Testing a Power Transistor Replacement

[holden]

First post here by a newbie in amp repair...

Anyway, I bought a Peavey SC212 amp that had some intermittent loud humming problems. A look inside and some internet searching pointed to the two burnt looking power transistors as the problem. I just replaced all eight transistors and would like to test it with my multimeter before plugging it in and turning it on.

When I test one of these transistors NOT installed on the board with the diode check, things make sense and agree with the instructions you find on the internet for testing.

When I try to test the finished, installed transistors, it seems like it conducts in both directions. Also, using the ohmmeter test, there's a closed circuit between just about everything. Is this normal now that the circuit board is in play? Should the plastic gasket between the transistor and rail insulate them?

Are there any ways to test this better?

TIA.

[Jazz P Bass]

Do you have the schematic?
If so, you need to study how the separate transistor leads are connected.
The two (high side & low side) final stage output transistors are in parallel.
So they will read as such.
The base of the drivers has resistors that connect to the output transistors.
Study it & check everything.
Resistors, diodes, connections from point to point.
If all appears satisfactory, power up the amp with a lamp limiter, without a speaker load connected.
If the voltages are stable, then you can do away with the limiter.
Send a signal through & see if it appears at the unloaded output.
At this point it is best to use a scope, so you can see the waveform.
If all is well, hook up a speaker.

[holden]

Thanks for the response. I wish I were skilled enough to follow your advice. I do have the schematic, but I'm not an electronics hobbyist. Just a tinker who can follow directions well! This is basically a $35 hail mary attempt to fix an amp I paid $80 for.

I'm sure my soldering is good and I did not create any shorts with solder. My most likely mistake, if I made one, is one or more of the transistor leads touching the side of the holes in the rail they go through.

[holden]

OK... let me try a simpler question:

When replacing power transistors, does special care need to be taken to keep the leads from shorting out in the tunnel through the base plate?

I am getting solid conductivity between all 8 transistor cases and the external heat sink (they all connect to all). This seems very wrong since I understand half the transistor cases carry negative and the other half positive.

[Jazz P Bass]

OK... let me try a simpler question:

When replacing power transistors, does special care need to be taken to keep the leads from shorting out in the tunnel through the base plate?

I am getting solid conductivity between all 8 transistor cases and the external heat sink (they all connect to all). This seems very wrong since I understand half the transistor cases carry negative and the other half positive.

The rear casing on "most" power transistors is typically the "Collector" of the transistor.
If the schematic does not indicate that the output collector is tied to chassis ground, then it cannot be.
To isolate the collector from the heatsink, there is an isolator pad between the transistor & the heat sink.
If it was there at the start of the repair, then you must reinstall them.
If the isolator (mica, teflony rubber) is cracked, crumbly, torn or punched through, install new ones.
The mica ones require a smearing of silicone thermal grease.

[holden]

OK, call off the dogs!

I opened up another Peavey SS amp of the same vintage and got the same results with my multimeter. So I plugged the repaired one in before putting it back in the cabinet. No sparks, no smoke, power led lit up. Put it all back together and it's working great! Loud as hell too!

Thanks again for trying to help. I may be stupid, but I managed to do this repair correctly.

[g1]

Not sure what you mean by "base plate", but you should not have continuity from the transistor cases to the heatsink (unless the heatsink is completely isolated from ground which is not common). Each transistor should have an insulator (usually mica or some kind of rubbery/plastic coated with heatsink grease) between it and the heatsink. Often the transistors mounting screws also have an insulating bushing. Chances are one or more of your output transistors are physically shorting to the heatsink.
As far as the leads touching the "tunnel through the base plate" if you mean the heatsink (or another plate touching the heatsink), no they should not touch it.
With your regard to your first question about the transistors reading differently in circuit than out of circuit, this is normal. But you do not want to see any collector to emitter shorts or shorts of anything to the heatsink with the transistors in circuit.

[g1]

Well, please ignore my last post then, & thanks for posting your results.

[holden]

There were clear plastic insulators between the transistors and the heat sink (the thick metal piece between the transistors and the circuit board - I've referred to it as "base plate"). I kept the insulators in place during re installation.

When I took apart my functioning mid-80s Peavey Bandit, it had the same layout (except only two transistors). With an ohmmeter, I got good conductivity between both transistor casings/collectors and also between the casings/collectors and the heat sink, just like I was seeing on the SC212 I was repairing. I can't explain this, but both amps work fine and I definitely know how to use an ohmmeter.