on the border: Radio Craftsmen C-810

[mhardy6647]

I think this radio, and the question I have, are more an "antique radio" than a "hi-fi tuner" question, so I am posting here.

I have a very old Radio Craftsmen C-810 AM-FM tuner given to me by a friend from our church (who most likely bought it new). Long story short, it had audio but no "radio functionality" when it was passed on to me. Turns out that it had several truly bad tubes, which I have now replaced. AM is now working quite well. FM is now working, too, though sensitivity seems to be poor.

My question, though, is (I think) AF not RF. This tuner has a volume control. Even with the volume all the way down, there is considerable static/noise. Signal will over-ride it, and there's plenty of AF gain, but the noise just ain't right. On 'dead air', the noise is independent of the volume control setting.

The noise strikes me as the kind that can arise from old/bad carbon comp resistors. Is there a best place to start troubleshooting? I don't have a schematic, but I could start near the output and work back. Thought one of the 'old salts' around here might be able to save me some sweat equity. :-)

Thanks for looking!

[mhardy6647]

sorry for the 'quality'... could only find the old, old Olympus 1MP camera, and no one else is home.

[Tom Bavis]

One possibility is silver migration making leakage paths inside the IF transformers. Many of these used silver mica capacitors without any insulation. Silver plus moisture plus high voltage gives tiny slivers of silver "growth" across the insulator surface... often giving a "thunderstorm" static among other symptoms. If there's a short from B+ to ground, it's not in the siganl path per se, but of course will couple into audio through power supply... "K-Tran" IF transformers are bad for this.

But noisy resistors are a possibility too...

[mhardy6647]

'Thunderstorm' static is indeed what we're dealing with, Tom. Thanks for the insight.
Is there any reasonable cure?

[Tom Bavis]

The best cure is to remove the IF can, open it, and remove the mica cap. Then replace, solder 100 pF (usual value) on the terminals underneath.