Check whether the windings of the audio output transformer are really symmetrical.This is how the two anodes of the 6v6 are fed and the remaining hum in the end circuit is eliminated. The hum is eliminated by the two (opposing) windings.
I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps.
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Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away.
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Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference.
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I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another?
