Zenith 7H920 AM but no FM - Printable Version

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+--- Thread: Zenith 7H920 AM but no FM (/showthread.php?tid=6439)

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Zenith 7H920 AM but no FM - Eric Adams - 04-12-2013

OK, here's my latest project. I replaced all electrolytics and wax/paper caps. Checked and replaced all resistors. Checked all coils. All tube test good.
AM sounds great. FM just hums. While turning the dial, I was able to disrupt reception on an FM radio on my bench. So I guess the oscillator is working. I checked the AM/FM switch (had a problem with one on a Philco once) and it appears to be OK. Any ideas where to look?
Here's the schematic: http://www.nostalgiaair.org/Resources/312/M0025312.htm

Thanks,
Eric

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RE: Zenith 7H920 AM but no FM - BrendaAnnD - 04-12-2013

Before any of the below is considered, you may want to clean all the tube pins and sockets by using DeOxit or similar and rocking the tubes a bit in the sockets. FM is very picky about signal path.

My first suspect would be the 19T8. These have in my experience a very high H-K leakage failure rate. If you have a scope and an isolation transformer, I would trace the signal beginning at the input to the 1st IF and go downstream from there until you lose it. If it's not there at the 1st IF, then you may need to replace the 12AT7.

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RE: Zenith 7H920 AM but no FM - Eric Adams - 04-12-2013

Thanks. I'll check this out and get back to you.

Eric

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