I’m recently the proud owner of a copy of the original dial-stringing guides for several Stromberg-Carlson radios from 1939-1941. This information is extremely hard to come by – it wasn’t included with the original engineering/service data, nor is it anywhere else on the web. Back in February when I was starting to restore a SC 520-PG for gift, I was looking for this dial-stringing information. With the help of a fellow radio collector Jody, I was able to come up with a photocopy of the original service data sheet. In six months of intensive searching and consulting every collector’s resource I know of, he was the only one able to help. Most other collectors and hobbyists, including several who were around when they were making these new, didn’t believe the data was ever published in the first place so I’m quite excited to be able to share it here.
I’m making these available free of charge to any collector or hobbyist who can use them, in the hope they’ll save someone the amount of frustration I had to go through to get my 520 dial restrung without a guide. So, without further delay, here they are:
These diagrams obviously cover the Stromberg-Carlson models 519, 520, 512, 522 and 523. If you’re looking for schematics and engineering data, check out Made in Rochester where you can find high quality scanned data for servicing and aligning these receivers.
My Stromberg-Carlson 520-PG (previous parts 1 2) is finally finished! The radio came out ahead of my expectations, even, and was delivered to its new home yesterday where I expect it will continue to serve for years to come. See these in-progress photos, or jump to the bottom for the “after” picture!
The radio was fairly beat up when I found it – dirty, scratches, missing the speaker cloth, and completely original electrically.
After assessing the condition of the coils and transformers, it was time to mount the chassis to the bench and start the real work.
After replacing pretty much everything that needed replaced or probably would soon, the radio was back to full electrical integrity and ready for alignment:
Alignment completed, it was time to work on the cabinet:
And, at last, time to re-cover the speaker and reinstall! The final results:
It plays beautifully on line input, and picked up distant AM and Shortwave broadcasts (including Radio Australia, a Russian CW and an Entertainment station, Japanese music, English-language religious programming, and a Cuban broadcast. Now, it’s on to the next project!
My current main project, refurbishing my Stromberg-Carlson 520-PG radio (which has been a fair amount of fun so far) is nearing completion. I’ve just completed an IF alignment, which greatly improved the sound quality and volume and general performance.
Radios contained tuned circuits, and like any tuned circuit it can drift over time and become un-tuned. Poorly aligned tuned circuits will result in all kinds of problems – stations showing up at the wrong place on the dial, low volume, distortion, and poor reception. Superhetrodyne receivers, the modern type made popular in the early 1930s and still in use today, convert the incoming radio signals from their radio frequency (RF) level, down to an intermediate frequency (IF). By injecting a signal at the same frequency as the IF and adjusting the fine tuning on the IF transformers, you can measure the changes in output using a meter or even by ear – the volume change is very obvious.
The radio is ready to reinstall:
The last steps are to clean up the cabinet a bit, and repair the grill cloth. I’ve purchased a new grill cloth from Grill Cloth Headquarters (linked on the right sidebar of my blog) and have finally dismantled the mounting board. I’ll be using Howard Restor-a-Finish, #0000 steel wool on scratches, rings and marks and a regular finish application pad for the flat surfaces. This particular cabinet is in excellent shape for its age, and the finish needs just a little touch-up to look great again.
If I’m lucky, I’ll be able to send this beautiful piece of history to its new home in only a few more days.
In this photo, the shortwave loop antenna for distance reception, the cabinet with speaker board removed, the original Webster-Chicago 78 rpm drop changer, and the speaker board. The record changer is still functional, in fact – the motor started up and the mechanism clicked when I turned it on, and pressing the action button made the tone arm lift up, move over and set itself down. Quite a surprise!
I’m still working on my 1941 Stromberg-Carlson 520-PG and am running into a few new problems I’ve never come across before. Back in the era of tube radios, everything was serviceable on a component level and most people had enough aptitude and desire to learn that it wasn’t uncommon for people to attempt repairs themselves. You also had “radio technicians” who may or may not have been reputably trained and were frequently turned loose on people’s equipment for the lowest bid.
This was definitely the case with my radio, as I’m discovering evidence of someone else having been inside and attempted repairs that were of dubious quality and may never have worked properly in the first place.
It’s pretty easy to tell if someone has made repairs before. Component brands is the easiest method, at least for major-brand radio sets from that time period. Most of the capacitors in this Stromberg-Carlson set were branded with that name, but Mallory capacitors made a couple of appearances too. These were clearly replacements, S-C .01uF capacitor on top and a Mallory .01uF capacitor on the bottom. Making it worse, the solder job was so bad I was able to slide the entire joint up and down along its wire – I doubt there was ever an electrical connection between the wires, even though they were physically fixed together.
There’s also an extra part not listed on the schematic, bypassing the high voltage plate resistor for the audio amplifier tube to ground. I speculate this was done to eliminate some interference from getting into the audio, but it’s another modification that is of unknown quality. The capacitor, the block with colored dots, has started to fail after ~70 years and occasionally introduces some static into the audio as it’s playing. (The capacitor is across R-6 on the schematic snip slightly lower on the page.)
Finally, there are wiring changes made that don’t match the schematic. Is the schematic wrong, or is the wiring in the radio wrong? Schematics of the day were hand-drawn by draftsmen who frequently worked long hours revising and publishing schematics and service data, and mistakes are not unheard of.
I’ve been in other radios that have shown evidence of previous repairs, like this Packard Bell 35-Late which has 5 different brands of capacitors from 3 distinct eras of materials, but were all wired correctly. Pretty much every one of the cylinders except for the tiniest ones is a capacitor in this photo:
or this Zenith 7-S-363 which used both Zenith- and Mallory-branded capacitors from the factory, and also contained Aerovox and Solar later replacements but also worked perfectly after repair:
This Stromberg-Carlson is the first radio I’ve serviced where there were clear mistakes made along the way. It’s an entirely new set of challenges on top of the already-difficult antique radio repair process, but it does add a level of fun and discovery that a straight-up easy “recap” repair doesn’t offer.
Edit: After some peer review, it turns out it was a draftman’s error between the schematic diagram and the wiring diagram! Annoying. I should publish some errata, maybe I’ll do that soon.
If anyone reading this has information about a Stromberg-Carlson 520-PG from 1941, I would appreciate if you contacted me. I’ve been trying to find a photo of one that someone else owns, or even a picture of one in a printed advertisement, to help determine how it needs to be refinished – but as far as my research has indicated, I might have the only documented example of this model.
The Radiomuseum doesn’t have any more information than I do (in fact, those are my photos on their site). There’s nothing in the Radio Attic Archives, a huge collection of photos of restored radios from all different models. The Antique Radio Forum gallery of Stromberg Carlson radios has a bare chassis photo of another model in the 520 family. And even “Made in Rochester“, a site dedicated to a lot of S-C audio equipment, has only the scanned engineering data. A fellow radio collector has identified it as having a highly desirable Ingraham cabinet, which is a plus.
There’s this ad, showing the 520-PL which has the chassis mounted under a hinge in the top, and the record player is accessed from above:
Another radio enthusiast on the board found a 520-J which uses the same chassis but in a tabletop format, that’s the closest I’ve found so far: