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.

Comments

[exabopper]

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!

[knife141]

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.

[Phil B]

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!

[knife141]

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.

[Phil B]

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. 

[Chris Van Ihinger]

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.

[knife141]

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.

[Chris Van Ihinger]

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.

[wobbler]

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.

[knife141]

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

[wobbler]

"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.

[dsandds2003]

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.

[knife141]

I agree, and did replace the wax capacitors (Step 3). I've heard of using a variac to apply initial power when testing these old radios, but unfortunately I don't hav access to one. Guess I should consider getting one some day. Thanks for your comment.

[invisiblelight386]

My grandfather worked at Zenith after WW2 and he received this same radio as a retirement gift. The radio still works and I love it. It's nice to see an old radio like that brought back to life like you have done with this one.

[knife141]

Glad your old radio still plays! Thanks for the comment.

[Thereyouhaveit]

Did you think of upgrading the speakers ? :P

[knife141]

The speaker was in great shape, so I left it as it was.

[thewoodcarver]

Very nice work ,I have several old Tube radios some I have restored others I have as works in progress ....very well written.... tubes can be found at flea markets along with the whole radio and on Ebay but shipping on a radio is way too much

[knife141]

Thanks for your comment. I have found some pretty good deals on old radios at flea markets and occasionally at antique malls. I've bought one or two off of eBay, but as you say, shipping can eat your lunch.

[mistic]

I used to repair tube radios vintage 1944.Tubes were hard to get account of the wwII effort. Do they still sell those tubes today.. just curious.

[knife141]

Yes, you can still buy tubes for most of these old radios. Before I started on this radio I checked to see what the tubes would cost if I needed some, and for this radio they ran $3-$6 each. I'm sure there are probably some really ancient & exotic tubes that might be hard to find, but I've not run into a problem getting tubes.....yet!

[bbetsinger]

Is there supposed to be a picture of a sailboat on the ocean on the left thing that is a speaker? If not, that is amazing that there is a perfect picture with birds and clouds and a perfect sailboat.

[knife141]

Yes, this particular model of the Zenith Universal Wavemagnet radio is often called the sailboat radio because of the design of the speaker cloth.

[vader0ne]

I do know caps can be a big problem.I do have a old Zenith AM,FM tube table top that my friends grandpa had ,after he passed Albert gave it to me, It still works like new I dont play it much I just have it on the shelf for looks.

[woodNfish]

The asbestos mat in the radio was to reduce the hazard of fire. By painting it with varnish you probably made it flammable. This may or may not be a problem, but it would have not been a problem at all if you had just left it alone.

Yeah I know the idiotic mantra of how evil asbestos is. Don't believe all the government propaganda you hear. Most of it is BS.

Nice job on the caps. I'm glad your radio works good.

[NaTeB1]

Great Job! Very interesting and meticulous. I didn't realize restoring radios was so intense. How long did all this take? It looks incredible.

Recently Ive been really wanting to get an old tube radio and restore it, if needed. Ive been looking for the perfect one but haven't found anything decent yet. We want one of the plastic body models with very round corners.

I picked up an old 60's Monitoradio in almost perfect condition for a few bucks but I can figure out if its a scanner or a radio. Eh, whatever it is it looks great on the shelf next to the old Brownie camera I just found for $2 at a thrift shop.

[knife141]

Its kinda hard to say how long it took with this radio. I worked on it off and on over a couple of weeks to get it up and going. I probably had about 8 hours or so in the rebuilding, not counting the time spent running down the right capacitors. I was lucky that I didn't have to recover the case -- all the case needed was a good cleaning & waxing. I have an even older radio similar to this on its way, and it will require re-upholstering the case with fabric in addition to the "real" radio work. The first radio I ever rebuilt was a 1951 General Electric in a plastic case. I have it in my study and listen to it every morning when I check my email. Thanks for the comment.

[Wildrat]

Nice job! I have two old tubers my Dad gave me before passing. One radio works and sounds good after warming the tubes up for awhile, still has a little 60 cycle hum though. The other radio does not work, but I'm sure it's repairable. My Dad also gave me a large box of tubes he was going to throw out, but of course I accepted them to add to my collection of might get used one day supplies. Anyway, good job. I think I'll bring my two radios to my radio room.

[jomaro]

Very nice and radio and good work!
You were a bit luckier than me. I have a nice Atwater Kent that has four trimming capacitors and those are hard to find and to replace. It also came with a dynamic speaker.
Anyway there is a lot of information in the web for those radios. I got the original schematics for mine in a publication known as the Ryder Manual.
Those interested shoul d check this instructable:
http://www.instructables.com/id/How-to-fix-a-classic-American-AM-tube-radio/

Thanks for sharing

[Tom Hargrave]

I have restored many vaccum tube radios and your article shows that you did a great job.

One thing I do different is I pull paper caps and check for leakage then I replace all of the same valye with mylar caps. I discovered a long time ago that some caps in these old radios become leaks and some don't depending I guess on construction.

Electrolytic caps that have lost their value can be reformed by applying the rated DC voltage across the cap through a 30K resistor. If the resistance across the resistor does not drop to 10% of the applied voltage in 10 minutes then the cap can not be reformed. You need to hold the voltage across the circuit until less than 1% of the rated voltage is across the resistor.

[knife141]

I've heard of reforming electrolytic's, but have never tried it. I just replace them, along with the paper cap's without testing. Electrolytic cap's and Mylar caps are relatively inexpensive, and all will tend to eventually fail. Also, if a resistor happens to be buried underneath a paper cap, I generally replace it while I'm in there, since occasionally a resistor can drift over time. Thanks for the comments!

[Tom Hargrave]

Vacuum tube radios are somewhat tolerant of varying valued caps and resistor. It was common to have carbon composition resistors at +/- 20% tolerance and paper caps at even wider tolerance. I've yet to find a resistor outside the original tolerance unless it was cracked.

I'm of the belief that there is a certain amount of risk involved in just replacing parts. I also found out a long time ago that if a particular value & construction cap in a radio is leaks they probably all are. That's why I only replace caps when one in the seriels I pull loose tests leaky.

Some of the radios I worked on this way are still running fine 30 years later.