Re: {Collins} 75A-4 Curious Issue

But note the above. The hum changes with the position of the AM/SSB switch. If its lack of filtering on the ouput stage one would think the hum would be constant. The OP also said, perhaps in a personal message, that the hum was 60hz not 120hz which would suggest heater current getting in somewhere. I think the bias rectifier is half-wave so it might also be ripple from that.

You're right! Richard,

I forgot  the hum changes with cw/am to ssb mode so the
problem is more likely around V-11 the cw/ssb product detector.
C94-C is the plate filter for both plates in V-11 so probably not
the cause of the hum if it is absent in the cw mode. Pin 6 of V-11 is the plate for the ssb detector and is also filtered by LC network
C-90, L-28 & C-91 could be there.. I will check afurther.

Deric, VY2DA

Richard Knoppow
Los Angeles

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