I just bought a sansui g3000 amp at a garage sale it worked fine but after I got it home only 1 channel works (left side out) I checked all the fuses and their ok. I put a meter on the speaker terminal screws and the best I can tell (I'm a rookie at this ) It showed a minus power draw. I did notice that the balance switch seemed rough kind of a sand paper feel when I turned Knob left? Just my luck the next day I received a vintage fisher amp I bought off of e-bay (the seller claimed it worked fine before shipping ) and the right side is out on it the difference being the balance knob is smooth and you can hear faint sound on right side with a bunch of popping an hissing?
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I would bring it to a repair shop who is qualified to do repairs on vintage equipment. Are you in the NYC area?