Radio Boulevard
Western Historic Radio Museum

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

Part 3

 Restoring the WMI SP-10, Restoring the SP-100X, Restoring the SP-100LX
Competition Performance Comparisons
 Photo Gallery of Collector's Super-Pro Receivers
COMET-PRO DETAILS in Appendix A at the end of this article

by: Henry Rogers WA7YBS/WHRM


photo: Swedish reporter using Hammarlund SP-100 (no crystal filter) - ca: 1940


Restoring Pre-War Super-Pro Receivers

The following are write-ups of three early Super Pro restorations. The SP-10 write-up is from 2008 and the SP-100X write-up is from 2007. The write-up for the SP-100LX is from 2019, so it's in a different style. For the past few years I've done these restoration write-ups as journals that track the day-to-day progress of the restoration. Of course, sometimes there are inevitable delays and sometimes these delays might run into several months or even years. At any rate, the SP-100LX restoration write-up is a journal-type and that allows the reader to become aware of how much time is spent on finding proper parts or special materials necessary for a proper rebuild. Also, the time it takes to accomplish the tasks involved once all of the parts become available. - May 2019


Restorating the WMI-Hammarlund 'Super-Pro' SP-10   SN: 576

photo left: The WMI SP-10 before restoration

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

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

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

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

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

The Planned Restoration Work

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The SP-10 Restoration Work

The photo to the left shows the front panel before restoration. The physical modifications were going to be difficult to repair. Most of the time, amateur modifications have little regard for symmetrical layout or quality machine work. The WMI SP-10 mods, even though performed by "professionals," were very much like amateur workmanship. I knew from an earlier restoration of an SP-200LX receiver with a repainted aluminum front panel that stripping and repainting wrinkle finish on this aluminum panel would be a disaster. Trying to remove the paint from the engraving is next to impossible. Touch-up is the only method that preserves the original look of the receiver panel. Filling the holes presented a challenge because of their size. First, the panel was thoroughly cleaned using Glass Plus and a brass "suede" brush. This is done to clean the smoke and grime out of the engraving so it would look silver again. The only practical hole repair method was an epoxy fill. I placed masking tape over the holes on the front side of the panel and then backed that with blocks of wood clamped to the panel. This gave a very flat surface and the masking tape assured that the epoxy would not stick to the backing blocks. I fill from the back side using slow curing epoxy to make sure all of the bubbles have a chance to rise to the surface (the back of the panel.) Because of the thickness of the panel, two layers of epoxy were required for the complete plug. The epoxy was left to cure overnight and then the blocks and tape were removed. This leaves a super-flat surface on the front of the panel. The back side is leveled using a Dremel Tool. The front side is painted using Krylon Black Wrinkle finish applied with a brush and "wrinkled" using a heat gun. Only the plug is painted and if everything goes okay, the match is virtually undetectable. The backside was painted with silver paint.  

The photo to the left is a shot of the chassis underside before restoration. There was so much that was non-original  I created a list to keep track. Some of the notable deviations under the chassis - the shielded cables routed next to the RF box. These carried the audio output to the phone jacks mounted on the front panel. The transformer matched the 8 ohm Z output of the receiver to the 600 ohms required at WMI. None of the capacitors are original, most having been installed in the 1970s after the receiver's tenure at WMI. Many modern style components were installed in the various circuits.

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

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

The two 60K resistors are replicas that are repainted to match the BED coded originals. There were several JAN type resistors installed and even some modern metal-film resistors that were removed. To keep everything looking the correct vintage I used similar to original size and shape resistors of the correct value. These similar style resistors were marked in banded color codes. Also, some of the replacement resistors found had leads that were too short for the Hammarlund "thru-the-eyelet" type of mounting. I had to carefully add a length of TC wire to be able to mount the resistors correctly. When the leads were installed, I then painted the body of the resistor in the BED code style. When mounted, only a close inspection reveals that these are replica resistors.
I initially installed the three IF screen bypass capacitors as the replacement, non-original caps had been installed - on top of the component board. This had the capacitors on top of the 10K resistors. This didn't seem likely and I remembered that in the SP-100 rebuild, the IF screen caps had been mounted at the tube sockets. Looking at the manual drawing, it was apparent that the caps shouldn't have been mounted on the component board. I removed them and moved the component boards in order to have access to the IF amp tubes. There I found the evidence I was looking for - the actual remains of the original capacitor leads left behind when the receiver was re-capped years ago. I installed the rebuilt caps at the tube sockets, as was original manufacture. The photo to the right shows the RF-IF section with all rebuilt capacitors installed in their correct positions. The Aerovox rebuilds installed in the RF section can be seen. The LO capacitor that looks like an adjustment trimmer is actually a fixed capacitor that is the correct value when the screw is tight. Though this one is a replica I made, it is very close to the original in size, value and appearance.

The photo to the left shows the bandspread condenser side of the RF Tuning Unit and the rebuilt paper-wax capacitors. These five capacitors are completely hidden and require the removal of the RF TU from the chassis and then the removal of the side covers to access the capacitors for replacement. The photo to the right shows the replacement fiber tube socket (the one on the left) that was installed to replace an incorrect bakelite socket that had been installed in a repair done many years before. Luckily, I found an exact style fiber socket with the correct "42" tube ID in one of my junk boxes. Nowadays, we probably shouldn't call them "junk" boxes since the parts they contain are so necessary for restorations and are so difficult to find otherwise.

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

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


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

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

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

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

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

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

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

Restoring the 100 Series "Super-Pro"  -  SP-100X   SN: 3387

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

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

photo right: the restored SP-100 chassis - note the differences in this chassis and the SP-10 chassis. The lack of adjusters on the Det and AVC transformers, the metal octal tubes used and the different style audio transformers. Also, note that SN: 3387 is a "table model" version with an 18" wide panel and no rack mounting slots.

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

Rebuilding Capacitors

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

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

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

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

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

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

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

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

Other Restoration Work

Once all of the capacitors were rebuilt, it was necessary to replace all of the grid leads from all of the IF transformers and AVC Output Transformer and the BFO coil. The Crystal Filter assembly is rather complicated in its construction and was removed from the chassis in order to easily disassemble, replace the grid lead and the connecting leads to the crystal and the phasing condenser and then reassemble. These grid leads were originally rubber insulated stranded wire but the rubber had become "lumpy" and had hardened, becoming brittle. Any flexing would break the rubber off of the wire. I used a cream colored cable jacket that was off of old telephone hook-up cable (I now use the cream-color, rubber insulation found on the Neutral wire in AC power cables - much easier to find and looks correct.) 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 (I now don't install grommets on the IF and AVC cans as it appears that they weren't used originally.)

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

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

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

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

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

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


Restoring the SP-100LX "Super Pro"   SN: 2730

photo above: SP-100LX sn: 2730 showing the "as found" condition. Although the external appearance of this receiver is very good, inside was a different story. Coil changes, weird mods and strange wiring errors awaited our underchassis restoration.

July 22, 2016 - Purchase - I got a call from my old friend and fellow Dayton, Nevada ham, KB6SCO, inviting me over to his garage because he had just brought some radio equipment back from Quincy, California. The gear had belonged to a ham in that area. Among the parts, accessories and test gear was this early Hammarlund Super Pro along with its original matching power supply and interconnect cable. A quick check revealed that this was the "LX" version of the Super Pro that has the two low frequency bands. A very reasonable price was agreed upon and I took the SP-100LX, power supply and cable home.

Initial Inspection - The serial number on this SP-100LX is 2730 which is from about the middle of production meaning it's probably from late-1937 or early-1938. Series numbers on the SP-100 series start at about 1000 and run up to about 4000. This SP-100LX is a rack mount version with the 19" wide front panel. The receiver has indications that it might have been used by the Signal Corps during WWII since there's two MFP date stamps (4APR1945) on the top of the RF box and bottom of the coil box but oddly there isn't any MFP except on these panels themselves. There appears to be some "overspray" of MFP on the back of the band spread dial. However, there aren't any SC acceptance stamps on the receiver which probably indicates it wasn't originally supplied to the SC under contract and may have been acquired by other means. Nearly all of the components under the chassis appear to be original. The dust cover SN matches the receiver SN. The power supply that came with the receiver seems to be somewhat newer with a serial number of 5164 and it does have a SC stamp on its chassis. The band spread knob is certainly not original. The white pointer type knobs aren't original although all the knobs do match. These are generic pointer knobs that were available from many sources. These same types of knobs were found installed on the WMI SP-10 receiver. On the rear apron of the chassis are two Jones' plug sockets, one for remote relay and one for B+, Fil & AVC outputs. Neither are original and certainly aren't Signal Corps either. As for tubes, the Mixer and LO have later type tubes that are installed into elaborate adapter sockets to preserve the original sockets. Also, 6V6 tubes installed substituting the original 6F6 tubes. 6V6 and 6F6 are "pin for pin" compatible and will work as substitutes however the 6F6 is a standard pentode and the 6V6 is a beam-power pentode. Generally, a 6V6 will sound "brighter, almost harsh" when sub'ing for a 6F6. The Jones' sockets and tube adapter mods appear to be from the late-fifties.

Jan 21, 2017 - First Attempt to Get Started Uncovers a Major Problem - I have inspected this SP-100LX and found it is mostly complete and original with the exception of the coil box. It seems that the Antenna/RF Coil for the 100kc to 200kc band was removed and the antenna input connected to the 2nd RF coil. I'll need to find a good LX coil box to harvest a replacement coil. I'm sure that a BC-779 coil box would work (a lot easier to find.) No other serious problems found. Original audio transformers and the meter checked okay. Restoration on hold until 100kc-200kc coil parts found.  Turns out more was missing than just L5/L10.

Update Apr 3, 2019 - Trading Parts - A routine e-mail reporting a serial number for a SX-28 receiver had a surprising turn when I mentioned in a reply e-mail that I was looking for a "junk" BC-779. It turned out the e-mailer had the RF box for a BC-779 available for "parts removal." This meant that I had to specify what I needed and he would remove the required part. I ended up trading SX-28 T-2, (2) T-3 and a '28 tuning condenser box cover for BC-779 coils L5/L10.

Update Apr 4, 2019 - Getting Started (Finally) -  In anticipation of actually obtaining a replacement L5/L10 for this receiver, I performed another "closer look" inspection and found that not only was the 100kc to 200kc Antenna/RF coil (L5/L10) missing but the remaining three coils (L15, L20 and L25) weren't original. They weren't even close in appearance to originals. It looks like someone in the past removed all of the 100kc to 200kc coils and installed other types of coils in the LO, Mixer and 2RF positions in an attempt to have the 100kc to 200kc band tune the AM BC band instead. The ceramic bases are present for the LO, Mixer and 2RF with different coils mounted using small brackets. Luckily, my source for L5/L10 could also supply L15, L20 and L25 (I offered to add a SX-28 AVC knob to the return trade list.) The rest of the receiver is relatively stock. Only three resistors changed and eight capacitors changed. Also, missing is the stock RELAY pin jack terminal fiber block on the rear of the chassis. Unfortunately, the SP-200 series changed this part to a screw terminal block that is physically much larger.

April 8, 2019 - Disassembling the Receiver - Removed top and bottom covers, removed coil box cover,  removed all tubes, dismounted the meter, the crystal filter assembly (requires desoldering two wires,) all knobs, front panel, dial index assemblies, mask drive gear and the dial escutcheons. All parts were put into plastic bags and then placed in a large plastic bin for safe storage.

RF BOX REWORK - Photo to the right shows the LX coil box before any restoration work was performed. The coils on the left side of the box should be the 100kc to 200kc set of coils. It can be seen that the Antenna coil set (bottom row) is entirely missing. The RF and Mixer coils are oddball replacements with internal brass slug. The Oscillator coil is small and has a compression trimmer associated with it. All of these coils are mounted on the original isolantite coil bases using small brackets. Note that the twisted lead from the Antenna terminals has extension wires added to route the antenna input to the RF coil and no Antenna coil is used (typical use of white medical tape for insulation.) Judging by the size of the coils it appears that this modification was to change the 100kc to 200kc coverage to the AM BC band (540kc to 1700kc.)

photo above: Shows the BC-779 100kc to 200kc coils installation. Note that the BC-779 coils have plated brass parts while the originals are unplated brass parts.

The remaining four sets of coils are original and provided the receiver with 200kc to 400kc coverage and 2.5mc to 20mc coverage.

The RF/Coil box has to be removed from the receiver chassis to allow side panel removal which will allow better access to install the replacement coil assemblies. Also, there are five paper-wax capacitors inside the box that will need rebuilding. Also, the four grid leads need to be restored. All of the grid leads on the IFs, Amplified AVC, Crystal Filter all need to be replaced due to drying out of the natural rubber insulation that is now "falling off" with the slightest provocation.

April 9, 2019 - Dismounting RF/Coil Box - Only a few wires need to be unsoldered under the chassis. There are obvious screws to remove but there are two hidden screws that are inside the back Antenna Input housing. The two small shields have to be removed to access these two inner screws. The RF/Coil Box can then be lifted off of the chassis. There are 25 screws on each side panel and both sides have to be removed. One side to access the area for the 100kc to 200kc coil work and the other side to access the five paper-wax capacitors. It's better to remove the two dials since they aren't protected when the RF/Coil Box is out of the receiver. Additionally, the four grid leads need to be replaced and the connections aren't accessible unless that side panel is off.

April 11. 2019 - Installing the BC-779 Coils - The BC-779 coils arrived today. Other than minor finishing, they are identical to the coils in the SP-100LX. Even the same identification number is used for the isolantite bases. The obvious change is the cadmium plating used on the metal parts where the SP-100LX coil metal parts are brass. I installed L5/L10. I had to make an extension for the L10 secondary connection since the original wire had been cut. But, the twisted pair from the Antenna terminals switch to L5 could be routed exactly as original. The BC-779 coils do have wire ends left on since they were "cut" when the coils were extracted. This helps in using the correct routing and the correct connections. 

April 12, 2019 - RF Box Restoration - Completed the installation of all 100kc to 200kc coils (L5/L10, L15, L20 & L25) into the RF Box. Two wires needed extensions. I salvaged the correct color cloth insulation to cover the extensions. The four grid leads were restored using cream-color rubber insulation salvaged from old AC line cable. The proper stranded wire was inserted into the proper length rubber insulation to create original looking grid leads. These were installed into the RF Box. The plate leads to the 1RF and 2RF tubes were too short. Bare wire extensions had been in the receiver that connected pin 3 of the 6K7 tubes to the wires from the RF box. I plan on using the same connections but covering the bare wire with vintage black vinyl sleeving.

April 13, 2019 - New Grid Leads - Finished the installation of new rubber insulated grid leads by installing new grommets for the feed-thru holes and installing the original grid caps. New grommets were installed for the Plate leads and the LO cathode lead. Installed the side panel and the mounting bracket. This completed the tuning condenser side of the RF Box.

April 14, 2019 - Hidden RF Box Capacitors - There are five capacitors inside the RF Box that are accessed by removing the panel on the bandspread condenser side. To my surprise the capacitors weren't the normal C-D TIGER (Cornell-Dubilier brand) paper wax caps but were molded C-D caps that were probably WWII vintage. These were the brown bakelite square package about 1.25" x 1.25" x 0.325" which may have been installed when the 100kc to 200kc coil change was accomplished. While these capacitors are probably many times better quality than the original TIGER paper caps, these are still C-D brand and are still 75 year old paper caps. Replacing these caps with rebuilt C-D TIGER caps will require three .01uf 400vdc TIGER shells and two .02uf 400vdc TIGER shells to install new polyfilm caps inside.

April 20, 2019 - Weird Stuff Inside the RF Box - I rebuilt three .01uf and two .02uf C-D paper wax shells with new polyfilm caps inside. The rebuilds were coated with bee's wax to duplicate how the original C-D caps looked. I began to install them by removing the non-original molded C-D caps. I had wondered about those molded C-D caps and when removed I noted that all five were 0.01uf. Not critical but not correct. After I had installed the new rebuilds I happened to notice that there was a resistor installed in the black vinyl sleeving of one of the plate leads. I checked all five wire leads (B+ and AVC) and found that all had series resistors inside the sleeving. Two were 2K 1watt and three were 10K 1watt. These resistors are already installed the receiver chassis although the plate loads for the RF tubes are 5K on a SP-100 (but are 2K on a BC-779.) On a SP-100, the plate load resistors are located on a component board adjacent to the RF box. Also, the AVC series 100K resistors are also located on a component board also adjacent to the RF box. I think the person in the past that modified the RF box and changed the 100kc to 200kc coils only had a BC-779 schematic to go by. That's probably why the duplication or addition of resistors. It's also possible that the extra resistors were part of a mod that involved the different RF amp tubes (that were on extension sockets) that were installed when the receiver was first examined. I removed the duplicate resistors to have the RF box have component values and connections that are correct for the original SP-100 version.

Installed all of the covers. Installed and synchronized the main tuning and bandspread dials. The RF box was now ready to install back on the main chassis but the paper wax caps in the chassis now needed to be rebuilt.

photo above: The rebuilt C-D TIGER capacitors installed in the RF Box. The air variable is the band spread tuning condenser.

photo above: The RF box ready to install. Note the new grid leads with the cream-color rubber insulation - salvaged neutral wire from old AC power cable.

Apr 21, 2019 - Front Panel and Caps - Cleaned the front panel with Glass Plus and a brass wire brush. This has to be done with a very light touch to work the dirt out of the engraving and the convolutions of the wrinkle finish without damaging the paint (no scrubbing, in other words.) Followed with more Glass Plus and a bristle paint brush, cleaning until the paper towels didn't show any discoloration. Let the panel dry for a couple of hours, then touched up the edges and minor chips with black nitrocellulose lacquer. After that dried, the panel was rubbed with a clean cloth saturated with "3 in One" oil followed by wiping with a clean dry cloth. The end result is an excellent looking panel with a soft luster and bright silver nomenclature. Cleaned and installed the dial escutcheons.

Checked the chassis and eight capacitors weren't original. The total quantity of paper capacitors used in the SP-100LX would have been 35. Minus the five already installed in the RF box would leave 30. Minus four other paper capacitors that are hidden in the AVC and Detector Output transformers leaves 26 paper caps in the chassis itself. All non-original capacitors were .05uf, so I checked my C-D TIGER capacitor box and found eight .05uf 400vdc capacitors to harvest their shells. Sorted through the polyfilm capacitors and found all of the values that were going to be required for the rebuilding of the remaining paper-wax capacitors.

Apr 22, 2019 - Close Checking the Resistor Values and Other "D'Mod'n" - Measured all of the resistors in the chassis. All tested within 20% which is their specification. The screen load on the first IF had obviously over-heated in the past due to a shorted bypass cap (also obvious since the bypass cap had been replaced.) The 5K value was a little high and I will have to replace it with a vintage part. The plate load for the Mixer was a 40K resistor (a replacement part) but the value should be 25K so that will also require replacement with a vintage component. Removed the square Jones' receptacle and associated wiring. I had to leave the round Jones' receptacle installed since I didn't have the correct Hammarlund "relay" receptacle. Also, the Jones' receptacle used the original mounting holes, so when an original receptacle becomes available, replacement will be easy. The square hole will have to be covered with a butch plate. Dismounted the shield between the RF and IF sections to allow access to the screen bypass caps on the IF tube sockets. The chassis is now ready to rebuild with the goal of returning the circuit, the wiring, the component values and the lead dress back to original specifications. However, the IF and AVC transformers need to be rebuilt first. 

Apr 23, 2019 - More New Grid Leads, IF and AVC Can Rebuilds - Started with the BFO coil can. Removed can to install new grid lead. AC Neutral cream color rubber insulation with new stranded wire inside. Moved to Amplified AVC Output can. This one has three paper wax capacitors inside plus seven resistors and one fixed mica cap. Checked all of the resistors for value drift - all okay. Rebuilt the paper wax caps with new polyfilm caps inside the original shells. An interesting note was that the .02uf 400vdc cap was original and it was an Aerovox indicating that Hammarlund was still intermixing C-D TIGER and Aerovox caps at this time (1938.) No grid lead used on this can. Next was the Amplified AVC Input can with no hidden components inside. Only a new grid lead was required. Next was the Detector Output transformer which also has a hidden paper wax cap inside. Also, two resistors which measured within tolerance. Rebuilt this .05uf cap with a new polyfilm cap inside. Most of the can shield tops were dented so while they were off I removed all of the dents using wooden blocks to straighten the aluminum without scratching it.  Apr 24, 2019 - Variable-Coupling IF Cans and Xtal Filter Rebuilds - The Detector Input/Last IF transformer only required a new grid lead. The second and third IF transformers are the variable-coupled types so these have to be disassembled carefully. The upper fiber board is the stop for the compression spring for the moveable lower coil. If you just remove the can, the upper board can be pushed off of the guide rods allowing the spring to launch itself off of its guide rod. Slowly removing the can, the upper fiber board can be carefully slid off of the guide rod and the spring allowed to expand slowly. Then the new grid lead can be installed. Reassembly is easiest by compressing the spring, sliding the upper fiber board onto the guide rods and then installing the can while guiding the new grid wire out its hole in the can. It sounds difficult but it just requires a little dexterity to accomplish. The first IF transformer is the Crystal Filter on "X" version SP-100 receivers. There are four rubber insulated leads that needed replacements. The lower coil is inside an aluminum shield housing that mounts inside the outer can. You have to dismount the Crystal Filter IF can off of the chassis in order to access the coils. Five wires connect this IF can to the chassis which have to be unsoldered. Once the IF can is off of the chassis the lower coil can be removed. Then the shield is removed followed by removal of the the upper coil. Four new rubber insulated leads were installed and then the Crystal Filter IF reassembled and reinstalled onto the chassis.
Apr 25, 2019 - Restuffing Paper Wax Capacitors - I'd already restuffed the five paper-wax (pw) caps that were inside the RF box. Also, I'd already restuffed the three pw caps inside the Amplified AVC transformer and the one pw cap inside the Detector Output transformer. All that remained were the pw caps in the chassis. I already had eight .05uf 400vdc C-D shells to use for the eight non-original caps in the chassis. I went ahead and restuffed these eight shells so they would be ready to use. That leaves 18 pw caps left in the chassis to do.

My procedure nowadays is to make a simple drawing of the of the underside of the chassis showing the pw caps, their value and the location of the outside wrap band. With the drawing as a reference, I can remove a fairly large number of caps, rebuild them and then reinstall. Since there are 18 left, I'll probably do two sessions and remove nine caps for each rebuilding session. As a reference for time involved, it took one and half hours to rebuild the eight pw caps that I did earlier. This time didn't account for capacitor removal or reinstallation. Those tasks would have increased the time involved to probably two hours.

Photo right shows the rebuilt capacitors in the RF/IF section of the chassis.

Apr 26, 2019 - More PW Caps and Vari-coupled IF Problem - Installed the eight .05uf 400vdc rebuilt C-D TIGER caps. Rebuilt three .01uf 400vdc and three .05uf 400vdc C-D TIGER pw caps and installed those. 14 total installed with 12 left to do.

The Variable Coupled IF fiber board cam levers that actuate the IF plungers were warped. One was already broken and I broke the other one trying to mount it so it would ride on the cam correctly. These are easy to make since they are just .060" thick fiberboard material. The originals were riveted to the mount but if I can't save and reuse the rivets, I'll have to use a screw and nut to replace the rivet. Unfortunately, I've robbed these parts out of my Super Pro "parts set" a long time ago. I'll have to make these fiberboard pieces.

Apr 27, 2019 - Vari-Cpl'd IF Levers - I measured the thickness of the levers at 1/16" or .063" and the material is actually garolite which is a woven cloth that's impregnated with resin and is typically brown in color. Everything I had in the "junk boxes" was too thick. Since the slotted-end of the lever has to fit in the slot of the IF transformer plunger, the thickness is important. I ordered four 12"x12" sheets of .063" garolite off of eBay. I'll have plenty to make lots of mistakes in building these levers. 

Photo left shows the rebuilt capacitors in the AF/AVC section of the chassis.

Apr 28, 2019 - Mod or Wiring Change? - Since the garolite won't be here until next week, I continued on with the other problems that needed repair. In checking the schematic closely versus how the receiver was wired, it looks like some minor changes were installed. The stock standby function for all pre-SP-600 Super Pro receivers is to remove the B+ from the two RF amplifier plates along with the plate and screen B+ on the Mixer. The wiring to the SEND-REC switch was changed to have only the RF Amp plates turn off. Changed this wiring back to original.

Reconditioned the Crystal Filter front panel. Cleaned with Glass Plus, dried and applied "3 in One" oil. Remounted Crystal Filter assembly to front panel.

Removed six pw C-D TIGER pw caps, rebuilt and reinstalled.

May 1, 2019 - Making New Variable-Coupled IF Levers -  Garolite came today so I made the levers for the Variable IF Bandwidth adjustment. I scribed the dimensions into the .063" thick Garolite sheet and then cut out that section out using a Dremel tool and a thin cut-off wheel. Using a diamond impregnated file, I evenly shaped the overall dimensions required for two levers. Using the Dremel tool with a drum abrasive tool, I rounded the one end of each lever piece. I then used the Dremel tool with the thin cut-off disk installed and cut the slot in the other end. From the broken lever, I marked where the hole should be placed and drilled a 7/64" diameter hole. I then used a round diamond impregnated file to enlarge the hole to the diameter that allow the original rivet to just fit thru. I assembled each lever adjuster and then used a conical tool to reset the rivet. I was able to reuse the original rivets and this is important because the depth of the solid part of the shaft determines how the rivet seats and that allows the lever to move while the rivet is tight.

Shown to the left are the lever adjusters with the left-most being the completed assembly with new Garolite piece installed. Next is the second Garolite lever not yet assembled. Next is one of the broken original levers. The right-most piece is the second adjuster not yet rebuilt and showing the original break on this lever that's at the notched end. After both rebuilt lever assemblies were complete they were installed into the receiver chassis and tested for functionability with no problems encountered. Since all of the pw capacitors in this section of the receiver chassis have been replaced, I went ahead and reinstalled the RF-IF metal shield.

May 2 & 3, 2019 - A Very Interesting Problem - When removing these last six caps I noticed that the .25uf 400vdc cap that's the CW AVC time constant cap wasn't grounded. This was because, although the outside foil end of the capacitor was soldered to the ground lug, that ground lug wasn't mounted under the component board standoff-to-chassis which is how the chassis-ground connection is achieved. The interesting part is what else was connected to this "ungrounded" lug, 1 and pin 2 of one of the push-pull 6F6 audio output tubes. Pin 2 is the tube's heater return to chassis and it wasn't going to chassis ground, meaning that the this tube's heater wasn't operating. This mechanical connection is fairly well-hidden by the component board so it may have been that way for quite awhile. Makes one wonder what the former user/owner thought of the SP-100 audio which must have been anemic with only half of the "push-pull" working. Of course, I remounted the ground lug under the standoff.
More Caps - Rebuilt the remaining pw capacitors and reinstalled those rebuilt pw caps. I pulled the P-P 6F6 cathode bypass 50uf 75vdc electrolytic to measure it and found it completely open which wasn't totally unexpected. A modern electrolytic capacitor is dwarfed next to this gargantuan C-D unit. This capacitor consisted of a non-waxed cardboard tube with paper label. The electrolytic capacitor inside was kept in place by having the ends "capped" with sealing wax. This had to be chipped out on one side and then the capacitor could be pushed through leaving the cardboard tube empty. I reused the cardboard end caps that are recessed and under the sealing wax. I used a NOS Sprague 50uf 150vdc capacitor that I reformed and tested prior to installing into the tube. I had to install lead extensions because of the smaller size of this capacitor. Flexible cardboard spacers were used to keep the new capacitor from moving while the end caps were installed and then covered over with brown sealing wax. The lead extension joints were below the sealed ends and not visible. The rebuilt cathode bypass cap is virtually indistinguishable from the original in appearance.

Additionally, all of the C-D TIGER pw caps were coated with bee's wax as I rebuilt them. I've found that this really enhances their "original" appearance. All replaced resistors were vintage resistors that were measured for correct value and had the same physical size as originals. The resistors were just a few years newer so they had the color code bands. I repainted these resistors to use the "body-end-center band" code that most of the original resistors in this receiver had. Photo right shows the rear part of the chassis just behind the back of the RF box.

May 4, 2019 - Installing the RF Box - There are a couple things to watch when reinstalling the RF box. This first is that all eight wires, five on the AF side and three on the RF/IF side are lining-up with the semi-circle notches in the chassis. When dropping the RF box into the opening, you have to make sure it will clear the shielded AC power wire cable. Finally, you have to be very careful of how the dials engage the pinch-wheels. For the most part, the RF box drops in very slowly so watching and guiding it isn't that difficult. It might be necessary to loosen the chassis nuts for the pinch-wheel bushings to allow them to move around for better alignment to the dial edges. Once the RF box is setting on the chassis, the mounting screws can be installed. The antenna input section has two screws that are internally mounted, then the two outer covers can be installed. Once the RF box is mounted, then the dials and the pinch-wheels can be adjusted. Tighten the pinch-wheel bushing nuts when solid engagement is achieved. These older receivers don't have the nut on the back of the pinch-wheel disk, you can only move the bushing a little and then tighten the bushing nut.

Once the RF box is mounted then the eight wires are soldered to the correct locations. The AF side of the box has two red wires that are plate voltage and three blue wires that are the AVC connections. The RF side of the box has one green wire that connects to the LO cathode at the tube socket and two yellow wires that are the plate voltage connections at each RF amplifier tube socket. Screen voltage is connected externally to the tube sockets, not through the RF box.

May 5, 2019 - Installing the Front Panel - The front panel mounts using threaded extensions on the five toggle switch barrels, the bushing on the Bandwidth control and the bushing on the AF gain control. Additionally, there are two steel brackets that fit into notches in the chassis and bolt to the front panel in each upper corner. There are spacers that assure that these brackets mount tightly and also keep the panel perpendicular to the chassis. Once the panel is mounted, then the Crystal Filter has to be connected to the first IF transformer wires. The upper connection uses a lug and the two lower connections are soldered. Next the knobs can be installed. When found, this SP-100LX had replacement knobs for all of the controls except Main tuning. I installed all correct, original type knobs.

May 6, 2019 -  I installed the Carrier Level meter and that completed front panel installation. I made the butch plate to cover the square hole in the rear of the chassis. Next, all of the vacuum tubes had to be tested and replacements had to be found for the two 6K7 RF amplifier tubes (other tube types were installed there using adapters so the original sockets remained unchanged.) Also, three 6F6 tubes had to be found since the receiver had 6V6 tubes installed when I got it. I also got the power supply that came with this receiver out of the garage storage so I could start on its rebuild. Upon looking over the power supply I saw a "SC-101" Signal Corps' stamp on the chassis. This is an older, rack mount power supply with the cut-out on the rear chassis for the terminal strip to connect a field coil speaker. An original butch plate was riveted in place to cover the opening for the terminal strip. This indicates the power supply probably is for the SP-100 series but optioned to operate a PM speaker. The serial number is 5164 which indicates it could have originally been with a very late SP-100 or maybe with a very early SP-200. In either case, it will work with this receiver.

May 8, 2019 - SP-100LX Operational - I couldn't wait to test the LX and see if it would function with the BC-779 coils for the 100kc to 200kc band. Also, all of the weird additions and minor problems found had me wondering how this receiver had operated in the past. Since any later SP power supply will power up a SP-100 series receiver, I got out an early SP-200 power supply that was one I used regularly and was known to operate correctly. I used an eight inch, 8Z ohm speaker and a 20 foot long wire on the floor for the antenna. One terminal of the antenna input was grounded to chassis for an unbalanced input. Upon power up, the background noise sounded pretty normal. I noticed that I had gotten the dial mask set to the wrong band. This is very easy to correct without removing the front panel. The drive gear on the band switch has a set screw that can be loosened. Then the drive gear can slide on the band switch shaft forward to disengage from the idler gear. Now the mask can be rotated to the correct band. I just tuned to the band that received WWV 10mc at the bottom of the range and WWV 15mc near the middle of the range. This had to be the 10mc to 20mc band and the mask was set accordingly as was the band switch knob. The drive gear was then slid back on the shaft until it engaged the idler gear and then the set screw tightened.

Tuning around (mid-afternoon) I heard several hams on 40M, SW BC around 12mc and several other signals. All controls seemed to be working. I turned on the HP 606B so I could have a signal source of known frequency and amplitude. Checking all bands, everything was pretty close indicating that alignment was pretty good. The big test was how did 100kc to 200kc work? I tuned the receiver to 150kc and set the HP 606B on its lowest range I adjusted its frequency to 150kc. I tuned the receiver until I heard the signal - at 146kc - pretty close. Lots of house electrical noise with the short antenna laying on the floor on the lower frequencies, which is normal.

The next step will be to align the LX. It should be very close but I'd expect the 100kc to 200kc coils to need more adjustments since they were formerly installed in a BC-779.

Photo left shows the completed SP-100LX from the bottom. Note the MFP stamp on the RF box cover. It's dated 4 April 1945. Although the receiver doesn't have any military acceptance stamps, the power supply is stamped "SC101" which probably is a Signal Corps acceptance stamp but it's not in the typical orange paint but is instead black paint. In comparing the LX chassis to the SP-100X chassis, there are some slight differences in component mounting of the bypass capacitors in the amplified AVC section. Also, the Mixer screen load resistor is connected to the B+ on the Rec side of the Send/Rec switch rather than crossing over the shield and connecting to the B+ on an IF component board (that's how it's mounted in the SP-100X.) The LX with serial number 2730 and the SP-100X with serial number 3387 are 557 serial numbers apart but, since the power supplies were still being sequentially serialized, that would seem to indicate that the two receivers are 278 units part in manufacture.

Alignment Procedure Error - The SP-100L(X) manual that I copied from BAMA has an error in the LO alignment of the 100kc to 200kc band. The error is in the reference to the padder capacitor (works with the L and varies the low end frequency adjustment) as a "trimmer capacitor" on coil Y adjusted at 100kc. The next error is referencing the actual trimmer capacitor as the "inductance" adjustment on coil Y adjusted at 200kc. The padder is shown as the inductance adjustment in the alignment component drawing. Confusion is sure to happen unless you look at the BC-779 alignment instructions in TM11-866, which are correct. Oddly, in the LX manual, the 200kc to 400kc band, which also uses a padder capacitor for low end frequency adjustment, is correct. Additionally, the alignment instructions for the remaining bands are correct. But, on to the alignment,...

May 11, 2019 - Alignment - As expected from how the receiver performed before alignment, the IF was very close in adjustment. Only very slight movements were necessary to have the IF agree with the crystal's active frequency, which was 464.9kc. Amplified AVC was also very close only requiring the slightest tweaking. I did go thru the IF adjustments twice just to double check.

RF tracking was significantly off, as expected, on the 100kc to 200kc band. However, it adjusted into specification very easily. The remaining bands were all fairly close in alignment only requiring slight adjustments to bring each band into specification.

I used an audio output meter for the IF alignment but I used the SP-100LX carrier level meter for the RF tracking adjustments.

May 12, 2019 - Reassembly - The bottom cover was installed using all binder head screws. The dust cover was thoroughly cleaned and then given the "3 in One" oil treatment. It was then installed on the receiver using the original capped nuts and rear thumb screws. This completed the receiver's reassembly.

SP-100LX  SN: 2730

May 13, 2019 - Power Supply - Although I'm using the early SP-200 power supply to run the "LX" and it does a fine job, I did want to use this SP-100 power supply. There are slight differences between the 200 and the 100 supplies. For example, the DCR of the input choke, some of the wire wound resistor values and the obvious opening for the field coil terminal strip that had the butch plate riveted in place to cover the hole. So, this was a power supply for a late version SP-100 or perhaps a very early SP-200.

Unfortunately, the 100 Series power supply was in deplorable condition with the front panel having been painted "sparkle gray" and also a lot of .25" holes drilled in the cabinet over the 5Z3 rectifier tube. Inside, none of the filter capacitors were original and one filter cap on the bias supply was missing altogether.

I looked thru my stock of old style can electrolytics to see if I could find three 16uf cans that matched, luck. I looked thru more junk boxes to see if I could find a box-type cardboard housing for the three 8uf caps,...again, nothing. I decided to install a vintage component board under the chassis and mount new filter caps there. If, in the future, I do find three matching vintage cans or a vintage cardboard cap housing, I can replace the component board and install the new caps inside these cans and the box.

I had to substantially rewire the power supply because numerous repairs over the years had left most of the solder joints looking like "glop jobs." I used vintage cloth covered wire and, if vintage cans/box turn up, the wiring changes will be minimal.

I sanded off the crappy paint and repainted the front panel in black wrinkle finish (on May 14th.) I couldn't really do anything about the numerous .25" holes over the 5Z3, so I cleaned the cabinet and gave it the "3 in One" oil treatment. The holes might have been a Signal Corps mod since the layout was nice and square and the holes drilled with some precision (it almost looks stock,...except it isn't.)

I'm going to let the front panel wrinkle finish paint cure for several days before I reinstall the panel and complete assembly of this power supply.


Photo Gallery of Collector's Super-Pro Receivers

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



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


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



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


SP-200X with Faux Woodgrain Panel and Table Cabinet

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


SP-100S Special Diversity Receiver built for CODECO

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



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


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


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

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


WA7YBS - I came across this nice example of a SP-400-SX receiver setting on the floor at Ham & Hi Fi in Sparks, Nevada on April 17, 2019. There were four other receivers stacked on top of the SP-400-SX. The price was cheap and included the correct matching power supply but there wasn't an interconnecting cable nor was the bottom plate on the power supply. It's overall condition is very good but there are some issues. The chassis appears to have been "conservatively" modified to use single-ended octal tubes for the first RF amp (6AC7 in place of the 6K7,) the Mixer (6SJ7 in place of the 6L7) and the LO (6SJ7 in place of the 6J7.) The grid leads exit the RF box and, using wire extensions, are then routed though grommet-lined holes in the chassis to connect to the appropriate tube socket grid pin. All of the other tubes appear to be the original type. SEND-REC has an "unwired" potentiometer installed instead of a two-position switch (incorrect knob also.) The original two-terminal floating (balanced) Antenna Input block was replaced with an unbalanced SO-239 connector. On the rear chassis an RCA jack was added and marked "PAN" which probably was for a Panoramic Adaptor. Above this RCA jack a square notch was cut out of the back of the cabinet (connector clearance?) The cabinet is missing the handles. Four small holes were added on top of the cabinet (possibly for "top-mounted" carrying handles.) Luckily, most of the unique knobs (unique to the SP-400 and the HQ-129) are present but the tuning and bandspread knobs are National type knobs. The "SX" version of the SP-400 tunes from 1.25mc up to 40mc.


The SP-400, SP-600 and Conclusion

The Hammarlund Super-Pro of the thirties was just what Hammarlund had intended for it to be - a "standard" by which all other communications receivers would be measured. By the time WWII began, the Super-Pro had matured into a first-rate, ruggedly-built receiver that the military could use without reservation or modification. Immediately after the war, Hammarlund began to offer the new SP-400-X Super-Pro. The new receiver redesigned the wartime SP-200 by changing the frequency coverage to the more standard .54 to 30MC (or 1.2 to 40MC for the SX version) and changing the P-P audio tubes in the later versions to 6V6 types with audio output of 8 watts with dual audio output Z of 8 or 500 ohms (early versions used 6F6 AF output tubes and 600Z and 8000Z outputs - like the SP-200.) Also the IF frequency was changed from 465KC to the more standard 455KC. Other than those changes, the receiver was still very familiar to former SP-200 owners. The styling changes to the front panel using a very thin smooth finish paint and miniscule knobs (that are normally not found on surviving examples today) have resulted in very few SP-400 front panels remaining in good condition. Most are found with severe wear around the control nomenclature. However, some examples of the SP-400 will be found with original gray wrinkle finish panels and engraved nomenclature. These style panels are usually in much better condition because of the durable qualities of wrinkle finish. Unlike its immediate predecessors (that is, the WWII versions of the SP-200,) the SP-400 is usually found in the table top cabinet rather than in a rack mount configuration. The SP-400 was only around for a few years, 1946 to 1948. It wasn't built in large quantities and isn't seen too often. Hammarlund was biding their time and designing a totally new Super-Pro that would again set the "standard" for what a modern communications receiver would be - the famous SP-600 series. That the SP-600 owes a lot to its predecessors, especially in design approach, is obvious. The  SP-600 is a well-known receiver with a plethora of information available on the web, including our own article "Rebuilding the Hammarlund SP-600" - link to that article is in the Navigation Index.

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




photo right top: SP-400-X owned by W2EMN


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

Information Wanted:

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

We are particularly interested in the following:

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

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

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

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

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

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

7. Any R-129/U that is operational - what is the IF? Manual says the IF is 465kc but the receiver's lowest frequency band supposedly covers 300kc to 540kc which implies a different IF or non-continuous coverage on that range. I suspect the the R-129/U actually covers 200kc to 400kc on the lowest band and .54mc to 10mc on the upper bands, like the Swedish MRM-5 versions of the SP-100. This would use coil sets that weren't "special builds" and only substitutes standard 200kc to 400kc coils for the standard 10mc to 20mc coils. This would allow a 465kc IF and would provide a frequency coverage that is close to what is specified. But, that's just speculation. Perhaps an owner of a R-129/U could let me know what an actual R-129/U receiver's frequency coverage is.

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



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

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

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

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

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

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

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

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

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

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

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

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


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

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

Comet and Comet "Pro" Features

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

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

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

Performance Expectations Using the Comet Pro

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

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

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

Comet Pro Selling Prices

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

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

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

Patterson PR-10............. $70.00 - speaker included (dealer's price - list was $119)
Patterson Preselector.....~$25.00
National AGS-X............... $265.00 - price with all necessary accessories
National FB-7 complete...$90.00 - price with all necessary accessories
RME-9D......................... $112.00 - speaker included

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

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

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

Henry Rogers - June 2008

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

Henry Rogers - WA7YBS  February 2008 - 2019  Additional Information Added: March 2008, April 2008, May 2008, June 2008, Aug 2008, Apr 2009, Nov 2009, Aug 2010 (clarification on model numbers,) Nov 2010 (add'l info on Geisler Mods), June 2012 - Re-Edit Style and Appearance to conform to later articles, October 2014 - corrections to Comet Pro info, July 2016 - SP-100LX info added, January 2017 - SPA info added, Apr 2019 added info on SP-100LX restoration, also minor corrections and additions to all sections, 

Pre-War Super Pro Part 1                      Pre-War Super Pro Part 2                        Return to Home Index



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