Peavey 400BH

[lowell]

This PA head came in with 1 out of 8 blown power transistors. The positive supply was also non-existent. I found that C35 in the supply was bad and replaced it. I replaced the blown transistor. 2 traces for the rectifiers were burnt open, I fixed those. All ballast resistors are good. So here's where I'm at:

For now I'm using my bulb limiter, and it's still glowing very brightly when all is connected as normal.

Without the power amp modules connected, +-52 supplies are there (reading +-48v w/ limiter) and bulb glows dimly.
CR11 and CR15 protection diodes seem fine w/ diode test function on meter when the boards are disconnected from the supply. When I connect the module boards CR11 and CR15 read 0 on diode test function.
The +-52v supplies read 2v/-3v with limiter when everything is connected. Something's almost a dead short.

Q2 and Q12 test good w/ diode function... not sure how to test transistors under operation.

I'd think if a power transistor had gone bad during fixing testing that it would reveal itself when checking CR11/CR15.

Something's interacting when the power amp is connected to the supply but I can't figure it out.

[Enzo]

Go down the row of 8 power transistors and check again for shorted Collector (case) to emitter, just to be sure.

CR11,15 read shorted when the boards are connected? DO they still read that way when power is OFF?

I never disconnect the transistor board from the main board except when I need to physically remove the parts. There are the two thre pin connectors for the transistors, but there is also a very important two wire connector for the bias diode. Is that OK? That is dual diode CR13. If it is open, both sides outputs will turn on hard. And check CR9/R21 for open.

[lowell]

CR11,15 read shorted with power off when boards are connected.

I'll check the other stuff and report back.

[lowell]

I had not remembered that another way of testing transistors is collector to emitter shorts. whooops. Found 1 more bad transistor and voila! Thanks enzo.