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how come my am / fm tube radio works on am side but not fm side? Answered

i have an Arvin 480 tfm am/fm tube radio from 1950 that the am side works perfectly with the built in am antenna and has no hum. but when i switch it to fm mode it has a faint hum coming from the speaker and doesnt change with volume, no static, music or anything, just a hum and it just stays the same. all the tubes light up and i even switched around the ones with the same rating in the radio but it still does the same hum. i connected a wire to the fm antenna screw to make sure it wasnt just not recieving anything but it didnt change. im just confused why the radio works perfectly on the am side and the fm side has nothing but a faint hum. any ideas?



Best Answer 7 years ago

I would try hooking up a real antenna, not just a wire. Older radios were pretty sensitive to things like that. A TV antenna has an FM component built into it.so it would work good for this. Even a set of rabbit ears would work.
Another thought would be to check the tuner for dirt, dust and foreign objects.
The tuner looks like a set of meshing plates. As they move between each other it changes the frequencies it picks up. Make sure there is nothing shorting them together. They need to be unbent and clean and need to slide together without touching. Later on for the early transistor radios they used little sheets of insulation to keep them apart because the miniaturization made the metal, air ones to close together to keep from shorting out.

i just tried the rabbit ear suggestion and it does the same. i put one lead into fm and one lead into the ground like the label on the bottom of the radio says to do. as far as the tuner goes, it appears to be straight, clean with no dust or rust anywhere.

It is a Zenith B835R, from around 1958

I have a Zenith with the same problem. I have a small FM transmitter, and it won't even pick up signal from that. The AM side, however, picks up stations better than my Tivoli!

I shouldn't think that a radio that old was built to a (modern) standard FM design.
I.e. it's the wrong receiver for what FM you can pick up these days.


i kinda thought the same but the display shows the normal fm frequencies from 88 to 108 mhz. but has it changed since then?

It's not just the carrier frequency, the modulation may be different. 1950 is rather early for FM - does it actually say "FM" on it?


ill post a pic of it, it looks like it would be the same as it is now.

Does it actually say FM on it, or does it just receive in what is now an FM-band?


it says fm right on it, under, and on the online manual i looked up, it even says how to calibrate it for the 88-108mhz frequency back in 1950, maybe a capacitor or something dried up underneath it on the fm circuit?

I didn't know you had a manual.
I still think the FM broadcast may have been differently modulated in 1950, but you seem to have the best information about things.


Has it ever worked?

What has changed since then

It needs an FM antenna (aerial)

Dirty switches often give trouble on my kitchen radio.

there are so many variables in such a problem - It's a bit like asking why my car won't start - engine doesn't run, been like that for weeks, plenty of fuel, tires are OK - Turns out the battery is flat.

For the cost it is most likely worth buying another if the fault isn't simple or obvious.

im not sure if the fm has ever worked to be honest, i bought the radio from an antique store for $25, i tested it for 20 mins before i bought it, the am side was the only mode working, other forums ive been on say its the filter caps in the metal cans on top. im new to tube technology so im basically poking a screw driver into things (unplugged of course)

If never worked and second hand you don't really know what others have done to it - More than likely without some fairly sophisticated test equipment and knowledge your not going to get it working. I would look for a working alternative.


7 years ago

Take a closer look inside... the FM tunning section and receiver circuits in most cases are usually enclosed in a metal screening box, this includes the tubes used within the antenna / tunning sections. If in your case you can identify this section then you can substitute the tubes as they are used for FM only!
The tubes may even have their own individual metal screening cans. Always take care with tube equipment and be aware of high voltage circuits!!!

yeah i took out the chassis and looked above and below it, i cant tell which circuits are fm and which are am. it to me, looks like the two frequencies are intergrated with each other for the most part other than where the antenna wires go.

it might be the way you have the radio placed. The fm frequencies are slightly different than the am frequencies so even the slightest variation in the angle can cause that or you fm receiver and transceiver is shot and you need to replace it
I work with radios all of the time this is not uncommon

AM and FM need completely different circuits to "demodulate" them. Switching from AM to FM isn't just changing the frequency you're listening to.

While its failyl rare for anything but a valve/tube to fail, its not unheard of, something else has failed. There are plenty of folks who can fix them around still !!!