Ampeg STV 4 Pro MOSFET replacement and other Qs

[nsubulysses]

I'm about to get into something that will not be fun it seems. Not familiar with this amp or style of amps from Ampeg in the 90s at all.

R142 and R126 are burnt and open. When replacing them and turning on the amp the bulb still lights brightly. Without the bulb R142 starts to smoke.
I have not gone too far into this amp but it seems one or more of the MOSFETs in the output are shorted.

Any advice for how to go about this? Other threads say I will need matched MOSFETS for the output and that will be expensive and possibly hard to find.
Mouser's parts don't match even in specs. IRFP9240 is 12A with a drain-source resistance of 500mOhms and IRFP240 is 20A with drain-source resistance of 180mOhms.

I will have time Sunday to get back into this. I'm thinking I should try to isolate the A and B output sections and see if one works so I can compare it to the other. Any other helpful hints for strategy are needed and very appreciated.

Oh yes, and also the schematic is here in post #3.

[drewl]

You have to rebuild the output section, its a pain but fairly easy.
You need to replace all the mosfets , hopefully they will be from the same batch and closely matched.
You need to check all the emitter resistors, all the diver transistors, diodes, bias circuit- everything in the power amp.

Replace all the bad parts and it should work.