Fender PD-250 passport

[goldtop5]

I've been working on this Fender PD-250 Passport for the longest time. It keeps going into protect mode. When I pull the amplifier board out, I would find 2 shorted output transistors. I replaced them, the unit would work for about 5 minutes, then kick back into protect mode. So I replaced all the output transistors, including the driver transistors, and (not even 5 minutes) it still goes into protect. Is there a possible solution for this? Do I need to replace the whole amplifier board?

[Enzo]

No, you need to fix it completely.

Chances are only one side was blown. You need to not only check for failed transistors, but also for resistors that are open or damaged. Check things back at least as far as the bias circuit, and set the bias very cold first, then adjust it last.

[km6xz]

Be sure not to put a load on it until you see that everything is stable, and not getting hot or having unusual current drain, and no oscillations plus being able to drive a high Z load or no load with equal degree of clipping with a high output voltage. When it is working for those few minutes, is anything getting hot? Is the test signal displaying good symmetry or is one side clipping before the other? Any DC instability at the output?