schematic that had to be left in place. During the restoration, every step was guided by the desire to have this SP-10 be the most authentic, best representative of how the Hammarlund SP-10 appeared and functioned when it was new in 1936.

 To guide us, we used our experience gained during our "museum quality" restoration of the SP-100X SN:3387 receiver along with the evidence found during the SP-10 disassembly, the under chassis artwork in the SP-10 manual and the SP-10 and SP-100 schematics as references. Hopefully, this would result in a very authentic, restored SP-10 that could be used as a reference for how these receivers were built, how they performed and how they looked - both externally and internally.

The Planned Restoration Work

I had never encountered a receiver that had so many deviations from original wiring layout, in component placement or component type but that was still functional and still looked decent. This was primarily due to the WMI repairs done over the years. Since the receivers were used 24/7, repairs must have been a somewhat regular necessity. Originality was not a concern and neither was neatness. The end result of years of use and repair cycles is a receiver with very few original parts, several wrong value parts and many modifications to component placement or layout with some circuitry modifications thrown in. A lot of later post-WMI work was also performed on the receiver, also with little regard for originality. Here is the list of the deviations from original.

A. AC Power switch disconnected and wires connected to "Speaker-Phones" switch instead - repair of defective AC power switch

B. Send/Receive switch has another set of wires running back to the "Phones" terminal output - runs stand by switch operation to rear of chassis

C. 500-8 ohm transformer installed on inside wall of chassis connected between audio xmfr and spkr term. (matched 8 ohms standard audio output Z to 500 ohms Z)

D. Shielded cables run from 500/8 xmfr to front panel phone jacks and to speaker terminals  (allows phones use since the rear terminal was S/R function)

E. Two phone jacks installed on front panel marked inside panel as "600 ohm output" - brings audio output to front

F. E-W antenna switch front panel mounted - for switching directional antennas at WMI but was unwired

G. SO-239 front panel - connected to back antenna input terminals with RG-58 cable - part of antenna switching that was disconnected

H. #47 lamp sockets installed - should be #40 lamp sockets - "hamster mod" because of difficulty finding #40 lamps

I. Dial window indexes not original - look crude and home made

J. Tuning Meter is non-original movement inside original housing with repro scale - meter repair that was actually fairly well done

K. RF Gain mod per W4QCU - overloaded on strong sigs - two wires moved, not invasive

L. All small knobs were not original types - unknown reason

M. 42 tube socket replaced with bakelite type - burned up original?

N. Grounds are all connected together with small gauge wire - unknown reason

O. Small gauge plastic insulated wire used to connect 1st IF transformer to circuit - earlier repair?

P. Shielded cable from detector to BFO - non-original cable (modern) - original may have deteriorated

Q. BFO grid cap was 500pf, should be 100pf - repair with wrong value part - may have been WWII repair when parts not available

R. Screen load on 2RF amp was 2.4K should be 5K - repair with wrong value part - may have been WWII repair when parts not available

S. 220 ohm resistor added to B+ line to CT of audio output xmfr plate winding - to lower B+ from +385 to +365vdc at the 42s - allows using SP-100 PS

T. Though minor, someone wrote in black marker ink all of the circuit functions on the inside of the chassis. Additionally, dymo-lables were on the RF box indicating alignment functions. Also, all of the IF and AVC transformer cans were marked with marker ink. - Who knows why? Probably someone thought it would save time in troubleshooting if the circuit and component functions were obvious.

Planning and Finding Parts: I waited about 2 months before starting the restoration. This gave me time to do research, studying and planning of how to accomplish the rework. I was going to need 35 paper-wax capacitors to build the replica capacitors. The original paper-wax caps had been replaced eons ago but I wondered if the "hackers" had ever gotten into the RF box. When disassembled, I found three of the five capacitors were indeed originals. They were Aerovox brand, which meant I would have difficulty in finding enough correct vintage shells. After searching only turned up five or six correct style Aerovox shells, I decided I might have to go to Cornell-Dubilier TIGER brand, as they were standard originals for the SP-100 and later receivers. Fortunately, in searching around I made an important discovery. While looking at a junker HQ-120 receiver to harvest C-D caps, I noticed that the original capacitors were intermixed Aerovox and C-D brands. I was positive all the capacitors were original. I had discovered a precedent that Hammarlund did intermix capacitor brands sometimes, so I would do the same thing with the SP-10. Five Aerovox shells would be used where the caps mount on the tube sockets, three original Aerovox shells would be used in the RF box and all the other capacitors would be Cornell-Dubilier TIGER brand. The Importance of Having Parts Sets:  I had serious doubt that the SP-10 dial windows with index line were original. They were a crudely made lamination of plastic and the scribed index wasn't even straight. I convinced myself of their non-originality when I pulled apart the laminate and the glue was still somewhat tacky. Luckily, I had an SP-200 parts set. This poor receiver has served its fate as a donor for three different Super-Pro restorations and it was nearing its end as a useful "parts set" since there were very few parts left on the chassis. But, even though most of the parts were missing, this receiver supplied dial masks, dials, both dial window indexes, AC power harness, IF transformer parts, correct dial lamp harness, a multitude of screws and washers, some knobs, some wire and some meter parts. Additionally, an HQ-120 junker supplied many Aerovox and C-D capacitors. The junk box supplied a correct style fiber tube socket with the correct "42" ID, several of the correct style/value resistors. Without these junk sets and several junk boxes to rob parts from I doubt that the SP-10 restoration would have ever started since correct authentic vintage parts are so difficult to find.

The SP-10 Restoration Work

The photo to the left shows the front panel before restoration. The physical modifications were going to be difficult to repair. Most of the time, amateur modifications have little regard for symmetrical layout or quality machine work. The WMI SP-10 mods, even though performed by "professionals," were very much like amateur workmanship. I knew from an earlier restoration of an SP-200LX receiver with a repainted aluminum front panel that stripping and repainting wrinkle finish on this aluminum panel would be a disaster. Trying to remove the paint from the engraving is next to impossible. Touch-up is the only method that preserves the original look of the receiver panel. Filling the holes presented a challenge because of their size. First, the panel was thoroughly cleaned using Glass Plus and a brass "suede" brush. This is done to clean the smoke and grime out of the engraving so it would look silver again. The only practical hole repair method was an epoxy fill. I placed masking tape over the holes on the front side of the panel and then backed that with blocks of wood clamped to the panel. This gave a very flat surface and the masking tape assured that the epoxy would not stick to the backing blocks. I fill from the back side using slow curing epoxy to make sure all of the bubbles have a chance to rise to the surface (the back of the panel.) Because of the thickness of the panel, two layers of epoxy were required for the complete plug. The epoxy was left to cure overnight and then the blocks and tape were removed. This leaves a super-flat surface on the front of the panel. The back side is leveled using a Dremel Tool. The front side is painted using Krylon Black Wrinkle finish applied with a brush and "wrinkled" using a heat gun. Only the plug is painted and if everything goes okay, the match is virtually undetectable. The backside was painted with silver paint.  
The photo to the left is a shot of the chassis underside before restoration. There was so much that was non-original  I created a list to keep track. Some of the notable deviations under the chassis - the shielded cables routed next to the RF box. These carried the audio output to the phone jacks mounted on the front panel. The transformer matched the 8 ohm Z output of the receiver to the 600 ohms required at WMI. None of the capacitors are original, most having been installed in the 1970s after the receiver's tenure at WMI. Many modern style components were installed in the various circuits.

The photo to the right shows the SP-10 underside after restoration to original. The wiring layout follows the under chassis drawing in the SP-10 manual and also required several references to the SP-100 receiver. This was to assure that all wiring and component placement matched what was done at Hammarlund.
Note: The shielded cable most visible next to the RF box carries the AC power to the front panel switch. This is an original Hammarlund harness that was removed from a parts set and was installed during restoration.

The close-up photo to the left shows the AVC and BFO sections with rebuilt capacitors and replica resistors. These rebuilt capacitors happen to all be Cornell-Dubilier TIGER brand. This receiver restoration was different in that I had to find all of the shells first. This allowed me to rebuild the caps ahead of actually installing them. This seemed to make the job go much faster because I could do several capacitors at one time. Even so, the time per capacitor still was about ten minutes. My method is to use a heat gun to melt out the original capacitor and then wipe the shell with a paper towel to clean it. I then install a film capacitor of the correct value and correctly oriented and secure it with hot melt glue. After the glue has set-up, I fill each end with brown sealing wax. The end result is a capacitor that is new but difficult to distinguish from an original.

The two 60K resistors are replicas that are repainted to match the BED coded originals. There were several JAN type resistors installed and even some modern metal-film resistors that were removed. To keep everything looking the correct vintage I used similar to original size and shape resistors of the correct value. These similar style resistors were marked in banded color codes. Also, some of the replacement resistors found had leads that were too short for the Hammarlund "thru-the-eyelet" type of mounting. I had to carefully add a length of TC wire to be able to mount the resistors correctly. When the leads were installed, I then painted the body of the resistor in the BED code style. When mounted, only a close inspection reveals that these are replica resistors.
I initially installed the three IF screen bypass capacitors as the replacement, non-original caps had been installed - on top of the component board. This had the capacitors on top of the 10K resistors. This didn't seem likely and I remembered that in the SP-100 rebuild, the IF screen caps had been mounted at the tube sockets. Looking at the manual drawing, it was apparent that the caps shouldn't have been mounted on the component board. I removed them and moved the component boards in order to have access to the IF amp tubes. There I found the evidence I was looking for - the actual remains of the original capacitor leads left behind when the receiver was re-capped years ago. I installed the rebuilt caps at the tube sockets, as was original manufacture. The photo to the right shows the RF-IF section with all rebuilt capacitors installed in their correct positions. The Aerovox rebuilds installed in the RF section can be seen. The LO capacitor that looks like an adjustment trimmer is actually a fixed capacitor that is the correct value when the screw is tight. Though this one is a replica I made, it is very close to the original in size, value and appearance.
The photo to the left shows the bandspread condenser side of the RF Tuning Unit and the rebuilt paper-wax capacitors. These five capacitors are completely hidden and require the removal of the RF TU from the chassis and then the removal of the side covers to access the capacitors for replacement. The photo to the right shows the replacement fiber tube socket (the one on the left) that was installed to replace an incorrect bakelite socket that had been installed in a repair done many years before. Luckily, I found an exact style fiber socket with the correct "42" tube ID in one of my junk boxes. Nowadays, we probably shouldn't call them "junk" boxes since the parts they contain are so necessary for restorations and are so difficult to find otherwise.

None of the grid leads were the correct, rubber insulated wire. The correct color is cream. I found that the old style large gauge round AC power cables use rubber insulated wire and the neutral wire is a creamy white color rubber insulation material that provided a good match for the original grid leads. Fortunately all of the grid caps were original and were reused for the new grid leads.

In addition to the rubber insulated grid leads, there were two shielded cables that needed to be rebuilt. The output cable from the Detector to the First Audio stage had deteriorated and was on the verge of shorting. By using the same rubber insulated wire I was able to "push thru" a new center conductor that looked just like the original. A replica shielded cable had to be made for the BFO output to the Detector cable. This had been a piece of modern phono cable which we replaced with a replica that matched the original style.


photo above: the variable coupled IF system levers. The top one is good, the bottom one is broken

Rebuilding the Variable Coupled IF Transformer System:
   I noted that the Selectivity control shaft did not stay where set and that the action felt very loose. Inspection revealed that two of the three levers were broken and were not even moving the IF plungers. Removal of the variable-coupled IF transformers showed that earlier repairs had broken the fiber board guide holes on two of the transformers. The parts set provided the necessary replacement parts including two good condition levers. See the section "Guide to Restoring Super-Pro Receivers" in this web article for more details on general rebuilding of the variable coupled IF section.

Tuning Meter: The Tuning Meter has a more modern movement that replaced an open coil original. The case and glass are original. It was necessary to install a shunt inside the meter case so that the meter would have the proper range. This had to be selected after the receiver was operational since there are no specs for what the original meter movement was. Testing showed that 7.0mA fs gave the best action and range. If I can ever find an original functional meter for either an SP-10 or SP-100, I will replace this meter since it is not totally original.

Replica Dust Cover: The original dust cover had been discarded many years ago by WMI. I tested the fit of the dust cover from my SP-100 and found that it fit perfectly. I bought some 20 ga. sheet metal and did the layout for the dust cover. I carefully cut out all of the vent holes and marked all of the bends necessary. I had a local sheet metal shop do the bending and spot welding. The replica fit perfectly when I got it back from the shop. Next, I had to make the studs out of 8-32 threaded stock. I didn't have the necessary small rivets used originally to hold the studs in place. Instead, I used 2-56 screws and nuts to secure the studs. I did modify the screw head to look like a rivet. When installed only the nuts inside give away the fact that these are replicas. I was lucky that fellow ham KØDWC had some of the original 8-32 cap nuts and I made the rear 6-32 thumb screws. Painting was all that was left. The inside was painted gloss black and the outside painted with Krylon Black Wrinkle finish that is "baked" to force the wrinkle. The finished cover is next to impossible to distinguish from the original, except that we don't have the rear ID plate - but we did drill the holes so it would look as if one was there at one time.

photo right: the finished WMI SP-10 chassis which also shows the variable coupled Detector and AVC transformers

photo above: The finished WMI SP-10 SN: 576 receiver with dust cover installed

Final Testing and Alignment: I modified a late manufacture SP-100 power supply to use with this SP-10. By installing a 250 ohm 5 watt resistor between the +385vdc supply and the PS output terminal, the voltage is dropped to about +360vdc at the SP-10's P-P 42 audio output tubes. The other voltages are the proper level for an SP-10. The late SP-100 PS was the one that came with the SP-10 and probably had been with the receiver for a considerable period of time. Being a late build, this PS had a filter choke installed to sub for the speaker field which allows the use of a PM speaker. I also was using an SP-200 power cable which has larger tube heater wires for a smaller IR drop across the cable. Upon power up there was no signal, just audio hiss. Within a few minutes the smell of "hot resistor" was noted and the power was shut down. The Mixer plate load resistor had gotten very hot but what caused it was unusual. The wiring for the tube heaters in the SP-10 is unconventional in that some of the heater wires pass over the tube sockets instead of around them. In this case the plate pin of the Mixer tube was contacting the heater wire and the insulation was thin or gone - anyway, a short occurred and caused the hot resistor. I moved the wire and then applied a paint on insulation (black) to assure the problem didn't happen again. Power was again applied and this time the SP-10 burst into a wonderful audio sound experience. I was amazed by the sound quality of the AM BC station that happened to be tuned in. Unbelievably wide range, bass-laden music. I was impressed. Not that there weren't some minor issues, though. I still had to set the shunt in the Tuning Meter and perform a full alignment. After that, the audio from the SP-10 is just fabulous. It sounds very bassy and wide range when receiving AM BC or SW BC when conditions allow for great reception of the South America SW stations that play music. AM hams that run some power sound incredible. The entire operation of the receiver is exactly as described in the SP-10 manual and certainly a pleasure to listen to. Trying to imagine what an original owner must have thought of his new Super-Pro in 1936 is always interesting - too bad they were so expensive that few hams could afford them in 1936. Certainly signals are quite different today, but still it must have been thrilling to receive shortwave stations from around the world on a then new Super-Pro.

Restoring the 100 Series "Super-Pro"

I owned this 1937 SP-100 for about four years before I decided to restore it. It was an e-Bay purchase that happened to have been offered by a seller that was located only about 25 miles away. I e-mailed, asking if I could come over and look at the receiver before I bid on it. The seller was more than happy to agree so I drove down to Gardnerville, Nevada to have a look. The SP-100 was in good physical condition and was complete with the matching serialized power supply. I bid on it and won. So, with another trip to Gardnerville, I became the owner of this great receiver. I didn't expect it to work and a quick check over found several things that needed to be repaired before it was powered it up. I only did "quick fixes," just to see how the receiver would perform. I used the receiver a few times but never trusted it with long operating stints. I had planned to restore it long before I actually did - but delays on projects seem to be the norm around here.

At the end of 2007 I finally got some time to do this SP-100. A detailed inspection of the chassis showed that many capacitors had been replaced over the years - mostly using Sprague molded caps similar to "Black Beauties" but without the color-code stripes. Some of the resistors had also been replaced since they had burned up when the original associated capacitor failed. All of the other parts,...IF transformers, AF transformers, the meter, etc. were all original and in good operational condition. I wanted to perform a "museum quality restoration" on this SP-100 as it was an excellent example of this rare receiver. Our "museum quality restoration" results in a fully functional receiver using the original design circuit with the entire appearance of the receiver as close to original as possible with the patina of age preserved. The under chassis appearance has to look original, therefore all capacitors are "re-stuffed" with new film caps inside the original capacitor shell. Any resistors that are replacements have to be the original style part. Any defective parts are rebuilt and if that is not possible, a correct style and manufacturer part is used as a replacement. When the rebuild is completed, the receiver is fully tested and aligned. The completed receiver can be used as a reference, illustrating how the originals looked - on both the exterior and the interior of the set. Also, I had wanted to document the performance of this receiver, so it was necessary for it to function reliably at its design limits.

photo right: the restored SP-100 chassis - note the differences in this chassis and the SP-10 chassis. The lack of adjusters on the Det and AVC transformers, the metal octal tubes used and the different style audio transformers. 

photo above: The bandspread condenser side of the RF box showing the "hidden" paper-wax caps inside. Note the Isolantite material used for the coil mounts and the variable condenser mounts. This was a low loss ceramic material. 

Rebuilding Capacitors

When checking the schematic, the parts list shows that 35 paper-wax capacitors are used in the SP-100 circuit. But comparing that information with what can be seen under the chassis, it becomes apparent that nine capacitors seem to be missing. They aren't - they are located inside the RF box, inside the 2nd Detector Output Transformer and inside the Amplified AVC Output Transformer. The RF box caps are difficult to see let alone replace. Disassembly of the RF box is necessary to have easy access these five capacitors. Unfortunately, you can't just remove the side covers - you have to remove the entire RF box from the chassis first. This isn't as difficult as it sounds - eight wires must be disconnected, the front panel removed and 10 mounting screws taken out to remove the RF box.

photo above: The completed RF box fully assembled and ready to install. Note the new grid leads and grommets. There are 33 screws for the bottom shield, 20 screws for the two top covers and 8 for the back covers along with the 50 screws for both side covers. Total of 111 screws just to hold the shields and covers in place.

There are also three paper-wax capacitors inside the Amplified AVC Output Transformer. In the photo to the left the .02uf and one of the .05uf caps are visible. The other .05uf cap is on the backside of the fiberboard mount.

There is one remaining paper-wax capacitor inside the 2nd Detector Output Transformer. It is a .05uf shown in the photo to the right. What appears to be trimmer capacitors are actually an assembled fixed capacitor. There is one on the back of the board also. When the screw is tight, the capacitance is at the required value. These are original and are Hammarlund parts. Behind the board was a 5K ohm resistor that was burned and measured 1K ohm. This was replaced with a correct vintage part.

Also shown in the photo to the right is the deplorable condition of the grid leads. More on this problem below.

Since more than half of the original paper-wax capacitors had been replaced in the past with plastic molded style caps, I had to locate 18 Cornell-Dubilier "Tiger" paper-wax capacitor shells with the correct values to build my restored caps. I had an old SP-200 parts set that became the "donor" for these correct capacitor shells. I use a heat gun to melt out the old original cap leaving just the shell. I wipe the excess wax off while the shell is still hot to clean the surface. I then install a new metalized-film capacitor of the correct value inside the shell. I orient the caps all in the same direction with regard to the outer film marker on the shell though it really doesn't matter with modern film caps. I secure the new cap in place with hot melt glue and when that has cooled enough, I fill each end with brown colored sealing wax. The whole process takes about 10 minutes per capacitor. The results are shown in all of the under chassis photos - all of the paper-wax capacitors shown have been rebuilt. I install the rebuilt caps in the proper direction. This whole process is for cosmetics, it does nothing to help performance. If under the chassis appearance is not important, then just install the correct value, modern capacitor.

It is easier to work on the SP-100 chassis if the RF box is removed - you have to do this anyway to replace the five capacitors located inside. Also it is easier to work on the RF/IF section if the shield between those two sections is removed. The photo to the right shows the RF/IF area of the chassis with the shield removed. Also to access the bypass caps on the IF amplifier tubes it is easier if the small fiberboard component mounts are placed out of the way by removing the mounting screws and a few of the wires. This allows open access to all of the parts that need replacement. There are nine of these small fiberboard component mounts under the chassis but only the three over the IF amplifier tubes need to be moved.

photo above: The Crystal Filter assembly with new grid lead and new connecting wire for the Phasing Condenser and Crystal.

Other Restoration Work

Once all of the capacitors were rebuilt, it was necessary to replace all of the grid leads from all of the IF transformers and AVC Output Transformer and the BFO coil. The Crystal Filter assembly is rather complicated in its construction and was removed from the chassis in order to easily disassemble, replace the grid lead and the connecting leads to the crystal and the phasing condenser and then reassemble. These grid leads were originally rubber insulated stranded wire but the rubber had become "lumpy" and had hardened, becoming brittle. Any flexing would break the rubber off of the wire. I used a cream colored cable jacket that was off of old telephone hook-up cable. I stripped the outer jack off and then inserted a stranded wire into the jacket to build grid leads that had the correct feel and looked pretty close to the original. I was able to reuse all of the original grid caps. All new rubber grommets were installed also.

All resistors were checked for value and all were found to be within 20% of the correct value except the burned resistor in the 2nd Detector Output Transformer. 

While most of the assemblies are off of the chassis is a good time to clean the chassis. I just used Glass Plus and a horsehair brush since the chassis was in good condition. Also, this is a good time to thoroughly check the Sensitivity potentiometer. This part cannot be removed when the RF box is installed. In fact, replacement of this part normally requires removal of the front panel and the RF box to accomplish, so now is the time to check it (this is also true of the "ON-OFF" toggle switch.) I disassembled the Sensitivity pot and cleaned it but it was going to become a future problem after re-assembly.

photo above: The underside of the chassis complete except for the 33 screws that hold the bottom plate on the RF box. Also note that there is absolutely no clearance behind the Sensitivity pot if it needs to be removed. Same goes for the "ON-OFF" switch.

 At this point the receiver was ready to reassemble. When replacing the RF box, the two pinch wheel drive housings have to be loosened and then the two dial edges guided in between the drive wheels as the RF box is placed on the chassis. Once the dial edges are engaged then the pinch wheel housing can be retightened and the dial drives tested. There shouldn't be any slipping and the drive should be ultra-smooth. Then the screws that hold the RF box can be tightened. The idler gear for the dial mask drive needed to be mounted and adjusted - the assembly can be moved vertically for centering the dial mask and then the screws tightened. When the front panel is bolted in place then the Crystal Filter panel can be mounted followed by all of the knobs and the tuning meter. I tested all of the tubes and found them to test fine - at least in the tube tester. The receiver was now ready to test and align. I had a couple of problems turn up after a short period of operation. First was a noisy 6B7 tube in the detector stage. This showed up as a continual but erratic "rushing-thumping" noise that varied with the AF Gain control. Second was a "noisy" 6F6 in the push-pull AF stage. This showed up as soft, weird noises (erratic audio oscillating) that was present even when the AF Gain was reduced to zero. I guess this shows that even the best tube testers don't catch everything.

During the alignment another problem showed up. Audio distortion was noticeable while in AVC and the Sensitivity control didn't reduce the RF/IF gain when the receiver was in AVC. This problem was caused by a bad solder joint in the AVC line to the RF amplifiers and an intermittent Sensitivity pot that ultimately had to be replaced. The finished SP-100X has fabulous audio with plenty of power, formidable bass and a very wide audio response when in the 16KC IF bandwidth. Excellent dial accuracy - easily better than the 0.5% specification. Sensitivity is at the limits of what antenna noise is present and selectivity is sharp in the 3KC bandwidth and ultra sharp with the Crystal Filter. AM-BC and SW-BC stations sound incredible. Vintage AM Ham stations that run some power (like retired AM BC transmitters) are a pleasure to listen to. Simply a great receiver.

photo above: The finished 1937 SP-100X sn: 3387 (ps is sn: 3388)


Competition Comparisons

The Super-Pro was introduced in March 1936. What was the competition at the time? There was the famous National HRO, a great performing receiver at a slightly lower price than the Super-Pro. The RME-69 was available and by November you could also buy the DB-20 preselector - for a package price about the same as the HRO. Hallicrafters was offering an inexpensive and popular receiver in the contractor-built SX-9. Patterson offered receivers that were built at the Gilfillan Bros. plant in Los Angeles, California. RCA had the AR-60 Professional Communications Receiver but at nearly $500 nobody bought them and consequently only a few hundred were built. The following are some details on the best of the 1936 competition.

The National HRO

The HRO was an incredible receiver, especially when it came to its sensitivity and almost unlimited bandspread. The price of a new HRO was usually around $200 with a set of four coils, power supply and a speaker. If the new owner learned to use his HRO, he could get impressive results with the receiver. But, there were several odd things about the HRO. First were the plug-in coil assemblies that made changing bands a hassle. Even storage of the unused coil sets could be a problem. If general coverage was desired (for example, to check shortwave broadcasts) the owner had to move four small screws on the coil set and screw them into different locations. Then remove and replace the screws again to go back to bandspread. Not to mention that the coil assembly had to be removed from the receiver each time to change screw positions. It was such a hassle, that most hams just forgot about using the HRO for anything other than a "ham band" receiver. Then there was the micrometer dial,...elegant in design and super smooth to operate but the new owner had to read the dial then correlate that to a graph on the coil set to determine tuned frequency. AM BC coverage required the purchase of two more plug in coil sets. With all of these quirks, why was the HRO so popular? Because it delivered absolutely the quietest front-end performance meaning that weak DX signals were easy to copy. Also, the ham bandspread was endless - equivalent to a linear dial over nine feet long! If you wanted a receiver for performance first and convenience last, the HRO was for you. Compared to the Super-Pro, the similarity is of course the front-end with double-preselection. The HRO will have the edge on greatest useable sensitivity but not by too much. Setup for bandspread, the HRO is unbeatable for tuning resolution. However, the variable Selectivity of the Super-Pro, its direct readout dial accuracy and its powerhouse audio are its definite advantages over the HRO.

photo above: 1935  National Company  HRO  Receiver

The RME-69/DB-20

The RME-69 was a popular receiver with hams. It wasn't too expensive (at first) and delivered great performance when used with a pre-selector. So much depended on the pre-selector that finally, in November 1936, RME provided one of their own, the DB-20. This addition gave the user three tuned RF amplifiers, providing top-notch sensitivity and image rejection. The tuning and bandspread dial drives are velvet-smooth with plenty of vernier reduction. The dial illumination is great - it has to be since the dial nomenclature is so small it can hardly be read without magnification. Six tuning ranges gave the user coverage from .54 up to 32MC. The power supply was built-in. About the only accessory needed was the speaker. The RME-69/DB-20 was a great performing combo but it had its quirks. There are no trimmers for the RF or Mixer coils, a Compensator control keeps those stages peaked for any tuned frequency. But, it's very easy to tune the Compensator to an image on the higher frequencies - a disaster for correct front-end tracking. The standby function is accomplished by pulling out the Audio Gain knob/shaft but there is no remote standby provided. Some of the construction is a little on the cheap side - especially the compression trimmers provided for the LO alignment. All in all though, the great performance made up for any shortcomings and the RME-69/DB-20 gave the hams a relatively compact receiver packed with lots of features. Selling price was about $200 when the DB-20 and speaker were included. Compared to the Super-Pro, the RME-69/DB-20 combo will perform as well on sensitivity but the continuously adjustable bandwidth and the 14 watts of audio power is the Super-Pro's advantage.

photo above: 1937 RME-69 Receiver, DB-20 Preselector, RME speaker

Patterson PR-16C

The PR-16C was designed by Karl Pierson and is mostly remembered for using parallel-connected RF amplifier tubes. The argument was that parallel tubes doubled the transconductance and that increased gain and lowered the thermal noise. It also reduced the grid input Z but nobody (except James Millen of National Co.) seemed to realize that. Additionally, the PR-16C was advertised as having two RF amplifier tubes - Patterson may have even mentioned that the tubes were in parallel but didn't elaborate that only one set of RF coils per band were used and that the PR-16C was really just a single-preselection receiver and was just as prone to image problems as any other single- preselection receiver. The other notable feature in the PR-16C was the incredible audio amplifier - three stages of push-pull audio amplification. A 6A6 dual triode is transformer coupled to two 76 triodes that are in-turn transformer coupled to a pair of triode connected 42s. The audio power was rated at 18 watts and the receiver actually sounds incredible when driving a high quality speaker. The PR-16C also had two-speed tuning, band-in-use masking, chrome plated chassis and, of course, 16 tubes. Really, a lot of features for the list price of $101 - but its overall performance as a communications receiver couldn't come close to the double-preselection receivers,...the Super-Pro and the National HRO, or the RME-69/DB-20 combo. The PR-16C's audio section is certainly comparable to the Super-Pro, if fact, it even has more power. The PR-16C is an incredible audio machine but is somewhat lacking at the RF/IF section.

photo above: 1936 Patterson PR-16C

Hallicrafters SX-9

This was Hallicrafters' last offering of a receiver that built by contractors. After the August 1936, all Hallicrafters' receivers were built by Hallicrafters' assemblers at their new plant that had been purchased from the Echophone Company. It is likely that the SX-9 receivers were built by Howard Radio Co. or by Wells-Gardner Co. - they were the major contractors in the Chicago area, but RCA also would build radios as a contractor for some companies. The SX-9 is a single-preselection receiver that uses all metal octal tubes. It also had bandspread, built-in speaker, and variable injection BFO. The power supply was built-in. The hams bought SX-9 receivers in droves, mainly because it sold for about $90 with the crystal filter. It was popular and with an external preselector, like one built by Peak or similar, one could get pretty good results. By itself, the SX-9 has all of the image problems one would expect from a single RF stage receiver. Certainly price was the major factor in the SX-9's popularity. As one would expect, the SX-9 can't compare to several of its contemporaries, including the Super-Pro. That's not to say that an experienced operator couldn't get great results from the SX-9, especially with the addition of a preselector. Most hams know that experience at using the equipment, whatever it is, sometimes is more advantageous than the results gotten with the best receiver available used by an inexperienced operator. 

photo above: 1936 Hallicrafters SX-9

Hallicrafters SX-28  vs  SP-200 Super-Pro

Since the SX-28 is such a popular receiver and many hams and collectors are familiar with its performance I am including a direct comparison of its features and performance with those of the SP-200 Super-Pro. Both were available in 1940.

At $159, the SX-28 was a bargain compared to the $275 that the SP-200 usually sold for from discount dealers. With a built-in power supply, frequency coverage from .54 to 43 MC and a respectable 8 watts of P-P audio from a pair of 6V6 tubes, the SX-28 did offer a lot for the low price. Additionally, the Lamb Noise Silencer circuit was the best noise limiter circuit available. Also, a ham band calibrated bandspread was included. About the only accessory needed was the speaker. Performance of the SX-28 was at the limits of the designs of the day. But, the SX-28 had its quirks. The Antenna Trim doesn't function on the lower two bands, double pre-selection is only used on the upper four bands and the receiver initially had a noticeable hum level in the audio. Both the SX-28 and SP-200 have a long warm up time, resulting in drift that lasts for hours. Certainly, as far as sensitivity and the ability to successfully allow world wide communications, the SX-28 performs equally as well as the SP-200. When the selling price is thrown in, then it becomes obvious why most hams bought the SX-28 rather than spend an additional $116 for the few advantages the SP-200 offered. What advantages? The SP-200 has better quality construction, it is certainly more a more reliable receiver built with better quality components, the infinitely adjustable selectivity is a better system, the front-end double preselection on all bands with a more accurate dial readout due to the better tuning unit construction and, of course, the more powerful and superior quality audio. But, in the long run, hams felt that it just didn't make up for the SP-200 disadvantages of having less frequency coverage per single receiver, dealing with a separate power supply and the fact that the bandspread is not calibrated in direct frequency readout. But certainly the most important factor was cost - $275 versus $159.

photo above: 1940 Hallicrafters SX-28

Photo Gallery of Collector's Super-Pro Receivers

Here are some photos of Hammarlund Super-Pro receivers that belong to collectors, hams and other Super-Pro enthusiasts. Interesting variations, extra nice condition, rare models, etc. Send in your photos with your comments on your receiver's performance, its acquisition or on your restoration.       E-mail to:  WHRM - SUPER PRO PHOTOS   



W8TOW - Steve picked up this great example of the SP-10 at Dayton 2007. It is serial number 720 with the matching sequentially serialized power supply. It is the table top configuration. This SP-10 was once owned by W2KW. Steve has gone through the receiver and it is in excellent operating condition. A characteristic of the SP-10 circuit is that it can easily overload on modern, powerful AM signals when the receiver is operating in AVC with an efficient antenna system. Steve's SP-10 exhibited this typical behavior. Steve changed the bias source for the RF Gain pot, removing it from the AVC line to eliminate the overloading issue. Steve says that now the SP-10 is a pleasure to use on all signals and, that in most cases, it will actually "out hear the 75A-4."


W9JDT - Bob has owned this very nice example of the Super-Pro for about two years. It has the optional crystal filter installed and is properly designated as the SP-10X. This receiver's serial number is 979 and the matching power supply is serial number 987. Sometimes non-sequential serial numbers do show up. This particular pair was probably originally purchased from one of the many radio discount-dealers of the thirties rather than from the factory which accounts for the "close" but not sequential numbers. Bob's SP-10X is partially re-capped and he uses it with a Heathkit DX-60 on the 80 meter AM net.



SMØAOM - Karl-Arne owns two of these Swedish Markradiomottagare MRM-5 receivers. These are Hammarlund SP-100 receivers built especially for Swedish government customers. The frequency coverage is different from the standard Super-Pro, covering 200kc to 400kc and .54mc to 10.0mc. The MRM-5 shown is a table top version with a serial number in the 16XX range indicating 1937 as the year of manufacture. The photo shows the receiver before clean-up and alignment. Karl-Arne uses the receiver in his "thirties station." He also owns MRM-5 sn 1352, a rack mount version along with the Swedish manuals dated October 1937.


SP-200X with Faux Woodgrain Panel and Table Cabinet

KG5V - Chuck sent me this old photo of an SP-200X he owned from 1957 to 1965. It was purchased when Chuck was a novice, KN2TPU, out of a New York City newspaper ad for the price of $75. The seller was not a ham and it was obvious that the receiver was not out of a ham shack but had probably been in the living room or den of a genuine radio enthusiast. Of note is the faux woodgrained front panel. It was previously thought that only the SP-150 console was fitted with a faux woodgrained panel but it is obvious from the photo that this receiver is an SP-200X. Note the knobbed switches over the phone jack and the AVC switch. Also the crystal filter is integrated into the panel as the SP-200X. The special cabinet was also faux woodgrained to match the panel. All of the knobs were brown color and the band switch skirt was faux grained. Speculation is that this receiver was built by Hammarlund for a high ranking employee (engineer? manager?) or for some NYC executive that warranted the extra cost of producing this "special" SP-200X. Over the years while the receiver was in Chuck's ham shack, visitors would comment on the unusual look of this SP-200X receiver. Unfortunately, while Chuck was away in the service his parents sold his faux woodgrained SP-200X (1965) via another newpaper ad and for the same price of $75. Does anyone know where this receiver has ended up today?


SP-100S Special Diversity Receiver built for CODECO

KE7SE - Jack and his father own, this SP-100S (SN:4167B) - a special build from Hammarlund for the company Codeco in conjunction with the Civil Aeronautics Administration - CAA. The receiver has two controls that are indications of its intended use. The knobbed-switch to the left of the Main Tuning knob is marked "HF OSC" - "INT" - "EXT" with another knobbed-switch to the right of the Band Spread knob marked "DIV OUTPUT" - "ON" - "OFF." These controls are indicators this SP-100S was part of a Space Diversity receiving system. Two or three receivers would be used, each with their own antenna separated from the other antennas by great distances (usually 1000 feet separation in commercial/military set-ups.) To achieve the diversity effect (reduction of fading) all system receivers have their AVC lines connected together and their Diode Load lines connected together. The result is that whichever system receiver is responding to the strongest signal at any given time, that receiver controls the AVC line and also has the maximum voltage on the Diode Load line. To keep the system stable, one common Local Oscillator can be used for all receivers allowing equal drift and common tracking of the system frequency. The DIV OUTPUT switch allows individual receiver balancing for equal response to a test signal. Does anyone have information on a late-thirties Hammarlund Diversity Receiving Set-up?



N2QEI - Pete has found this very early SP-200SX receiver with the matching power supply. The power supply has the terminal strip for the field coil connection for an electro-dynamic speaker. The SX version of the SP-200 was built specifically for the ham market since the receiver covers 160M up to 10M, 1.25mc to 40mc. Pete is beginning the electronic restoration of the great example of the early SP-200SX.


SP-100X with Original 12" Jensen Hi-Fidelity Speaker


N6YW, William Yates - owns this SP-100X (sn 2746) with the original power supply (sn 2785) and the rarely seen, original Jensen Hi-Fidelity 12" speaker. The standard speaker included with the purchase was a 10" speaker but an optional speaker was the 12" Jensen. The speaker has two cables with the standard Hammarlund "spade-lugs on terminal board" connectors, one for the audio output from the receiver and one for the field coil, which is connected to the power supply.

Also interesting is the addition of a cathode-ray tuning eye tube to compliment the "difficult to read" Carrier Level meter. This appears to be a professional installation as the proper "eye lid" bezel is used in the mounting of the eye.


WA7YBS - July 22, 2016 - I picked-up this very nice condition SP-100LX from KB6SCO who was helping to liquidate an estate of a long-time ham. The receiver is shown "as found" without any cleaning. The serial number on this "LX" is 2730 which is about in the middle of production meaning it's probably from mid-1938. The receiver has indications that it was supplied to the Signal Corps and there's an MFP date stamp (1940) on the chassis. The dust cover SN matches the receiver SN. The power supply that came with the receiver seems to be somewhat newer with a serial number of 5164 which probably originally was for an early SP-200. It also has Signal Corps stamps so I'm assuming that while the "LX" was "in the Army" the original power supply was separated from the receiver. For the 100 Series and later SPs, all power supplies are basically interchangeable. The band spread knob is certainly not original. I also suspect that the white pointer type knobs might not be original although all the knobs are the same. It might have been something that the Signal Corps wanted. The paper tag on the front panel is the "Junior Achievement Company" sticker. On the rear apron of the chassis are two Jones' plug sockets, one for remote relay and one for B+, Fil outputs. Neither are original but the installation is neatly done. As for tubes, the Mixer and LO have later type tubes that are installed into elaborate adapter sockets to preserve the original sockets. Also, 6V6 tubes installed substituting the original 6F6 tubes.

Restoration details will be coming in the next few months,... 


The SP-400, SP-600 and Conclusion

The Hammarlund Super-Pro of the thirties was just what Hammarlund had intended for it to be - a "standard" by which all other communications receivers would be measured. By the time WWII began, the Super-Pro had matured into a first-rate, ruggedly-built receiver that the military could use without reservation or modification. Immediately after the war, Hammarlund began to offer the new SP-400X Super-Pro. The new receiver redesigned the wartime SP-200 by changing the frequency coverage to the more standard .54 to 30MC (or 1.2 to 40MC for the SX version) and changing the audio tubes in the later versions to 6V6 types with audio output of 8 watts with dual audio output Z of 8 or 500 ohms. Also the IF frequency was changed from 465KC to the more standard 455KC. Other than those changes, the receiver was still very familiar to former SP-200 owners. The styling changes to the front panel using a very thin smooth finish paint and miniscule knobs (that are normally not found on surviving examples today) have resulted in very few SP-400 front panels remaining in good condition. Most are found with severe wear around the control nomenclature. Unlike its immediate predecessor, the SP-400 is usually found in the table top cabinet rather than in a rack mount configuration. The SP-400 was only around for a few years, 1946 to 1948. It was not built in large quantities and is not seen too often. Hammarlund was biding their time and designing a totally new Super-Pro that would again set the "standard" for what a modern communications receiver would be - the famous SP-600 series. That the SP-600 owes a lot to its predecessors, especially in design approach, is obvious. The  SP-600 is a well-known receiver with a plethora of information available on the web, including our own article "Rebuilding the Hammarlund SP-600" - link below in Navigation Index.

Today, most hams would choose the SP-600 receiver for a vintage ham station since it is the most modern version available. Most collectors are interested in either the SP-10, SP-100 or SP-150 because of their rarity. But, those receivers in between - the SP-200s, both military and civilian, and the SP-400, are incredible performers and are now finally being recognized as such. A desire to build "the best" regardless of cost defines all of the Hammarlund Super-Pro receivers. They are examples of what one of the best manufacturers in the United States could build during a time when this country produced absolutely the finest radio equipment in the world.




photo right top: SP-400 owned by W2EMN

photo right bottom: SP-600JX-21, early version from 1953 (no product detector)

Information Wanted:

As with our other articles on the National HRO, the Hallicrafters SX-28 and DD-1 and the RCA AR-88 Series, we are always updating this webpage to provide the most accurate information available. We depend on hams, collectors and others interested in preserving our radio manufacturing history. We are always interested in any receiver or any information that seems to contradict any of the information presented here. Accuracy is our goal, so let us know what you have or what you know.
Send us your Super-Pro serial numbers, we will add them to the Super-Pro Serial Number Log. Eventually, as more and more numbers are gathered, a more accurate picture of the Super-Pro production will be possible.

We are particularly interested in the following:

1. Any matched set of Super-Pro and Power Supply sequential serial numbers or any matched sets that don't have sequential serial numbers - this will help to clarify how some sets have sequential numbers and others don't. If you know your receive-ps history it will help.

2. If you have an operational SP-10 - please e-mail your opinion of the receiver's performance.

3. Any SP-10 with the 600 ohm Z audio using in-line resistors and a phone jack - confirms that this version was built and what its frequency coverage was.

4. SP-100LX versions with non-standard LF coverage, standard covered 100kc to 400kc in two bands. Some ads and Riders' suggest that 150kc to 300kc was the LF tuning using only one range. Has anyone seen any LX with non-standard LF coverage?

5. Any variations seen in the Super-Pro receivers that are factory original - like odd tuning ranges, or non-standard parts that are original installations. Please also include serial number.

6. Any SP-200 or BC Super-Pro with a serial number higher than 30000 - more accurate estimate of the total production.

e-mail Super-Pro info to:  WHRM - SUPER PRO INFO



1. Hammarlund Owner's Manuals for SP-10 and SP-200 - These provide information on design intent and expected performance in addition to schematics, alignment and other information.

2. Rider's Perpetual Troubleshooter's Manuals - Various volumes contain information on the Comet -Pro, SP-10, SP-100 and SP-200. Many times this is the exact same information from the owner's manuals.

3. U.S. Army Signal Corps - TM-11-866 - This Army manual contains a wealth of information on the SP-200 series, specifically the BC-779, BC-794 and BC-1004 receivers. It also includes the various power supplies, the R-129/U receiver and detailed circuit descriptions and drawings. TM11-896 provided details on the 1948 Wickes' modified BC-794 receiver.

4. QST - Various issues from 1936 to 1940 - Contain original ads that provide a time line for engineering and model changes, company performance expectations, sometimes interesting users.

5. Communications Receivers - The Vacuum Tube Era: 1932-1981. Raymond S. Moore - The best reference book on communications receivers, provided Hammarlund history and general specifications on the receivers.

6. The Hammarlund Historian - Website - Provided the Oscar Hammarlund history. The website provides a lot of information on the SP-600 series.

7. Popular Mechanics - December 1937 issue contains the article on the SP-100SX - shown in its entirety above in the SP-100 section.

8. BAMA - Boat Anchor Manual Archives - source for Hammarlund Super-Pro manuals on-line.

9. Thanks to Todd KA1KAQ, Steve W8TOW, John W3JN for their help and detailed information on their Pre-war Super-Pro receivers and variations they have encountered.

10. Thanks to Steve Bringhurst for providing info on these websites showing the foreign military Super Pro copies, the Australian AMR-200,  

       and the Russian KV-M,       Thanks to Karl-Arne Markstrom, SMØAOM, the the information on the Swedish MRM-5/SP-100 receivers.

11. Thanks to Inland Marine Radio History Archive for the photo of WMI in 1937. Here is their website URL:


Appendix A  -  More Information on the Hammarlund Comet "Pro" Receiver

photo above: Ad for the Comet-Pro installed in a console cabinet. From "Radio News and the Short-Wave", March 1934. Note that the plug-in coils are mounted in a coil storage rack located under the lid on the left side of the cabinet. This is a later version of the Comet-Pro shown with AVC.

Comet and Comet "Pro" Features

Introduced in 1931, the first version of the Comet Pro was called the Comet "All Wave" Receiver. This version used 24A, 27, 35 and 47 type tubes. The power supply rectifier was a type 80. The IF frequency was 465KC and there were two stages of IF amplification. No RF amplifier was used so only two plug-in coils were used  for each tuning range. The plug-in coil set covered 250M to 16M in four pairs. A 240M to 550M AM BC band coil set was available rather early in production. Initially, the main tuning (WL and OSC) may have had the dials on the outside of the panel. Later they were fitted behind the front panel and viewed through cutouts in the panel. The Comet receivers were sold as a chassis or in a console cabinet. Later, a table cabinet was available that was a combination of wooden back and sides (painted black) with a metal lid and face plate, though some receivers were still sold as console "entertainment" radios. A change to the audio output tube came in 1932 with a type 27 taking the place of the type 47. Additionally, the field coil speaker connections were eliminated and an earphone jack installed with output terminals for a loud speaker with output transformer. When this version was used and where thunderous volume was desired, a separate audio amplifier was provided. By 1932, the Oscillator Coil wiring was slightly changed and individual coil shields were added for both plug-in coils. Possibly at this time the Comet was renamed the Comet Pro.

In 1933, the entire receiver was given a complete upgrade and the new name, Comet-Pro, implied that the receiver was now "professional-quality" in both performance and design. The tubes were changed to type 57 and type 58 in the front end with electron coupled oscillators used in the LO and BFO. The audio tube was changed to a 2A5 with an audio output transformer added with a 4K ohm Z output and a tapped output for earphone operation. The Wave Length coils were redesigned slightly for the new antenna input connections that allowed a dipole feedline to be used. The cabinet was changed to an all-metal fabrication, though the wooden version was still available on request, as was the console cabinet. A short time later, a Lamb-style crystal filter became an available option, followed by a 10 Meter coil set and by the end of 1933, Amplified AVC was added to the options. This required the addition of another tube, a 2B7 duplex diode-pentode for the AVC functions.

From 1934 through 1935 not too many changes are incorporated into the Comet-Pro since Hammarlund was primarily working on the Super-Pro receiver. A "Stand by" switch function may have been added since some late Comet-Pros appear to have an extra toggle switch installed on the panel. Coil IDs were changed sometime between 1933 and 1934 with the engraved wooden handles changed to now use a recessed paper ID tag protected by a plastic cover. There were probably more late improvements to the Comet-Pro and I'll add to this description if more information surfaces.

Performance Expectations Using the Comet Pro

The 1933 and later Comet Pro receivers tune using separate OSC and WL condensers and a four section condenser in parallel with both the OSC and WL condensers that is used as a bandspread tuner. The power is turned on with the lower left knob which also is the Tone control. Since AVC was not optioned on my Comet Pro, the Sensitivity control (lower right knob) has to be adjusted for each station's particular strength because the audio gain is always at maximum and is not adjustable. BFO is tuned on with the toggle switch and the frequency control is the "swing arm" on top of the BFO coil can. The Crystal Filter has an "on-off" switch and a Phasing control. The design of the coils places the ham bands about in the center of each range, so setting the OSC and WL at "50" is a good place to start. The actual "peak" for the WL dial will somewhat depend on the antenna impedance but it should be fairly close to the OSC setting. The knobs nearest the arc scales control the OSC (left) and WL (right.) The bandspread (center larger knob) can then be tuned in search of signals. The Bandspread dial is illuminated and is projected onto a frosted window. All tuning scales are 0-100 and require "scale versus frequency" graphs to determine exactly where the receiver is tuned. The graphs were in the Hammarlund Comet Pro manuals. The Comet Pro has good sensitivity and the bandspread allows for easy tuning on all amateur bands covered. The 80 and 40 meter bands are especially well spread out with the 40M band covered by a little over 100% of the bandspread and 80M uses over 200%. Surprisingly, SSB signals demodulate quite well since we are using the Sensitivity control to set the volume. This provides the correct ratio of BFO injection to signal at the detector for good CW and SSB copy. CW signals are great on a Comet Pro. AM signals sound a bit different since there isn't any AVC. This results in the operator running the receiver with minimum front end amplification and maximum audio gain. This is great for noise reduction and QRM but full-bodied AM isn't really possible except on the AM BC band where signals are at a constant high level. High power ham AM signals also sound great but sometimes QSB (fading) will make enjoyment difficult as the Sensitivity control must be constantly adjusted for rapidly changing conditions. 

photo: The 1933 Hammarlund Comet Pro with Crystal Filter option but without the AVC option.

Images are not a problem with the coil sets DD, CC or BB but the highest frequency coil set AA (10MC to 20MC) is plagued with images. This isn't unexpected in a receiver without a TRF amplifier ahead of the mixer tuning above 10MC. With the AA set, it is best to use the graphs to set the desired frequency since the images are about the same strength as the tuned signal. The addition of an aftermarket pre-selector would all but eliminate the image problem on the AA range (see last photo in this section.) Drift is rapid for the first 10 minutes of operation, then settles down to a very slow drift that is standard for pre-war receivers. For a 1933 receiver, the Comet Pro is a fine performer and it would certainly be possible to use it for vintage ham communications today although earlier versions do not have a "stand-by" function.

Comet Pro Selling Prices

The Comet Pro list price was $150 without tubes. However, it was usually offered in several different configurations. Generally, prices were as follows:

A. Comet Pro Standard Chassis.......$79.38
B. Comet Pro Metal Cabinet..............$8.82
C. Comet Pro Crystal Filter option...$23.52
D. Comet Pro AVC option..............$17.64
E. Comet Pro tube set.......................$7.17 or $8.35 for AVC

With all options the price was usually around $140 in 1934. However, Leeds did offer a complete Comet-Pro for the total price of $117 FOB, in 1934. The 1934 competition's asking prices were as follows:

Patterson PR-10............. $70.00 - speaker included
Patterson Preselector.....~$30.00
National AGS............... $265.00 - price with all necessary accessories
National FB-7 complete...$65.00 - price with all necessary accessories
RME-9D......................... $112.00

photo left: The 1933 Comet Pro chassis. Antenna terminal is center of chassis. Speaker terminal is near the AC power cord exit.

Rebuilding Capacitors in the Comet-Pro Receiver: The Comet-Pro uses mostly metal box multi-capacitor packages along with two bath tub types and three paper wax type caps. The bath tub caps are black wax filled and have no metal bottoms so the wax is easy to "dig out" to remove the original cap and install a replacement inside the metal tub. The tub can be refilled for authenticity. The Aerovox paper wax caps have to have the rolled end "unrolled" and then the internal cap just slides out and a new cap can be installed and the end re-rolled. Cardboard circular end-covers go over the leads so when the new capacitor is installed the capacitor ends are not visible. A small amount of wax can be added for a more secure seal on the rolled end. The four multi-caps in metal housings are more difficult to rebuild. The housings are removed from the chassis and the wires disconnected from the circuit. Carefully unbend the flare on the eyelets that hold the bottom fiber cover to the can and remove it. Using a heat gun to get the can hot enough and using the wires to hold on to, pull the internal cap assembly out of the can. When hot, this pulls out easily. Make up a replacement cap of either three .1uf caps (four leads - three caps with one common) or the single .5uf with two leads. Use the original wire from the old caps and adhere to Hammarlund's color code in building the replacements. These can now be installed inside the cans and waxed in place. Reuse the eyelets to hold the fiber bottom to the can and be sure that the correct wires exit in the proper order out the holes. Crimp the eyelets and the cap is ready to remount and rewire. Be sure to note that the multi-cap that connects to the mixer tube is wired with the common connected to chassis and the other two multi-caps connect with their commons to the appropriate IF tube cathode. The multi-cap with the single .5uf connects to the detector tube. The Comet-Pro doesn't use very many capacitors when compared to the Super-Pro receivers and only the four multi-caps mounted in the square metal housings are time consuming to rebuild.

Comet-Pro filter capacitors are "wet electrolytics." Theoretically, they are self-healing and as long as they still have their liquid inside, they are still usable. The trouble is that the liquid can leak out, or it can dry out but most often the solids settle out and remain at the bottom of the can and the liquid no longer has the same dielectric properties. Though I have found a few wet electrolytics that seem to work okay, I usually replace them for reliability's sake. I mount new dry electrolytics inside the original can. This can be done many different ways depending on the tools available. First mark an index line on the can and then cut the bottom off about .25" up from the bottom. I drill a counter-sunk hole in the bottom piece of the can to install a 4-40 flat head screw that holds a solder lug inside the can. The center conductor is then tapped to hold a 4-40 screw and lug. The new electrolytic is installed between the two lugs. Use a piece of heavy paper rolled into a tube that just fits inside the upper can piece and the lower can piece. Epoxy the paper at the joint being sure to get the epoxy on both the paper and the inside of the can at the joint. Join the two pieces using the index mark as your guide for correct fit and hold in place with masking tape until the epoxy cures. Remove the tape, cover the joint with silver paint and then the rebuilt cap is ready to reinstall.

Henry Rogers - June 2008

photo above: The Comet-Pro with the PEAK P-11 Pre-selector. The P-11 adds two stages of RF amplification ahead of the mixer-oscillator of the Comet-Pro which greatly reduces images and adds a little gain in the sensitivity. The Comet-Pro becomes a splendid receiver for its design period when used with a separate RF pre-selector.

Henry Rogers - WA7YBS  February 2008  Additional Information Added: March 2008, April 2008, May 2008, June 2008, Aug 2008, Apr 2009, Nov 2009, Aug 2010 (clarification on model numbers,) Nov 2010 (add'l info on Geisler Mods)

June 2012 - Re-Edit Style and Appearance to conform to later articles, October 2014 - corrections to Comet Pro info, July 2016 - SP-100LX info added,


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omprehensive History, Serial Number Analysis, Restoration Details. Includes info on the Hammarlund Comet Pro

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