I won't attempt to get into radio theory in this instructable, nor attempt to provide troubleshooting info on all that can go wrong. What I'll focus on, however, is what I've found to be wrong in the last five radios that I've resurrected, which is not vacuum tubes, but bad capacitors. Old electolytic, paper, and wax capacitors were never designed to last more than a couple of decades, therefore when you find a 50-70 year old radio that doesn't work properly, replacing the capacitors is a great place to start! Although a tube can certainly go bad, they can last almost indefinitely unless they are either subjected to rough handling, or subjected to excessive voltage from another component failure.
Also, if you attempt to work on one of these old radios, be very careful. It is not uncommon for some of these radios to use voltages in the hundreds of volts, and that can hurt, or worse! So, exercise caution. Don't work on it when it is plugged in, and be sure to discharge each capacitor (by shorting across the terminals) even when it is unplugged.
The radio I'll show in this instructable is a Zenith Wavemagnet radio that was manufactured sometime prior to March, 1942, making it almost 70 years old at the time of this Instructable. When I first powered up this radio, it would receive some stations, but had a loud hum, and the sound became very distorted after just a few minutes. The hum was a good indication that the filter cap's were bad, and the distortion turned out to be due to one bad paper capacitor.
Step 1: Remove the Radio Chassis From the Case
Once I have the radio out of its case, my first step is always to replace the power cord. Even if the power cord looks good, I replace it anyway.
Step 2: Replace the Electrolytic Capacitors
I was fortunate that the values of the capacitors in these two cans were clearly marked on the can. If they had not been, I would have to have reverted to a schematic and parts list for this particular radio.
Modern capacitors are very small in size compared to capacitors made 70 years ago, so I decided to simply cut the leads to the old capacitors and place the new electrolytics underneath the chassis. The third photo shows two new electrolytics mounted under the chassis as a replacement for one of the cans.
By the way, most electrolytics are polarized, meaning that it is important to get the positive and the negative side installed correctly. Otherwise, things can get a bit too exciting when you power up the radio! Also, I make it a practice to power up the radio after replacing each capacitor to ensure that I haven't made a mistake.
As I work my way through the chassis, I always replace any wire that has brittle insulation. Once all the filter cap's were replaced, the irritating hum was no longer there when the radio powered on.
Step 3: Replace the Paper and Wax Covered Capacitors
Once these capacitors were replaced, the radio's distortion stopped. Actually it stopped when I replaced the 6th capacitor, but I went ahead and replaced them all. These were never designed to last 70 years!
You may notice that several pieces of the radio's original wiring have been replaced in this third photo. Where a wire was originally rubber coated, I either replaced it or made a sleeve for it from heat shrink tubing. Rubber coated wire in an old radio like this is very brittle, and should be either replaced or covered by more insulation.
Step 4: Radio Is Now Working Correctly
Step 5: Asbestos -- Nasty Stuff!
Step 6: 70 Years of Neglect!
The face of the radio was wood, and I touched it up with wood stain.
The lens over the dial had become cloudy, so I buffed it out with a very fine metal polish.
Step 7: The Results
I can't help but wonder where this radio has been, and who has listened to it over the years. Did they hear the news reports from WW2 on it? Did they listen to the old radio shows?
I don't know where this radio has been nor who has listened to it, but I do know that this fall I'll be listening to the games on it!