91-121 push pull output hum - Printable Version

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+--- Thread: 91-121 push pull output hum (/showthread.php?tid=17493)

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91-121 push pull output hum - grandntl - 11-07-2017

Hi everyone, my 91-121 has a hum that occurs after tube warmup. It is not changed by the volume or tone control and is superimposed on the very slight hum at turn on. When I pull either one of the 42 output tubes, the hum disappears and replacing with other 42 tubes makes no difference. The radio was recapped and had many resistors replaced before I bought it  and the job looks well done. I tried reversing the field coil connections and the radio acted the same as if one tube was removed (no hum but lower volume). The field core wires were originally correctly connected according to the schematic: 
http://www.nostalgiaair.org/Resources/884/M0013884.htm   I can find no wiring errors and the only capacitor not replaced was the no. 47, 0.001mf that was bolted to the chassis by the speaker wires. I replaced it (connected to the 42 plates) with no effect. Where should I look for a solution? The sound is never distorted, just an excessive hum. Thank you.
Bob

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RE: 91-121 push pull output hum - morzh - 11-07-2017

It does not look as if your speaker(s) have a humbucker; if they do, the field coil orientation does not affect hum, but the humbucker relative to the voice coil does.

Check the 53 cap (the AC line filter) for the ground lug being grounded (check from actual lug, not from the screw).
Check electrolytic caps for proper capacitance, filter choke, the cap across it - you might even try to disconnect this cap and increase the electrolytic after the choke.....

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RE: 91-121 push pull output hum - jcassity - 11-07-2017

using an oscope if you have one , connect to your expected dc voltage points and determine where the highest AC ripple is.
A stray excessive ac ripple could honestly be just about anywhere.

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