Western Historic

RADIO MUSEUM

HAMMARLUND MANUFACTURING CO.,INC.

The Incredible Pre-war 'Super-Pro'

 Production History ~ The SP-10 Series ~ The SP-100 Series ~ The SP-200 Series ~    
Military SP-200 ~ Power Supplies & Cables ~ Serial Numbers and Production Estimates ~ Serial Number Log ~ Restoring 'Super-Pro' Receivers ~ Restoration of the SP-100 Series ~ Competition Performance Comparisons ~ Photo Gallery of Collector's Super-Pro Receivers

by: Henry Rogers W7HTR

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.

'Super-Pro' Production History

Oscar Hammarlund came to the United States 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 Hammarlund-Roberts Co. folded - not that it affected 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.

Hammarlund's entry into the shortwave and ham receiver market began in early 1932 with the introduction of the Comet-Pro, an eight tube, plug-in coil, AC operated shortwave superheterodyne. Generally credited as the first commercially-built shortwave superhet available to the public, it was a well-designed receiver that was found in many ham shacks of the thirties and was also used by many professionals, including the military. Some Comet Pro receivers even made it to the Arctic and Antarctic with various expeditions. Compared to the scant competition, the Comet-Pro was one of the better communications receivers of its time. Its limitations were due to the lack of an RF amplifier which made the Comet-Pro susceptible to images as the received frequency was increased. Also, calibration was vague, the BFO control was under the lid and changing bands require the manipulation of two plug-in coils per band. The Comet-Pro continued to be improved with a Crystal Filter becoming available in 1933, followed by AVC the next year. Though it was a popular receiver, as receiver design evolved the Comet Pro's limitations were becoming obvious to Hammarlund engineers and many of its users. Production ended in early 1936.  

photo: The 1933 Hammarlund Comet-Pro with Crystal Filter option

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

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

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. 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.

It is apparent that the last of the SP-10 production had some of the characteristics of its successor, the SP-100. The most visible was the elimination of the Tone control. 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. Hammarlund, like several radio manufacturers of the time, would incorporate engineering changes into production as they were developed so  variations within a model type are common.

There seems to be some evidence that some SP-10 users had problems with overloading and subsequent distortion when receiving only moderately strong AM signals when the receiver was in AVC and the RF Gain was only slightly advanced. Customer complaints probably influenced Hammarlund's engineers to re-vamp the RF-IF Gain control circuit for the new SP-100 Series. The circuit changes corrected the problem as the SP-100 will handle strong AM signals without any noticeable distortion while in AVC. Today's powerful signals on the crowded AM Broadcast band are difficult for the stock SP-10 to receive without overloading. In the mid-to-late thirties, broadcast stations were much less numerous and very few operated at power levels over 1000 watts. The same is true of thirties shortwave stations and thirties ham stations - most ran significantly less power than today. It is likely that the original SP-10 circuit performed quite well dealing with lower signal levels received on the less-than perfect antenna systems commonly used then. However, receiving strong modern signals with a well-matched antenna system will likely result in an easily overloaded front-end using the stock SP-10 today. Dave Aabye, W4QCU, came up with a very simple rearrangement of the bias source for the RF Gain that improves the SP-10's ability to handle today's high-level signals without overloading. It's too simple to call it a modification and, of course, it is easy to return to stock at anytime. If you own an operational SP-10 and are experiencing overloading with the RF Gain only slightly advanced while tuning in moderately strong signals, maybe this little circuit rearrangement will help. Click on SP-10 Update for schematic and SP-10 Inst. for description.

 
Photo above: WMI operating room in 1937. Note the three SP-10 'Super-Pro' receivers in the racks. SN:576 is the receiver in the farthest rack nearest the door. Photo from: Inland Marine Radio History Archive

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

The 100 Series "Super-Pro"   1937-1939

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 would overload when tuning in moderately strong AM signals when operated in AVC. 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 the SP-100L 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 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 left blank or are stamped with some kind of factory identification - usually two letters. 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.

Rider's Perpetual Troubleshooter's Manual mentions the SP-100L version and indicates that it substituted the 2.5 to 5.0mc band with a 150 to 300kc band. Interestingly, 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, (but all known SP-200LX receivers are the standard "L" coverage.) Another interesting "L" feature is the dual secondary windings of the output transformer and the front panel "Phone" jack for the earphones. This became the standard for the later SP-200 models but it was first found on the SP-100L models. Though not confirmed, it is likely that the SP-100L version is 600 ohm Z speaker line and Hi-Z phone output windings on the dual audio transformer.

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.

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.

Photo left: 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.

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. 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.

photo right: The SP-150 pictured in an advertisement from QST, July 1938

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. 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 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. 

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-below 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.

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.

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. 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.

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. The chassis may be MFP'd depending on its service. 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. Later 200 Series receivers will have a greenish-gray paint on the front panel of the receiver and power supply. There are variations in the exact 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.  

 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 shown above were destined for Allied service but never shipped. ASP = Allied Super Pro? Under the tag, the panel is ink stamped "BC-1004" but there are no Signal Corps stamps anywhere on the receiver.

Super-Pro Power Supply and 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 back to can-type electrolytic 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 has 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 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 of the SP-10 with a later power supply. 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. Usually the original power supply serial number is one off  from the receiver serial number on early sets. 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.  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. 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 above: The power cable from a military SP-200 Series Super-Pro receiver-power supply showing the unique spade-lug type connector strip used on the original cables

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. This 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 was starting to supply Super-Pros for the war effort, the use of sequential receiver-power supply serial numbers probably ended. 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. 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 probably around 8000-9000 receivers, perhaps more. Total Super-Pro production from 1936 up to 1945 should be around 12,000 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 ~ 8000+

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 5800......1939..........SP-100, SP-200, PS, HQ-120X

                           5800 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.

 * = sequential serial numbers                                                                                                                                                                                                                                            

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:  Western Historic Radio Museum

                       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                        
                         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
                         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, front panel "Phone" jack, dual secondary windings
                           on the audio output transformer
                           SP-150 console Super-Pro introduced in July 1938

1939 - SP-100 - SP-100 Crystal Filter panel eliminated on last of series
                           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 likely 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
                           Militarization of power supplies with heavy duty parts                   

1943 - SP-200 - Painting of front panel changed to red oxide primer coat with light greenish-gray paint, color of the panels are highly variable
                          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 - Rectifiers changed in the power supply, 5U4 and 5Y3, also oil filled capacitors replaced with can electrolytic filters
                           Late 1945 probable design date for SP-400 as it is introduced in early 1946

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. 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, 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:  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 of the information that is in the manual 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.
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. 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: 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. In a comparison to other receivers, the 'Super-Pro' is just about 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.

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, though in theory they are just as prone to current leakage as paper-wax capacitors. They should be tested at their operating voltage for 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) 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, check 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. Sometimes you might have to bend the metal mount slightly to get the right engagement angle. 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. There is a nut on the back of the pinch 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 27 to 50 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 sometime 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 returned to using can electrolytic filters. These were of very high quality construction and in many instances still function fine. 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 a great receiver. 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. 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.

Probably the most infamous Super-Pro modifications are derived from that most infamous of articles, "Souping the Super Pro" by L.E. Geisler, published in the Dec.1957 issue of CQ magazine. Also 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. All of 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 modifications. I have only seen one Super-Pro with these modifications incorporated and it was a wreck. No doubt, the end product failed to impress the owner and the receiver was afterward relegated to the junk pile.

Initially, I though 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 this area 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.

Using the Super-Pro as a Communications Receiver Today

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 use this receiver in our 1937 Vintage Ham Station for some time, so it was necessary for it to function reliably at its design limits. 

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 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

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 may have the edge on greatest useable sensitivity but not by much. The variable Selectivity of the Super-Pro, its direct readout dial accuracy and its powerhouse audio are its definite advantages over the HRO.