Intro: Rebuilding an Old AM Radio
What could be better this Fall than listening to the ball game on an old vintage AM radio? This instructable focuses on how I rebuilt an old, non-working pre-1942 AM radio.
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
Most of these old radios are fairly easy to remove from their housings. Generally the chassis will be mounted with 2-4 screws, usually from the bottom. With this particular radio, the chassis was secured with two screws -- one on each side of the case. After removing these two screws, removing the tuning and volume control knobs, and unplugging the antenna and speaker, the radio chassis simply slid off its shelf from the back.
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
Electrolytic capacitors are often called filter capacitors in these old radios. Sometimes they are mounted underneath the chassis; other times they are mounted on the top in "cans." The filter cap's in this old radio were mounted on top in two cans, and are what is called "stacked" capacitors. With stacked capacitors, each can will contain two or more separate capacitors in one can. In the case of this radio, one can held three capacitors, and the other had two.
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
The round objects in the first two photos are the paper and wax covered capacitors. All were replaced with modern Mylar capacitors (shown in the 3rd photo). I carefully matched each capacitor's capacitance rating, but used Mylar cap's with a 630 volt rating, regardless of the original capacitors' voltage rating. Going high on the voltage rating is safe. You never want to go lower in voltage than the original.
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
By this stage I have replaced the filter (electrolytic) capacitors and the paper and wax covered capacitors, replaced the power cord, and fixed all wiring that needed insulation. Upon powering the radio up, it now worked fine. So, my next step was to tackle the case.
Step 5: Asbestos -- Nasty Stuff!
This radio contained a thin sheet of asbestos on the shelf where the radio chassis sits. Rather than attempt to remove it, I sealed it with several coats of varnish. When asbestos is encapsulated, it is safe. You don't want these fibers floating around in the air. I made sure to wear a mask while I worked on this shelf.
Step 6: 70 Years of Neglect!
The case for this radio was covered in leather, and paint spots, and dirt! I began by giving the case a good scrubbing with saddle soap, then gluing down any places where the leather had begun to come loose. After the case was good and clean, I went over it with wax-based shoe polish, then buffed it like a pair of shoes!
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
This was a fun project. I enjoy taking old, nostalgic things and making them usable again.
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!