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Truetone AM/FM record player

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Post by willy3486 on Fri Nov 23, 2018 6:49 pm

Winter is back so now I can get back into radios. I have a radio I am working on, I think it is a Truetone d1234b AM/FM/Record player. I have got the AM working great but I am stuck on the FM part. I replaced all the wax caps and the main can cap. I am able to inject a 10.7 mc signal into the input of the 12at7 tube that the output goes into the 1st IF transformer. I can get it tuned up and get the signal from there and it goes all the way to the speaker. But I can not turn the tuning cap and pick up a station.

Am I correct in thinking that the FM works similar to the AM in that if I inject a signal into the first IF transformer then if it plays through the speaker then all the parts are ok from the IF to the speaker? Any suggestions on what could be bad from the antenna to the input of the IF transformer?  The tubes have been tested as well. There is a schematic at the link below in the bietmans repair book.


https://www.americanradiohistory.com/Archive-Rider/BEITMAN-SUPREME/Beitman-1953.pdf

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Post by jukeboxman on Fri Nov 23, 2018 10:44 pm

i would start by swapping the 12at7 and the 6be6 with known working tubes (-do not trust 100 percent a tube tester-)---both are in the fm signal path also check the resistor values around both tubes--the 6be6 is the fm oscillator tube so off value resistors may kill the oscillator the 12at7 is the rf amp and mixer for fm--did you clean the contacts on the band switch?--let us know if this helps --others here may have better info also

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Post by willy3486 on Sat Nov 24, 2018 12:57 pm

I tried a few 12at7 and 6be6 tubes. They should be good. One thing I was able to do was to put a scope on it after the 6al5 detector tube. I then injected a 10.7 mc signal to just before the first IF transformer. I was able to send that signal and it looked like a FM wave signal. It wasn't perfect but did look like one.

The one thing I did notice on the 6be6 oscilator tube on grid pin 6 it has about 65 volts with a digital meter. It is suppose to be about 85 volts per sams info. Would the digital meter be off that much over the meter type movement meter? Also would 20 volts low cause it not to oscilate? I also noticed on grid pin 7 it says it should have .7 volts but its closer to 1.5 volts. I believe the resistors here are probably off as I seem to remember them being off some. But it seems like most are off. Would the grid pins be the ones you were talking about being off and causing it not to oscillate.

Also if I put the 1.7mc signal to the antenna the scope shows the waveform going out of the last IF transformer and into the 6al5. But as soon as the 10.7 mc is taken off the antenna the waveform goes away. Also there is no waveform into the 1st IF transformer. With all this said I am thinking it is not oscillationg. Does this sound like the issue. This is why I wish I could spend more time on repairs. That way I could keep from getting rusty. I usually can figure out AM with no problems but FM has always knocked me for a loop for some reason. I am inclined to think Oscilator and that a good direction to go to next?

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