Today I traveled back to the Merrimack Valley to visit a high school buddy, Larry, who like me, has caught the vintage electronics bug.
An increasing number of vintage radios, phonographs and – gasp – 8-track players – have been finding their way to Larry’s home, and as you might expect, most need a little TLC to return them to fully functional status.
Larry was under the mis-perception that I am some sort of electronics genius because I was able to resuscitate those dead radios back to life. Ha! So when he recently called me asking for help with his 1940s era Admiral console radio and phonograph that had a low volume problem, I thought it was a good time to stop giving Larry fishes and give him a fishing lesson.
The Admiral 7CS5W is a 1948 6 tube superheterodyne AM band only radio receiver with a 78 RPM record changer in a side-by-each console arrangement. Larry’s complaint was that whether the radio or the phonograph was playing, the volume was too low.
We downloaded the set schematics from the www.radiomuseum.org website and I showed Larry how to read the schematic, working from left to right, identifying the different stages of the radio circuit. We focused on the final audio stage which included a 6SQ7 phase inverter and a pair of 6K6 power output tubes.
We started our work by testing all of the tubes, starting with the above named audio stage tubes, using my grandfather’s EMC 205 tube tester. We tested all tubes for shorts and quality and all passed.
It was time to pull the chassis and see what was going on underneath. Looking at the chassis, I immediately noticed a hole where the original canned filter caps would have been – clearly someone had already serviced the radio and had replaced the electrolytic capacitors.
I was mildly concerned that because the tubes tested good, as they did, and someone had recently recapped the radio, the low audio problem was going to be more complex to solve.
We discovered that while someone indeed had previously serviced the radio, and had replaced the two electrolytic capacitors, he only replaced about half of the wax caps. Also more interestingly, those replacement caps did not look that new. The electrolytic capacitors were a pair of large General Electric cap that I’ve never seen before. The other replacement caps were a variety of blue-green paper caps that I was also unfamiliar with. I would have to guess that this radio was last serviced in the 1970s or early 1980s.
So, starting with the filters, we recapped the entire set. Larry was a quick student, absorbing what I taught him about how capacitors function and why they need to be replaced – see my previous blog post about recapping The Glendon RCA receiver.
We needed to manufacture a pair of 30 MFD electrolytic caps as the stock I brought did not have that value. For each we wired a 10 MFD and a 22 MFD in parallel to produce a pair of 32 MFD capacitors. When working with old radio circuits, capacitor values aren’t critical. As long as the replacement caps has the correct voltage rating or higher, it’s generally o.k. to replace the original with a higher value. For the .002 MFD caps in the Admiral, we used .005 MFD replacements.
After about 2 hours work and a lot of good conversation, we had the two electrolytic caps and ten of the wax and paper caps replaced.
Now was the moment of truth – would the recapping improve the low audio? Only one way to find out. We put the tubes back in, reinstalled the chassis in the cabinet and reconnected the speaker, antenna, and phono connectors and braced ourselves for the ‘smoke test.’ How did we do?
Replacing the capacitors completely resolved the low volume problem. The radio and phonograph ought to be good for the next 30 years.
In the end we saved another beautiful old radio , we launched Larry on the road of vintage electronics repair while catching up and I also disavowed him of his mis-perception that I’m some sort of a genius – not a bad Saturday at all.