Testing Output Transistors...

[GibsonGM]

Here's a question (maybe the wrong forum, though, but there are people here that know everything):  I am debugging a 1977 Peavey CS-800 power amp for a friend. It has 2 amp sections.  Cleaned and tightened some of the Molex connectors - one had arced a little i think.   The symptoms he was having were loss of output and crackle, solved by him bashing the side of the unit.   That's gone away, but I am getting low output on the Left channel.   The Right channel seems fine - heat sink really cooks up and no problems...the left one doesn't get that warm.  Everything on the driver board appears to be OK,  so I am questioning the output transistors.   The voltages on their PCB pads don't match up with the ones on the side that works.

How do you go about testing them - remove them and check diode drops (there are 12...)?  It has 10 SJ6451's and 2 SJ6344's per side.   And, any idea where you can get replacements?  I've read that MJ15020 and MJ15024 are good subs for these amps...not sure tho.

Thanks if anyone has any info...the box is sentimental to him and it would be cool to get it back to him in good shape.

~Mike

[R.G.]

Debugging a high-power direct-coupled opamp (that's what all modern power amps are) is very challenging. Very often, any failure causes a chain of destruction to other devices as well.

As to how to test them: Do the ohmmeter thing, making sure that the base-collector and base-emitter diode junctions act like diodes, conducting one way but not the other, and that the collector-emitter doesn't conduct either way. That will tell you about gross flaws like being shorted or open. And if it ever worked, that's generally good enough.

But having one side with low volume is a little odd.

You have the schematic, right? How do the voltages compare with the schematic voltages?

[GibsonGM]

Hi R.G., thanks for the start on this...the schem Peavey sent isn't the right model, and has no test voltages.  I did find what I think is the right one here:  http://www.epanorama.net/schematic.php?file=schematicsforfree/Audio/Pro_Audio/Products/Peavey/CS-800.pdf
So far I've only cleaned things up, compared opamp voltages between boards, and they look ok.  Tested all diodes and electros, check.  The 'crackle' I was told about was solved by tightening the harness connections.

The low output might be an illusion - I am using a small guitar speaker rather than a cabinet and really can't tell.  But the right side amp is running really hot (but probably normal), and the left side is cool...its transistors aren't getting as hot as the right side ones.  So there is something going on - whether I can figure out what, we'll have to see!   Unfortunately (well, maybe not) I'm going on vacation for a week, so it will have to wait a while.  I'm sure I'll be back on with a few other ?'s before this is figured out!!   

I would assume if 1 or more output transistors blew, I should get no signal, but that's not the case.

[R.G.]

Ok, that's more to work with. The output level thing was confusing.

If I were doing this, I would check the transistors for shorted/open, and also for connection to chassis ground. Assuming that is good, I'd go look at bias.

Bias on solid state complementary outputs is determined by the voltage between the pre-driver bases. In this case, it's the voltage across C103; that's the voltage across the string of diodes CR127-CR130 that is actually doing the biasing. The only adjustment available is to insert/remove diodes, or to change the value of R173 (100R) across CR130.

If that string is lower voltage than needed, the amp will run cold and sound harsh from the crossover distortion. If it's higher than normal, the amp will run hot. If it's open, the amp will smoke and burn within milliseconds.