A/D/S PQ20: issues when the chassis is grounded.

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On my latest build I got my hands on two PQ20s. They are both mounted to a grounded metal bracket. One of the amps is having issues with turn on and turn off thumps. I also found 28 volts on the shield for the RCA inputs, even when it is supposedly off (remote wire unplugged). Otherwise it plays fine.
Once I take the amp off the bracket and lay it on the trunk carpet, it works as it should (0 volts at RCAs and no on/off thumps). Anyone have an idea? No obvious visual issues inside the amp though someone has changed all the electrolytic capacitors. Comparing to the other amp, all the values and polarities look correct. Even the original brand on most. Solders look good.

In other amps I have seen, the heat sink is common with the main ground cable terminal. This is not the case with either of mine (the good one or the trouble maker). From searching this site I have found mention of different heat sink grounding schemes that sometimes involve a cap or diode between the heat sink and main ground terminal. Is that what I should be looking for? I see two places where the power supply board is directly "grounded" to the heat sink by screws. Should I be tracing back from those locations until I find a cap or a diode?

Thanks in advance for any insight.
 
Impressive, Mr. Babin. Very impressive. There are only 2 devices on the power supply that use the mica insulators (I think they are rectifier diodes). Sure enough, first one I test was dead shorted to the heat sink. Apparently at some point in this amp's life the insulator was shifted and a new screw hole poked thru the insulator, leaving it to short thru the original screw hole. I guess I didn't encounter the problem before because the amp was mounted to wood in my previous car. Thanks so much for the help! I'll put it back in the car tomorrow to confirm but I am quite confident.
 
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