Peavey Deuce SS preamp section

[Euthymia]

I have an old Peavey Deuce hybrid on the bench right now and it has a problem in the "Normal" channel. "effects" channel works fine.

What's going on is I get a great sine all the way to the base of the NPN transistor in the stage marked "intermediate gain stage." After that, it's pretty much a flat line from the collector (direct coupled to the base of the PNP after it).

Here's what I have tried so far: pulling and testing both transistors (good, of course). Replacing the PNP. Replacing the 39pF cap and all the resistors connected to the emitters and collectors. Replacing the 0.1 cap going to the base of the NPN. Replacing the 250pF cap on the collector of the PNP.

The dc voltages on the NPN are E(24.7V) B(3.06V) C(2.51). If I pull the PNP out of the board, the voltages change to E(14.5) B(3.3) and C(2.76), and the sine wave appears loud and proud at the collector of the NPN.

I have also tried lifting the 250pF cap and 47K resistor that connect the PNP's collector to the tone stack. Didn't affect anything.

Somehow, having the PNP installed screws up the biasing of the NPN, but damned if I can figure out how, or how to cure it.

I figured that this would be a relatively straightforward SS amp stage to troubleshoot, as it can be isolated by pulling the 0.1 cap and the tone stack input, but no such luck.

Any ideas? I'm working from this:

http://www.schematicheaven.com/newam...vy_deuce-b.pdf

[Enzo]

First all those little 2uf/35v lytics are suspect.

Your voltages must be on the wrong pins, you have a +24v rail, so how can the NPN have 24v on its emitter and 2 on the collector?

If the 1M and 150k are OK, then you get about 3v - which you do - on the base. With the PNP out you get signal at the collector. So the NPN stage works.

Was the amp doing this from the start - it was brought to you like this? Or did it used to work and the channel died on you? This speaks to assumptions. If it worked then died, you know all the parts inside were as they should be, just something failed. if it was this way when you met it, you don;t know that. Kinda the difference between working on a Fender Deluxe and someone's home built copy. Your starting assumptions must ne different. ANy possibility of this: you tested a transistor and even replaced it, and installed the new one as the old one was installed. Perhaps the old one was in backwrds and we keep perpetuating the error? I worry about that sort of thing anyway, but especially so in light of your reversed readings.

When the PNP is in, what voltage appears at its collector? That collector has a 10k load resistor. You may have checked it or even replaced it, but is there 10k or resistance from the PNP collector lead to ground. Good resistors can have open traces connecting them to things. SO can transistors. An open load would turn that PNP into a diode.

Those are my first thoughts. Bat that back over the net when you get a chance.

[Euthymia]

Your voltages must be on the wrong pins, you have a +24v rail, so how can the NPN have 24v on its emitter and 2 on the collector?

Yes, you are right; I meant to write for the NPN:

E(2.51) B(3.06) C(24.7)

The amp was like this when I got my mitts on it, but I suspect from inspecting the solder joints, that no parts have been swapped. They all have that telltale Meridian kink where lead joins trace.

Hmm, yes, the resistance between the collector lead of the PNP and ground does need to be checked....

[Euthymia]

When the PNP is in, what voltage appears at its collector?

It has 15V on it.

I double-checked the orientation of the PNP and it's good. Also, the collector measures the 8.5K to ground you would expect to see for a 10K resistor with 57K effectively in parallel.

[Enzo]

As long as the base on the part conects to where it should on the board then. And the other two as well of course.

OK, 15v DC there but no signal? You didn't actually say anything about going farther than the NPN collector.

What are the numbers in the other channel, that stage looks identical.

I'm tired, I need to close up and go home, I'll check in next time.

Besides I still have to look at naked cheerleaders and get up to date on all the global conspiracies before logging off. Or maybe Landover Baptist or Betty Bowers. Or not.

[olddawg]

Yes, you are right; I meant to write for the NPN:

E(2.51) B(3.06) C(24.7)

The amp was like this when I got my mitts on it, but I suspect from inspecting the solder joints, that no parts have been swapped. They all have that telltale Meridian kink where lead joins trace.

Hmm, yes, the resistance between the collector lead of the PNP and ground does need to be checked....

I'm not looking at the schematic, but if you have a .6 -.7v voltage drop across the emmiter/base junction the transistor is biased on. I would be looking after that for something pulling the signal to ground.

[Euthymia]

Ah, okay, well, I put everything back together and tried it and now it works fine.

What had been a fuzzy mess on the base of the PNP is now a small sine, and a much bigger one on the collector.

I have no idea which of the half dozen components I replaced actually fixed it, or if it was a cracked trace that healed when I changed a component or what.

It works, and keeps working when I push on the PCB with a stick, so I can give it back to the client.

Kids, remember that part of the charm of working on solid state is that the mystery often stays intact after the repair is successful!

Seriously, thanks to those who offered suggestions. One of them may have helped.