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Question about hum from speakers

I know there's at least one amp engineer here... I'd like some help.  I bought a used solid-state amp from ebay; the ad read " McCORMACK POWER DRIVE DNA-.5 Deluxe REV B POWER AMPLIFIER - EXCELLENT CONDITION!"
Well, I swapped it in, replacing a Dynaco Stereo 120, and there's a hum from both speakers.  I have to believe that the seller was well aware of the hum.  What I'm wondering is, is the hum a result of some electronic component(s) in the amp failing (or having failed), such that it's salvageable by replacing the bad component(s)?  Or would that be hoping too much?
bhakti-rider
yogiboy's avatar
yogiboy
5,641 posts
 
"I have to believe that the seller was well aware of the hum. "

If that’s the case you should return it and get your money back! Why would you keep a defective amp?
deepee99's avatar
deepee99
86 posts
 
Your power supply cap(s) are shot. Easy to replace if you’ve any skill with a screwdriver and a soldering iron.Likely you had one bad cap, and nobody noticed. When the second one failed, that’s when the AC hum started. And in defense of the seller, these literally can fail overnight.I doubt if it's a ground-loop all of a sudden. That hum would have occurred  with your original amp.

bhakti-rider OP
44 posts
 
Yogiboy:  I've requested a return for full refund; I'm thinking in terms of what if I can't get that refund.
Thanks for the responses so far.  So it could be a ground loop even though there's no hum from the Dynaco, all else being equal?
yogiboy's avatar
yogiboy
5,641 posts
 
You should try a cheater plug and see if that works!
bhakti-rider OP
44 posts
 
Erik & Yogiboy:  I did disconnect the inputs, and there was no hum.  So I found a spare cheater plug and inserted that, connected the inputs again, and the hum was gone.  Thanks!
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