Radio Boulevard
Western Historic Radio Museum

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

Part 3

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

by: Henry Rogers WA7YBS/WHRM


Restoring Pre-War Super-Pro Receivers


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

photo left: The WMI SP-10 before restoration

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

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"

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

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

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

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

Rebuilding Capacitors

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

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

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

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

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

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

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

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

Other Restoration Work

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

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

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

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

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

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

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


Restoring the SP-100LX "Super Pro"

photo above: SP-100LX sn: 2730 showing the "as found" condition

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.

First Inspection - The serial number on this SP-100LX is 2730 which is about in the middle of production meaning it's probably from mid-1938. The receiver has indications that it was supplied to or was used by the Signal Corps during WWII since there's SC acceptance stamps and MFP date stamps (4APR1945) on the top of the RF box and bottom of the coil box but there isn't any MFP except on these panels themselves. 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. The band spread knob is certainly not original. The white pointer type knobs aren't original although all the knobs do match. It might have been something that the Signal Corps installed for some reason (but probably not.) 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. The Jones' sockets and tube adapter mods appear to be from the late-fifties.

Jan 21, 2017 - First Attempt to Get Started - 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 two resistors changed and seven 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 - Disassembled 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.)

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 - Dismount 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 - 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 L10secondary 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 - 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. I added extensions using the correct color cloth insulated wire. The splice was located inside the RF Box so as not to be visible once the shield cover is installed. LO cathode wire was okay for length. The wires on opposite side of the RF Box that connect to B+ voltages and AVC were okay for length.

April 13, 2019 - 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 - 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 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 paper caps. Replacing these caps with rebuilt C-D TIGER caps will require five .01uf 400vdc TIGER shells to install new polyfilm caps inside.


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.


The SP-400, SP-600 and Conclusion

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

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




photo right top: SP-400 owned by W2EMN

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

Information Wanted:

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

We are particularly interested in the following:

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

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

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

4. SP-100LX versions with non-standard LF coverage. Standard 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 covers 300kc to 540kc which implies a different IF or non-continuous coverage on that range.

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



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

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

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

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

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

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

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

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

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

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

       and the Russian KV-M,       Thanks to Karl-Arne Markstrom, 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  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,

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



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