Rebuilding an Old AM Radio

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Introduction: Rebuilding an Old AM Radio

About: I enjoy taking a pile of junk and making something unusual out of it. I like wheeled vehicles, and currently own two motorcycles, two electric bikes that I've built, and an electric scooter pushed by a soc...

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!


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    40 Discussions

    Ok im having trouble putting a new power cord on wich side is ground and power

    1 reply

    I would use a volt meter to determine what is what.

    I just wanted to mention that you don't have to use wax capacitor's for replacement. most can be replaced with a .01 if you cant find the others. the dipped capacitor's work the best and always a higher voltage that the original if you can. It improves quality.

    You can also use a variable ac transformer on these old radios. It is a 0 to 120 volts. start at 0 volts and (with radio power on) you vary slowly turn up the voltage and this helps to preserve the life of the capacitor. If the radio has not been used for a long time the capacitors become fully discharged. sudden surges on the old wax capacitors can weaken them. Not to say some caps are bad already.

    It's great to see and hear people are bringing life to history.

    Keep up the great work. Ever have any repair questions ask...I have tubes and old radio manuals as I have repaired many of these radios and would be more than happy to help.

    5 replies

    I have a question for you ive got an olx vacume tube clock radio that when ever i plug it in the radio starts right away with a tiny buzz on the speaker but as the tubes warm up the buzz gets louder and deeper to a point its very loud i cannot get the radio do anything but do this loud humming (very loud) the clock works perfectly the radios power supply seems to pass through the clock (the clock allarm can turn on the radio instead of a buzzer/bell) any clue as to why this is happening

    Check the filter capacitor. What you are hearing i believe to be bleed over. One cap is shorting with the other. also know as AC hum.
    At least in the old days...lol
    Look for either a aluminum can or big capacitor with 3 to 4 leads on it. Will also have 3 or 4 values on it.
    hope this helps.

    I have a question... If the can is marked with 2 different values with the same voltage, can I just add the values together and replace with one single?

    No. If a can is marked with 2 values it means that it contains 2 separate capacitors.

    Pardon my presumption (great project, by the way), but that asbestos was there to make that shelf so it would be heat-insulated and (I presume) fireproof. Applying varnish (flammable) may be a concern for future use. Not for nothing, but I'd hate for this fine object to burn your house down!

    1 reply

    The layer of asbestos was thinner than paper thin -- too thin to offer protection from the fiery destruction of an exploding capacitor. It may have been thicker 70 years ago, but there wasn't much left. After sealing it, I covered it with a layer of sheet aluminum covered with electricians tape. Thank you for your comment.

    In the late 1980s I bought a Zenith Trans Oceanic at a yard sale. The case was basically the same as the case on your radio. It was considered one of the first portable radios, even though it was almost as large as a small suitcase. My radio was probably made about 1950, even though Trans Oceanics became available quite a few years earlier and had minor variations in their appearance on knobs, buttons, and dial.

    My radio worked the first time I turned it "on," but not the second time. I finally learned to replace all of the paper and electrolytic capacitors. When one failed and was replaced, the next one down the line failed soon afterward.

    Seeing the photos of the underside of your radio chassis brings back many memories. I was working at learning to understand spoken German, and used that radio in my office to hear broadcasts in that language from Deutsche Welle, Radio Canada, and Radio Austria. After a few years I sold that radio to a younger guy who loved to collect old radios. He was especially pleased that the one I sold him actually worked.

    Thank you for posting this. Good job!

    2 replies

    Thanks, Phil. When I was a kid, I was given an old wooden cased radio that had a couple of shortwave bands. I strung an antenna between our house and an outside building, and used to listen to Radio Free Europe, Radio Moscow, and Radio Havana. This was in the late '50's, and you can probably imagine the propaganda that Moscow and Havana was broadcasting at the time. On some nights I'd pick up a ham radio operator, but rarely would get to listen to both sides of the conversation.

    I've rebuilt about 5 old AM radios, and one old AM/FM. I'm currently working on a 1939 (or 1940) Zenith, and so far it has me scratching my head. Tubes are all good, I've replaced all the capacitors, but still barely gets a station, and sounds terrible! I'll get it figured out eventually, though. Probably a bad resistor, or a bad thing-a-ma-jig.........

    Thanks again for your comment.

    In regard to propaganda from Radio Moscow, I did not hear it myself, but when the Berlin Wall fell in 1989 Radio Berlin went on the air and told its listeners, "We will no longer lie to you." At the risk of shameless self-promotion, you might enjoy my Instructable called "Listen to Shortwave Broadcasts on AM Radio," not because of any great technical insights, but because I shared some interesting stories about things I heard on shortwave.

    I wish you well with your restoration projects. 

    When shorting the capacitors to remove residual charge, use a resistor, say 100 K ohms, instead of a short piece of wire. This will slow down the discharge rate, resulting in less stress on the old components.

    Many restorers prefer to replace only components that are failed. This is the "If it ain't broke, don't fix it" approach. Others prefer to go in once, do everything. I guess it all depends on the cost and availability of replacement parts.

    A quick method of checking whether a power supply electrolytic capacitor is still working is to check for AC voltage across the terminals while the radio is powered up. Anything higher than, say, 5 volts AC between a positive terminal and the negative terminal indicates a bad cap.

    Some caps have more than one positive terminal. These are actually two (or more) caps in one can. Don't measure between positive and positive - that is not a valid measurement. Exercise the usual care when poking around inside a live chassis. And remember that the DC voltage across a powered up filter capacitor will be over 100 V DC!

    Another quick method to check whether a power supply capacitor is still working is to connect a known-good capacitor across it to see if the problem goes away. Remember to get the polarity right or you'll damage the test capacitor - spectacularly! Make sure the test capacitor is a similar value, with same or higher DC voltage rating as the capacitor under test. Power off and discharge the capacitor under test before hooking in the test capacitor. Discharge the test capacitor after the test is completed.

    1 reply

    My usual practice is to replace all the old capacitors, failed or not. Capacitors are relatively cheap, and when an old radio has not be in use for a long time, nearly always one or more are in the process of failing. So, while I have the chassis out of the case, I generally replace them all without testing them. I do use a test lead that contains a 100k resistor to discharge the caps. It is indeed a good proactice. Thank you for your comments.

    Many restorers prefer to leave the original electrolytic capacitor cans in place. It looks nicer and provides future restorers the ability to match the original capacitor values. Just remember to disconnect all wiring from the positive terminal(s) of the original cans.   If the original can is mounted on an insulator and/or has a cardboard cover (like the ones in this Zenith radio), then disconnect the wiring from the negative terminal(s) as well.

    A few restorers try to hide the mylar capacitors inside the original paper capacitor tubes. To do this, scrape off and set aside as much wax as possible. Using a small x-acto blade, carve out and dispose of the paper-and-tinfoil guts. Slide the new capacitor into the empty tube. Soften and re-apply the saved wax using a flat blade or paint brush.

    Old radios also sometimes don't have an isolating transformer and as a consequence they can have a live chassis if there is a fault anywhere. In the past when I've done an old radio like this, I've always added an isolating transformer to the sets just in case.

    2 replies

    I've never added an isolating transformer, but it is probably a good idea. Thanks for your comment.

    "I've always added an isolating transformer to the sets just in case" I should have added "If it didn't already have a transformer". I once had a Hallicrafters S-38 which had all of the heaters wired in series to the mains dropper resistor. The person who gave it to me used an auto transformer to drop the voltage from 240v (uk) to 110v. I learnt then the difference between an auto-transformer and an isolating transformer when I got a belt off the chassis. I quickly replaced it with a 240-110v transformer and earthed the chassis.

    I have a simular radio. I am restringing the dial. I works great. I would recomend replacing the wax capaciators. Also anyone who picks up an old radio i would power it up with a 0 to 120 volt variable power source. applying voltage slowlew will help from popping the capaciators. also pollyester capaciators will also work.
    It is great to see a CLASSIC radio restored, Kinda like restoring an old car.