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48-472-122 FM issue - Printable Version

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48-472-122 FM issue - Eric Adams - 02-14-2019

Hello Everyone,

I'm working on a really nice 48-472=122. I replaced all caps, tested resistors and tubes, aligned. The problem is, AM works fine. Fm, not so good. I can pick up my local stations fine except that they are very faint. Until I put a screwdriver to pin #8 of the 14F8. Then the volume goes up to a normal level. Any thoughts?


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Thanks,

RE: 48-472-122 FM issue - PhilcoMike - 02-14-2019

Just a guess that the local oscillator may not be working until you add a little extra capacitance by touching the pin.

RE: 48-472-122 FM issue - Eric Adams - 02-14-2019

Thanks Mike,

It looks to me like the oscillator is working because as I said, I can pick up my local FM stations, just at very low volume.

RE: 48-472-122 FM issue - PhilcoMike - 02-15-2019

grid bias issue with a resistor? Touching the screwdriver may be adding a little gain.

RE: 48-472-122 FM issue - Radioroslyn - 02-15-2019

Try touching pin 1 of the 12au6 fm rf amp to hear if the signal increases. If not use a cap connected to pin 5 of the 12au6 and touch it. his should give you the same result as touching pin 8 of the 14F8. Also measure the voltage around the 12au6 is see if they are reasonable. Methinks your osc is ok too.

RE: 48-472-122 FM issue - Eric Adams - 02-20-2019

Hi Terry,

Here's what I got: Touching pin 1 of the 12AU6 did nothing. Touching a cap to pin five was as you stated-just like touching pin 8 of the 14F8. I measured the voltages at the 12AU6. The only ones that were correct were the filament voltages. The others were 0 or negative. 

Any ideas were to look?
In the meantime I'm going to check more voltages.

Thanks for your help.

RE: 48-472-122 FM issue - EdHolland - 02-20-2019

Check the feed to and from R402, the 1000 ohm resistor. This is where the DC supply for the12AU6 comes from, and it seems that your problems lie there, and your RF stage for FM isn't operating for lack of power.