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Magnavox FM not working

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Posted on March 13, 2017 at 08:46:57
rickl
Audiophile

Posts: 533
Location: Twin Cities
Joined: February 7, 2002
I'm working on a 1949 Chairside tube radio and the FM is not working.

I have a scope and a wavetek function generator but not sure how to follow the FM alignment procedure.

I've tested the tubes but the original ceramic caps and resistors are still in place.

The attached link lists the procedure and the schematic. I'd really like to get this working. AM works and the TV input (I'm using a ipod source).

Thanks for any help.



looking for some jazz and a little libations - joe strummer

 

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RE: Magnavox FM not working, posted on March 13, 2017 at 17:23:33
elflow
Audiophile

Posts: 308
Location: Indiana
Joined: November 1, 2005
A quick review of the schematic reveals that the AM & FM tuners share much of the same circuit with different coils, RF and IF transformers switched in and out of the circuit to allow the circuit to function either as an AM or FM tuner. I would first uses a cleaner such as Deoxit to clean the AM/FM selector switch, next I would look at the 6AU6 limiter and 6AL5 discriminator part of the circuit which appear to only be used by the FM. My test device of choice to find the point of signal drop out in a tuner is signal tracer with a demodulator probe. You can signal trace with a scope but you have you know what is the proper wave form is for the part of the circuit. With the signal tracer you hear the demodulated audio signal the tuned FM station is broadcasting. It really is signal tracing for dummies like me.

 

RE: Magnavox FM not working, posted on March 14, 2017 at 09:06:12
rickl
Audiophile

Posts: 533
Location: Twin Cities
Joined: February 7, 2002
Thanks elflow. this might be enough to find the issue. I did clean the contacts but can hit them again.



looking for some jazz and a little libations - joe strummer

 

RE: Magnavox FM not working, posted on March 14, 2017 at 11:40:30
Michael Samra
Dealer

Posts: 35585
Location: saginaw michigan
Joined: January 30, 2005
Contributor
  Since:
June 24, 2005
Just to add to what elfow said,are you getting the hiss like a typical FM tuner between stations when the mute isn't on.I realize yours doesn't have a mute but I'm using it for reference.Since you have a generator,inject 10.7mhz at the IF and see if the signal gets thru.If it does,look at the oscillator in the FM or RF amp.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Magnavox FM not working, posted on March 16, 2017 at 07:29:49
rickl
Audiophile

Posts: 533
Location: Twin Cities
Joined: February 7, 2002
Thanks Michael for your response. I should have enough info to get started.

looking for some jazz and a little libations - joe strummer

 

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