My third antique radio project is a 1949 Sears Silvertone AM/FM radio (model 8020) that I restored for my father-in-law. This was his family’s radio when he was growing up.
As par for the course with radios of this vintage, it had a lot of electrolytic and wax paper capacitors that needed replacing.
I ordered replacement caps from DigiKey and replaced each of them. Wiring up the replacement electrolytics required a little creativity since there as a shortage of mounting tabs available.
This radio was the first I’ve come across that uses a solid-state selenium rectifier instead of a tube diode. The rectifier is the stack of orange plates on the left.
These rectifiers can be unreliable and when they fail they release noxious smoke that smells like rotten garlic, so it’s advisable to replace them with a silicon diode. Usually a high-wattage resistor is wired in series with the diode since it only drops 0.6V instead of the 10–15V drop of a selenium rectifier. I cut the leads off the old rectifier but left it mounted for appearance’s sake.
The final step in my initial repairs was to replace the power cord as well as several internal wires whose insulation had become brittle and started falling off. To remove the power cord, I had to break the rivets that were holding the connector to the back of the radio and unfortunately damaged the back in the process.
Once I finished the initial repairs, the radio powered up and the AM played pretty well, but the FM was totally dead. Upon closer inspection I found that the radio had a 6AK5 tube where it was supposed to have a 6C4 for the FM RF amplifier. These are not even the same type of tube — the 6AK5 is a pentode and the 6C4 is a triode — and they have different pinouts, so it’s unlikely that the radio ever worked with this tube. I ordered a replacement 6C4 from Tube Depot.
Replacing the 6C4 didn’t do the trick so I turned to the folks at Antique Radios Forum once again for help. They suggested that the local oscillator (12BE6) might be weak and unable to reach FM frequencies. I tested the radio with a tube borrowed from another radio and after confirming that it fixed the FM, I ordered another 12BE6 so I could return the loaner to its original home.
After replacing the tube, the FM was still very quiet. The video below was taken with the volume turned all the way up.
At this point, someone on the forum suggested aligning the FM coils. My signal generator isn’t capable of FM modulation, but I was able to perform a decent alignment by ear simply by tuning to a weak station and adjusting each coil up or down until I peaked the volume and minimized distortion. After the alignment, the FM band was as loud as AM and was picking up a lot more stations. The results can be seen in the video below.
The only remaining issue I haven’t fixed yet is the back of the radio. Unfortunately the particle board was quite brittle after all these years and began crumbling due to all the handling.
My plan is to find someone with a jigsaw who can cut out a replacement panel from new wood and then trim the old back around the AM antenna and attach it to the replacement back.