Originally Posted by
Notladstyle
one of the mosfets is bad - when mosfets partially fail from overheating or current overload, their current capacity is severely reduced. When you exceed that limit the mosfet no longer switches as it is designed and the result is popping, bumping, and a high or low pitched squeel.
+1^ Amp is probably not worth fixing. Most low to mid level amps aren't worth fixing anymore. That is why it is impossible to find companies that do it at all these days...