Zenith with problems

[UncleDave]

I have a Zenith AM/FM tube radio from the 50's, nothing terribly exotic, which I use in my office. It has been a nice little radio but one night I left it on all night and now there is a hum from it all the time.

Anyone have an idea what has broken?

[RetroHacker]

What kind of hum are you getting? Is it background hum, affected by the volume control, or is it a loud overpowering hum that drowns out everything and is loud even at low volume? If it's the loud kind of hum, you need to replace the electrolytic capacitors. Other sorts of hum can be other failures, many of them capacitors as well - have you recapped the set, or is it original?

-Ian

[UncleDave]

It is a constant hum, not affected by volume, although it is not too loud. I can get the volume up enough to hear the broadcast. No cap replacement has been done. That is somewhere on the project list - not at the top yet.

-David

[ekimetsok]

Save yourself the possibility of a lot of grief later. Leave it off until it's recapped.

[RetroHacker]

Quite often, hum can be caused by electrolytic filter capacitors that are no longer filtering. In table radios, they're typically a cardboard "can" under the chassis attached with wires, with two capacitors in one can. You can replace these with regular modern electrolytics. Electrolytics deteriorate with age, as well as heat. Leaving the set on all night may have been enough to push aging caps over the edge. If you're going to replace the electrolytics, do all the paper capacitors while you're in there. The radio will thank you.

Another thing to check, if you have spare parts or another working radio - is swap tubes around. Although not likely, I've heard stories of hum caused by a failing output tube (50c5).

-Ian

[Old1625]

Filter 'lytics going dry is the most common cause of hum, but there is a wide variety of other causes, some of which can be bizarre indeed.

I had a Zenith AM/FM table radio in the barn here which developed an elevated hum level. Also the dial drive had suddenly failed. Turned out a mouse had gotten into the set, bit the dial cord in two, and then sauntered over to the ouchput tube (35C5) and peed there. Such act turned out to be his last, because there he lay--dead. I had to restring the dial drive and replace the contaminated bakelite wafer tube socket. I treated the radio to a nice porcelain one instead, as that substance is designed to handle pee.

If you do decide to recap you should indeed do all of them, as suggested. Also most of the Zenith AM/FM sets of the time used a selenium rectifier. You can get a little more spirit out of your radio if you replace that as well, as those do fail, and when they do they emit fumes that smell as if someone broke wind. And those fumes are hazardous to inhale. I usually replace them, using a 2.5A 1000PIV diode in series with a 22 ohm resistor. And obviously one needs to observe polarity.