You could first check for a problem with the sound detector diode by checking from point "b" to ground with an ohmmeter. Should be about 10k one way and about 2.5k with the leads reversed. Bad tubes could cause it too. Anything to make the sound IF signal weak
Ok, heres a non-factory procedure from Carl Babcoke's "RCA color tv service manual". You'll need a couple bias boxes. 9v batteries and pots could suffice in a pinch probably. Bias is always negative here.
Quote:
(1) Adjust the fine tuning to any point that gives clear sound without buzz or noise, even if this point results in sound bars or a beat pattern.
(2) Adjust t203 (the sound detector transformer) for maximum volume and minimum distortion, using the second peak from the top of the coil.
(3) Apply a fixed bias of around -3 to the tuner AGC and approximately normal bias to the sound IF tube.
(4) Adjust the fine tuning for minimum buzz, then increase IF bias until the buzz and noise *barely* appear again.
(5) Now adjust the take-off transformer, t201, and the sound IF transformer, t202, as needed to eliminate the noise.
(6) Any time a transformer adjustment clears up the buzz and noise, increase the IF bias until it barely re-appears.
(7) Adjust the sound IF transformers again, then increase the bias until a change either way in the transformer cores makes the noise start again.
(8) Remove the bias supplies and adjust the fine tuning normally. Once more adjust t203 for maximum volume with minimum distortion.
(9) Try the fine tuning over a wide range, for a normal sound circuit with accurate adjustment should result in good sound without critical fine tuning adjustment.
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I havent attempted this in a long time. Your mileage may vary.
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