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Western Historic Radio Museum

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

Part 2

 Serial Numbers and Production Estimates 
Serial Number Log  ~  Expected Performance 
Guide to Restoring the Super-Pro Receivers
Modification Mayhem ~ Geisler Mods

by: Henry Rogers WA7YBS/WHRM

photo: SP-10 Super Pro in a late-thirties ham station


Hammarlund Super-Pro Serial Numbers and Estimated Production

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

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

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

One note,...the serial number on the military tag of all BC Super-Pro receivers will not match the Hammarlund factory serial number stamped on the rear of the chassis. The tag number was usually assigned for the Signal Corps by Hammarlund for that receiver to be part of a specific contract quantity. It will never match the chassis-stamped serial number that was issued at the time of assembly.

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

Serial Number Sequence with Estimated Production

           Model                     Serial Numbers            Quantity Built

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

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

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

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






Year to Serial Number Table

         SN Range        Year           Models

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

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

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

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

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

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

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

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

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

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

Known Serial Number Log

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

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

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

SP-200X:  4121, 4227, 4239, 4833, 4912, 4955, 5580, 5658, 6005***, 6239***, 7060(SC), 7334, 7853, 8295, 9419, 10664, 16135,

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

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

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

BC-779 (incl. Howard:) 6873H, 7416H, 9030H, 9828, 11162H, 12421, 23688

BC-794: 19828,

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


PS CIV: 721*, 804***, 987***, 2612***, 2615***, 2785*** 3388*, 3400***, 3545(100), 3649(100)***, 4215B (200), 4287, 5164, 6056(200)***, 6215***, 6331(200)*** , 7036(200SC), 9238(200), 11184, 11865, 14653***

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

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

Comet All Wave:  1006

Comet Pro:  7531, 7822, 8001, 8079

 * = sequential serial numbers - colors match receiver with ps                    
** = variation of standard model SP-10 or SP-100 (SP-200 listed as Special)                                          
*** = matched set with close but not sequential numbers, asterisk colors match the receiver with ps      
(Howard) = BC-779 receivers built by Howard Radio Co. as a contractor during WWII will have "H" suffix in serial number (Howard also built BC-1004 receivers)
(Special) = SP-200 Receiver with special frequency coverage or other non-standard features
(SC) = Signal Corps - military contract model                                        

Serial Numbers Needed

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


e-mail Super-Pro info to: 



   Characteristics and Engineering Changes per Model/Year

1935 - SPA (SP-10) - June 29, 1935 Order Number 10932-NY-35 issued for Hammarlund SPA Super-Pro Receiver for the Signal Corps US Army. SPA is identical
                                   to the SP-10 except for the installation of a data plate on the front panel. It's likely that these receivers were delivered before the official March 1936
                                   introduction presented in QST magazine that month.

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 may have been added to create a 600 ohm output Z on some receivers
                         Some receivers may have had a standard phone jack audio output on front panel
                         Aluminum front panel .187" thickness, engraved nomenclature
                         Paper-wax capacitors are usually Aerovox brand, sometimes intermixed with Cornell-Dubilier brand                        
                         Spacing rods in RF tuning unit are brass                        
                         Non-illuminated Tuning Meter measured total IF amplifier current
                         S-version introduced in June 1936 tunes 1.2 to 40 MC
                         Bias series resistor string modified - 600 ohm resistor replaced with 2-300 ohm, allowed moving detector input amp grid connection
                         to junction which slightly increased bias voltage
                         BFO Plate Load resistor changed from 5K to 50K (may be parts list error as all known examples use 50K)
                         BFO Grid capacitor (100pf) changed from grid to ground connection to parallel with grid leak resistor connection, (possible schematic error)
                         Tone control eliminated in late production
1937 - SP-100
- Introduced January 1937, uses eight metal octal tubes and eight large-pin glass tubes, Sensitivity Control replaces separate RF and IF gain controls,
                           Variable coupling AVC and Detector transformers changed to fixed coupling, Selectivity control renamed Band Width, engraved scales added to
                           Sensitivity, Beat Oscillator, Band Width and Audio Gain controls
                           Fixed-coupled AVC and Detector transformers allowed the component boards to be moved from under the chassis (SP-10) to inside the transformer cans (SP-100)
                           New style small knobs with metal pointers                  
                           Audio transformers changed from potted units to vertical mount-frame types - output Z is 8 ohms
                           Paper-wax capacitors are usually Cornell-Dubilier "TIGER" brand but could be intermixed with Aerovox brand
                           Spacing rods in RF tuning unit are steel                           

1938 - SP-100 - SP-100L introduced as low frequency version, 100KC-400KC and 2.5-20.0MC, production receivers have same audio output configurations as the SP-100X.
                           Late or Military versions of the "L" may have had front panel "Phone" jack, dual secondary windings - 600Z speaker and Hi-Z (8K) phones
                           "LX" version added a Crystal Filter
                           SP-150 console Super-Pro introduced in July 1938

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

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

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

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

1943 - SP-200 - Painting of front panel changed to red oxide primer coat with gray paint, color of the panels is highly variable with gray, blue-gray and green-gray commonly seen.
                          Tube layout charts are added to the top of the RF box
                          Schematic added to inside of bottom cover of the power supply
                          S-meter housing changed, full glass front replaced with partial glass with zero adjust mounted in metal
                          TM-11-866 Signal Corps Manual, first version. Manual covers BC-779, BC-794, BC-1004 and R-129/U plus power supplies RA-74, RA-84 and RA-94

1944 - SP-200 - Probable year for contactor built BC-779. Howard Radio seems to be the only company used as a contractor.

1945 - SP-200 -  Late 1945 probable design date for SP-400

Post -WWII   -   SP-400-X and SP-400-SX models introduced in 1946. .54-30mc for X and 1.25-40mc for SX. 455kc IF for both versions. Changes are mostly cosmetic
                           The Signal Corps continued to support and use the SP-200 Super Pro receivers. Some of the MWOs and manual additions date well into the 1950s.
                           Rectifiers changed in the RA-74D power supply, 5U4 and 5Y3, also 12 cylindrical oil-filled paper capacitors replace the two large block capacitors (1948)
                           Improvement Kit MC-531 shown in TM11-866 (Feb 1948 printing) - this is the three channel selectable crystal-controlled oscillator upgrade
                           Wickes Engineering R-270/FRR receivers supplied for AN/FRR-12 Dual Diversity Receivers - R-270 is a modified BC-794 receiver (1948)



Expected Super-Pro Performance

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

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

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

The high power, high fidelity audio provides fantastic sound when a good speaker is used and it is matched to the particular audio output Z of the specific Super-Pro. All SP-10s and nearly all SP-100s are 8 ohm Z output. Late versions of the SP-100LX versions use an audio transformer with dual secondary windings of 600 ohm Z and Hi-Z (8000Z) phones. A few early SP-200s are dual outputs with 8 ohm Z and Hi-Z phones using frame transformers similar to the SP-100 transformers but most SP-200s are 600 ohm Z and Hi-Z phones. If the SP-200 has potted transformers with the dual secondary windings then it is 600 Ohms Z and Hi-Z phones. The correct match is important for best fidelity and power. When everything is correct, shortwave BC stations sound beautiful, especially if conditions allow for opening the bandwidth up to 16KC. AM BC also sounds great if you can find a station that is playing music instead of incessant talking.   >>>

 >>>  Using a 60 foot long end-fed wire connected to an antenna coupler, I nightly copy ZL stations (New Zealand) on 40M CW on an SP-200X receiver on loud speaker. The ZLs are Q5 and about S6. At night (during the winter,) on 80M AM, stations on the East Coast can be easily copied, providing the QRM allows for it. 20M performance during the afternoon is incredible with lots of stations from South America and the South Pacific. DX is a daily or nightly occurrence using the Super-Pro and a decent 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's unfortunate that these incredible receivers have had to endure endless deriding from hams and SWLs over the past several decades. The low opinion of the Super-Pro probably originated from hams and SWLs who, in the 1960s, happened to obtain a well-used surplus BC-779 receiver (that like most Super-Pros was still operational) and began using it right away without doing anything to the receiver. The first thing noticed was that the highest frequency tuned was 20 MC. Then it was noticed that AM BC was not included but two long wave bands were. Also, a 600 Z ohm speaker was needed. Also, the bandspread only was provided on the top three bands. Complaints were numerous and mostly based on the surplus BC-779 version. The BC-794 or any "S" version would have ended almost all of the complaints but it was one of the most difficult models to find.

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


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

Manuals-Schematics:  Though original manuals are nice to own, they are not really necessary for the repair or restoration of an early Super-Pro receiver. In fact, most receivers do not exactly agree with the information in the manual anyway. This is because engineering upgrades are usually incorporated into production almost immediately while documentation may take quite awhile to catch up. Most of the information that is in the various Hammarlund manuals is also found in Rider's Perpetual Troubleshooter's Manuals. Also, several sources offer reprints of the original manuals and several on-line sources offer manuals that can be down loaded. When ordering a reprinted manual be aware that most manual suppliers reference the receivers with the Hammarlund ordering model numbers, thus an SP-100X receiver would be listed as an SP-110X (the SP-200 series may have the same ordering issues.) Hammarlund did identify the manuals using the speaker size modifier but since the SP-110X and the SP-120X are identical receivers, their reasons for this are a mystery. The SP-200 Military versions are covered extensively in the Signal Corps manual TM-11-866. This manual covers the BC-779, BC-794, BC-1004 and the R-129/U plus the power supplies RA-74, RA-84 and RA-94. Even if you have the civilian version SP-200, this TM is an excellent manual to have. Regarding the SP-10 schematic - the common published schematic for the SP-10 is for the very earliest receivers. Several engineering upgrades were added during production which, of course, do not appear on these early schematics. Hammarlund's documentation apparently never caught up with the SP-10, therefore, when working on an SP-10, you should also have an SP-100 schematic handy. You will find discrepancies in the SP-10 schematic versus most SP-10 receivers that are correct if you compare the circuit to the later SP-100 schematic. Also, if you are the owner of an SP-10 manual (hopefully along with the receiver) note that the manual drawing of the underside of the chassis is a "mirror image" and shows the RF/IF section where the Audio section should be and vice-versa - visually confusing.  Reworking Technique and Skills: Like many of the pre-war high-end communications receivers, the 'Super-Pro' is a challenging restoration project. Major disassembly is required to access some components that need to be replaced or at least tested. How well your restored 'Super-Pro' functions after a rebuild is dependent on your soldering technique, your ability to methodically perform mechanical and electronic work and to keep track of that work. Experienced technicians are always checking and rechecking their own work as it progresses. This ends up saving time in the long run as problems at the power up stage are minimal, if any at all. High-end receivers cannot be restored in a "rush job" manner. Take your time and recheck everything you are doing. The SP-10 and SP-100 Super-Pros have more mechanical complexity than the normal communications receiver and several of the fiber board parts can be very fragile after years of use. Don't force assembly or torque anything involving the fiber board parts or breakage is sure to occur. In a comparison to other receivers, the 'Super-Pro' is just as complicated of a restoration project as the Hallicrafters SX-28. Both require major disassembly because of difficult (or impossible) to access components that need to be replaced. Remember, the Super Pro has a rather high level of B+ in its audio section so care is necessary when doing any testing with the power on. Good troubleshooting skills are required when working on any type of vacuum tube electronics gear due to the high voltage levels required for this type of circuitry.
Super Pro Parts Sets - The Good News - Hammarlund had a difficult time with change. They didn't like it and that's to our advantage for restorations. When looking for parts you don't really need the exact part for, say, the SP-10 that you're restoring. A lot of the parts never changed from the SP-10 up thru the SP-200. When I was restoring the WMI SP-10, I needed to replace the tuning and bandspread dials, the index windows and the spacers. I found that those were exactly the same parts on a SP-200X "parts set" I had. I also needed the variable coupled IF levers which, other than having the brass plated, were exactly the same parts. When restoring the SP-100LX that was entirely missing the 100kc to 200kc coil set, I was able to transplant the 100kc to 200kc coil set from a BC-779 and they were exactly the same coils with the isolantite bases even having the same part number stamped on them. Of course not all of the parts are interchangeable,...the early S-meter being an obvious example. But, so many parts are interchangeable that it's worth having a SP-200 "parts set" even if you're restoring a SP-100 or SP-10.  >>> The Bad News - Unfortunately, the extremely high cost of shipping these "parts sets" makes us cringe. To actually pay two or three times as much for shipping as for what the "parts set" costs is really making us look for alternatives. One source would be other collectors that might be willing to remove specific parts and ship just the parts needed. This will pretty much eliminate any eBay seller and require finding a knowledgeable collector. Also, it usually has to be "worth" the effort for the donor to search for and extract the desired part. As incentive, I've found that "trading parts" works quite well. Hopefully, you can find a collector that's restoring maybe a Hallicrafters receiver that needs something you have in exchange for the Super Pro parts you need. This does require advertising or putting out "wants" and it does work once in a while. The only other recourse is to "suck it up" and pay the "way over-priced, eBay BIN price for a junker" and tack-on about $150 for shipping. I recently was looking at a decent SP-200LX parts set that wasn't too outrageously priced and was located in Colorado,...not too far away. It didn't matter, the shipping was still $150. If you're in an area that has lots of ham swaps per year, you'll probably be able to find a Super Pro parts set cheap without too much effort. Northern Nevada only has two ham swaps per year and early Super Pros just don't show up around here too often anymore.
Disassembly: The best procedure to rebuild a Super-Pro is to start with a major disassembly. It will be necessary to remove the front panel and some of the assemblies for access to parts that need to be checked or replaced. There is no way to access the capacitors in the RF Tuning Unit except by its removal from the chassis and then removing its shielding. There is no way to remove the RF Tuning Unit (RF TU) unless the front panel is off. There are five paper wax capacitors located in the RF TU of  the early SP-10 and SP-100 versions and three in the SP-200 versions. The early versions also have an RF shield between the RF and IF section of the receiver. It is easier to work in this section of the chassis if the shield is removed. In fact, it is easier to work on the chassis with the RF TU removed also. When the capacitors are replaced and the other rework completed, the receiver can be reassembled. On early models, if there is a problem with either the "On-Off" switch or the IF gain pot (Sensitivity pot on SP-100) you have to remove the front panel and the RF TU for access to remove these components. Check their operation thoroughly while everything is apart. Capacitors: All paper wax capacitors should be replaced for best reliability and performance. Hammarlund did locate some of the paper-wax caps out-of-sight in the RF TU and in some of the AVC and Detector transformers but these are easy to access during disassembly. Whether you use "orange drops" or "yellow jackets" doesn't matter - both are much better capacitors than the originals were when they were new and, of course, now the originals have about 70 years of leakage to deal with. If you want to preserve the under chassis original appearance, then the new caps should be installed inside the original shells. This procedure is covered in the section below "Restoring the 100 Series 'Super-Pro.' Restuffing capacitors is, of course, only for aesthetic purposes. If under chassis appearance is not important, then install either modern orange drops or yellow jackets. You should use all of the same kind of caps when rebuilding a receiver. It looks professional. Mixed types of capacitors look like you were working out of your junk box! Military 'Super-Pro' receivers will have "bathtub" capacitors installed around the inside edge of the chassis. These are multiple paper capacitors installed in a metal box. Some of these are oil filled units. Also, some of the individual larger paper capacitors are also oil filled. If the oil filled units are not leaking oil, they are probably okay. They should be tested at their operating voltage for possible leakage if you plan on leaving them in the circuit. Or, you can just replace or rebuild each tub with new capacitors - your preference.

Paper Wax Capacitor Locations:  

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

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

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

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

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

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

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

Front Panels:  The front panel of early 'Super-Pro' receiver is made from .190" 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 (no longer available, use VHT Black Wrinkle Finish,) 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 "3 in One" oil using a clean cloth after the touch-ups have dried. You may want to do two applications. Wipe off the excess with a dry clean cloth. The cabinet will look practically new with this treatment and it lasts quite a long time.
Alignment:  Any of the Super-Pro receivers are straight forward with no odd procedures or special equipment needed. The adjustment of the crystal filter may require a sweep generator and oscilloscope if the adjustment has been misaligned, but this is seldom the case. Since the adjustable 465KC IF transformers are over-coupled in the 16KC band width, the IF should be aligned in the 3KC position at the exact crystal frequency, if the crystal filter option is installed. Due to the extreme quality that was built into the tuning unit, all Super-Pros can be aligned to exceed the dial accuracy specification of 0.5%. If you can't achieve this accuracy then something is wrong with the oscillator section for that particular band. Power Supplies and Power Cables: Early power supplies used standard can-type electrolytic filter capacitors. These are almost always dried out and need to be replaced. Even if they check good, they will certainly fail within a short time if you try to use them at full voltage. The original cans can be restuffed with new electrolytic capacitors or the new ones can be installed under the chassis. For aesthetic reasons, I place the new capacitor inside the original can. The military power supplies usually have oil filled paper capacitors for filters. Unless these have shorted or are leaking oil, they are okay to use. Some of the late version military supplies will have 12 can filter capacitors installed. These appear to be electrolytics but they are actually 4uf oil-filled paper capacitors. They seldom have any problems since they were of very high quality construction. Test them thoroughly before relying on them, though. The power supply has two large tapped wire wound resistors and it is very rare for them to have any problems. Many times the power supply will not be found with the receiver. They are fairly easy to find and almost any 'Super-Pro' power supply will work with just about any 'Super Pro' receiver. The power cable is also not usually with the receiver or power supply. The original cable is not an easy item to find so most collectors just make one. The only thing to observe is the wires #1 and #2 are a larger gauge, usually 14 or so, to provide a small IR drop across the cable for the tube heaters. The remaining wires are around 18 gauge and not critical as the current carried is low. The tenth wire, if present, is not used in the receiver circuit. It was a switched ground for optional user applications.


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

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

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

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

The WWII Super Pro, the 200 series in its various Signal Corps designations of BC-1004, BC-794 and BC-779, are the receivers that are usually found in "hacked up," unrestorable and non-functional condition. Cheap post-WWII surplus had something to do with this and also the belief in the myth that the "ham radio operator is a competent engineer/technician" promoted by CQ magazine. This combination lead to the destruction of many of the WWII Super Pro receivers. The following is how the Super Pro Modification Mania began.

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

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

Initially, I thought very few Super-Pro receivers had ever been modified since it was such a great design in the first place. The Super-Pro receivers that I had found around Nevada were mostly all original. However, after talking to several collectors in different parts of the country, it seems that there are lots of modified Super-Pros out there. Many modified versions are now showing up on eBay with all of them being non-functional and sold as "parts only." Certainly, the rarity of the particular example will dictate how far the owner is willing to go to restore a modified receiver to its original specifications.

In a way, it's fortunate that the most common victims of the modification mayhem were the military receivers. They are also the most plentiful version around and that means that most of the SP-200 parts (many of which are interchangeable with SP-10 and SP-100 receivers) are usually easily available to complete a "restoration to original" on other Super Pro receivers that have fared better. It's unfortunate that the SP-200, especially the BC-779, were considered useless relics that were unusable "as-is" by hams and that, because of urging from CQ magazine and from the Surplus Conversion books, these great receivers were "hacked" to the point that now they are only useful as a "parts set."  However, a true "Geisler Modification" or a "Military MWO Modification" receiver should be left "as-is" since these modifications made sense, were conservative in scope, expertly incorporated into the receiver and generally enhanced the Super-Pro performance. True Geisler mod'd Super Pros are rare, however.


Using the Super-Pro as a Communications Receiver Today

Some hams are reluctant to use a pre-war receiver in actual "on-the-air" operations for fear that adjacent frequency QRM will limit their ability to successfully copy stations and that they will be unable to complete QSOs or Vintage Net operations. The Super-Pro might have problems today coping with adjacent frequency interference from powerful SSB signals when the user is trying to copy an AM signal (and this will be the case for almost all vintage communications receivers.) Even narrowing the "Bandwidth" IF passband or using the crystal filter doesn't seem to help much. However, here's a couple of "tricks" that work.

Probably the best way to "dodge QRM" is to tune the receiver slightly above or below the operating frequency. This, in essence, is selecting one sideband or the other of an AM signal, whichever has the least QRM. Most SSB transceivers have filters to keep the transmitted bandwidth at about 2.1kc on one sideband only. Most AM signals are at least 6kc bandwidth with audio information contained in two sidebands. It's very easy to tune the offending station "out of the passband" and still recover enough audio for solid copy of an AM signal.  >>>

>>>  Additional selectivity using the crystal filter will also help to reduce the ratio of adjacent frequency interference to tuned signal. You'll have to tune the desired signal "on the nose" for good copy. Broad, high fidelity audio fidelity cannot be enjoyed using either of these QRM-reduction modes. But, the goal is successful copy and a completed AM QSO, in other words - communications. 

What about in actual use? I use only vintage receivers for operations on the ham bands and I find that regardless of the model of vintage receiver used, these two methods all but eliminate most adjacent frequency QRM. You'll still probably hear slight indications of adjacent frequency activity but copy will be 100%.  Of course, it isn't enjoyable, "arm-chair" copy but it is a successful completion of the QSO or of the net operations, which is the goal.



Pre-War Super Pro Part 1                                             Return to Home Index




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