Hi everyone: I have a problem with this radio. I have a schematic that I ordered from AES because I couldn't find it here. Anyway, the problem is that until the set is fully warmed up (15 minutes or so), the FM will slowly fade away until it's gone. Switch it to AM and it slowly comes back. Anyone have thoughts about what it could be? Mark from Kalamazoo

Hi

Here is the schematic on the internet:

http://techpreservation.dyndns.org/schematics/56/56_Zenith_8Y02_186.djvu

It does require djvu program, available free on the internet.

Not sure what you have gone through on this radio but might try another 12AT7 tube.

Norm

:Hi everyone: I have a problem with this radio. I have a schematic that I ordered from AES because I couldn't find it here. Anyway, the problem is that until the set is fully warmed up (15 minutes or so), the FM will slowly fade away until it's gone. Switch it to AM and it slowly comes back. Anyone have thoughts about what it could be? Mark from Kalamazoo
:

Norm: I had switched out the 12AT7 with another used tube with no success. However, today was able to buy a NOS 12AT7 and that seems to have done the trick. Now the question I have is this: Is there something else wrong with this set that makes it malfunction with a less than new 12AT7 or is this typical for these Zeniths? Mark from Kalamazoo

:Norm: I had switched out the 12AT7 with another used tube with no success. However, today was able to buy a NOS 12AT7 and that seems to have done the trick. Now the question I have is this: Is there something else wrong with this set that makes it malfunction with a less than new 12AT7 or is this typical for these Zeniths? Mark from Kalamazoo
:
You may have a bad tube socket that is allowing the filament to lose power; check it in a darkened room.
Johnnysan-

So if the socket is bad, would that only affect the FM? The AM always works and both use the same tube for the oscillator. Mark from Kalamazoo

:So if the socket is bad, would that only affect the FM? The AM always works and both use the same tube for the oscillator. Mark from Kalamazoo
:
Hi Mmakazoo,
Good point, losing Filament would kill AM, too.
I have done quite a number of Zenith H723 radios. These use 12AT7 in a similar circuit. I have never encountered one that caused this problem, so I expect it is a rare failure mode.
You may want to check over the local oscillator one more time, with special attention paid to teh band switch. If all looks good, just toss the old tubes and move on.
I once had a radio that sounded distorted. I swapped out the type 42 tube with another I had and the problem stayed the same. It wasn't until a NOS type 42 was tried that I found out I had two bad type 42's.
Sounds similar to your problem, except distorted audio is a common problem in output tubes. I have not found your problem common in 12AT7.

One caveat, though, if your original 12AT7's were made recently, then you may have ones made for the audio market. These 12AT7 tubes are sometimes used as first audio tubes in electric guitar amps. If the 12AT7's were made for that application, they tend not to do well in mixer/audio applications in FM radios. That could be what you have run into.

Best Regards,

Bill Grimm

Okay, Bill, how would I know the age of the tube? And, I have an unfortunate follow up. The radio is acting up again! So, I guess when I get a chance to really dig into it again, I will spend some time in that stage and look for problems. This is one of the most challenging radios I have worked on, but what great sound and reception. Mark from Kalamazoo