n00b PA problem question

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I apologize in advance for being ignorant about these things, but i have a lantek (citronic) PPX 900 and after years of working fine it's having trouble that i can't explain properly to technicians.

when first powered on, the amp works fine. but after a few minutes it starts making sounds (through the speakers) that almost sound like wind blowing on a microphone. sometimes, when these sounds increase in intensity, the sound cuts out and distorts for as long as a minute. i've tried it on different speakers with the same result.

can someone tell me what the likely issue is?
 
I happen to have a PPX1200 on the bench these days and must say they are well build.
Mine has a different problem however.
As a rule of thumb I would check for bad solderings and tighten the TO3 cans as well as the screws of the big power supply capacitors. Check if all op-amps are well seated, too.

/Hugo
 
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