Peavey XR-1200 / CS-400 Blowing Fuses and Power transistors

[Enzo]

OK, so it is not the driver card, it is the power board itself.

You have pulled all the power transistors, and checked them individually, right? And you are aware that they are not all 8 outputs, the end one on each row up by the dual diode wires is a driver. So making sure the driver transistor is not swapped into one of the output trnasistor sockets.

We have by my count 7 10-watt resistors, 4 5-watt resistors, 7 smaller resistors, all to check for open.

Looking at your photo, there are three small resistors across the center, the lowest one looking lighter in color. That resistor ties two grounds together, and is 4.7 ohms. When the whole thing is bolted toge4ther, that might measure shorted across, but with the driver card off, you should be able to measure it.

And there are those two fat diodes - none shorted, right?

A handful of small caps, none shorted? Five of them I think.

And while we have seen photos of the component side of the board, you need to closely examine the solder side. There are no tiny traces on this board like there would be on a preamp or for that matter on the driver card, so I doubt you have a burnt open trace. However, it is very important the solder to the row of driver card pins is 100%, and of course the four Molex pins for the power connector.

[R.G.]

Advice for fixing power amps which have died:
1. SHORT the thermal compensation diodes for the initial power up and testing. Running with zero bias and crossover distortion until you figure out why the output devices want to be light emitting transistors is just fine. You can always bias it later.
2. Ditch individual thermal diodes for a Vbe multiplier with a trimpot. Use a high gain device in a TO126 or TO220 package so it gets good thermal coupling to the heat sink. Hook up the trimmer so an open wiper does not let them go open circuit.
3. Use a safety net. Stick a diode bias string one diode too big across the output bias network. This prevents an open wire to the real thermal compensators from opening the bias string entirely.
4. Put the thermal diodes the right place. On darlington cascades, put them on the output device heat sink. On complementary feedback pairs (one NPN and PNP per set) put the thermal diodes on the drivers. The output devices will follow the drivers because of the feedback.