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Buzz coming from power amp - not through speakers

Hi, first post here. I just picked up a Harmon Kardon PA5800 off ebay. It sounds great, no issues there. However, the amp itself makes an intermittent hum/buzzing noise (ie, the sound comes out of the amp unit, not through the speakers). It fades in, peaks, then fades back out with a period of about 5-7 seconds. Sometimes it's not noticeable at all, but occasionally it's loud enough to hear even with a moderate volume level through the speakers.

Anyone have ideas? DC noise on the AC line? Basic service needed, ie new filter caps? (I'm a tube guitar amp guy, not even sure this type of amp has filter caps).
logan7
czarivey
5,946 posts
 
Under chassis remove the screws from power transformer, treat them with tack and screw them back tight. Also would not hurt to reconnect/re-mount power transistors wit new tack to te heat sinks. Finally it's always good to inspect cold solder joints and apply fresh solder throughout PCB.
logan7 OP
4 posts
 
What do you mean by tack? Considering that the noise has a very distinct pulse to it, I'd expect there's a more fundamental underlying cause, and that just dampening the vibration would be going after it directly.
oldears
386 posts
 
If it has a toroidal transformer, they have been known to buzz. You can try to reorient the transformer and change the mounting tension or possibly replace the rubber damping between the coil and the mounting washers with better materials from Herbies Audio Lab. Some of this can also be tried for the mounting of c-core transformers.
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