Peavey CS800 keeps blowing same power transistor

[veganxxx]

Thanks for everyone's input. I've ordered MJ6451's to replace my mismatched transistors

I checked the Q10 insulator for metal/cracks and didn't see any.

I will check the binding posts

I think I previously stated this, but I did get the amp to run and produce signal through a speaker with the shorted transistor taken out.

[Enzo]

Yep, which is why I worried there might be something shorting the case of Q10 to ground.

So there is a test we can do. Bolt a transistor into Q10 position. No power. No load connected. Measure resistance to ground from its case. Should NOT be continuity.

[veganxxx]

Ok I have some results-

1. Both sets of banana jack posts measure about 4K ohms with the amp off nothing plugged in.

2. There was no continuity between the body of the transistor and the heat sink.

3. I put in all new transistors and put everything back together. Back on the light bulb limiter it is dim. The one thing I notice is a 4 second pulse. The clipping light on the "good" channel lights and then every 4 seconds it goes out and the light bulb gets bright for one second and then dims. It also did this previously when I had the "bad" channel unhooked all together.

[veganxxx]

Ok so I plugged it in to real power since I had a dim bulb and it immediately blew a fuse. This time it took out 3 power transformers. Once I remove them and put it back on the light bulb it's dim again. If there was a short causing the power transformers to blow why wouldn't that light the bulb bright?

[J M Fahey]

Then *maybe* there was not a short but one transistor was overbiased one way or the other and when fully connected passed too much current and exploded.

Which, just thinking aloud, might be caused by a shorted *driver* transistor which lets all the current in the World into those power transistor bases: you do not measure a _case to ground_ short but when you apply full power rails happens what I said above.

IF so, you should have applied mains power through a bulb limiter, "just in case".
Oh well, these things happen all the time, going slow and steady is the way to go.

As Enzo said, these amps **are** very repairable, just may take some time.

So to check whether my above suspicion applies or not, turn amp on again, thriogh a bulb limiter and no load applied, and measure Vbe at the last 3 blown transistor sockets or PCB pads.
* IF you measure anything above 0.6 or 0.65V there, they are overbiased ... let alone if you measure full volts there, then check backwards (drivers cough cough) *why* such a high Vbe voltage appears where it should not.
If drivers fine, continued backwards, maybe *they* are overbiased. .... why?
* IF you measure , say, between 0.6V and 0.5V (or even slightly less, Peavey often underbiases power transistors *on purpose* to increase reliability) , unless you miswired those power transistors (say inverted Base and Emitter wiring or put PNPs where NPNs should live) , then those transistors were fakes or poor quality leftovers from the bottom of the barrel, sorry.

And so on, that´s the way.

[veganxxx]

Not that I’m too concerned at the moment, but is there any reason why the good channel produces a pulsing on the light bulb limiter?

[veganxxx]

I couldn't get measurements with the pulsing light bulb, the readings would jump with the light flashing. I unplugged the "good" channel and got a consistent dim bulb. Here are the readings from the bad board-

Rail 1 (no blown transistors)
Base- .138VDC
Emitter- 0VDC
Collector- 31.49VDC

Rail 2 (3 removed blown transistors)
Base- -17VDC
Emitter- -31.6VDC
Collector- 0VDC

[Jazz P Bass]

Yikes!

Rail 2 base is way too high.

[veganxxx]

Is that indicating the driver is letting through too much voltage? Or does the problem lie elsewhere and what could be the cause/what should I test next?

Yikes!

Rail 2 base is way too high.

[Jazz P Bass]

Being a direct coupled amp, yes, the problem is at or before the drivers.

[veganxxx]

With the driver removed the 17VDC is gone. Put in a known good driver and the 17VDC is back. I'm guessing whatever feeds the driver is giving to much voltage. What should I check next, i.e. what produces that voltage?

[g1]

Q6, neg. pre-driver

[veganxxx]

Not sure what terminals are what, I couldn't find a data sheet for that part number, but on the + and - Q5/Q6 I get similar readings on each side-

30VDC
30VDC
1.3VDC

-30VDC
-30VDC
-1.3VDC

[g1]

Driver is back in?
How can you have -17V at the driver but not at the pre-driver? Blown trace or bad solder?

[veganxxx]

Unless I'm reading it wrong, according to the schematic the base of the power transistors are connected to the collector of the driver transistor. I'm getting -30VDC coming into the base of the negative driver from the negative predriver, which also has the same reading. Isn't that what the predriver is supposed to give?