Trying to fix a broken amp I bought. The issue is intermittent, but it's happening with increasing frequency. On power up or after a few minutes of working properly, the power amp module for one of the two channels quickly gets extremely hot. Sometimes it's the channel 1 module, other times it's the channel 2; tried swapping the modules, but it didn't make a difference. I rarely keep it on for more than a few seconds once the problem starts, but that's enough to get it to where you can barely touch the heat sink. Sometimes powercycling the amp resolves the issue, other times not.
The bias on the module experiencing the thermal problems, as it is experiencing them, is all over the place, from 200 to 600mV! I didn't adjust it. On the occasions when powercycling gets the module to operate normally, the bias is a few mV, which is what it's supposed to be, per the specs.
Any suggestions for things to try, or how to narrow down the problem? Perhaps this is a common failure mode? Amp is '97 vintage.
I've tried disconnecting the input board from the main board, to no effect on the thermal issue. Measuring the input DC voltage on the main board fingers where the power modules plug in, it is +/- 61.3V when the amp is on and the power modules are disconnected. When they are connected and working properly, it is +/-59V. When the thermal problem starts, the voltage drops to +/-45V. That could just be because of the power draw of the power transistors.
