Peavey Bass Version B

[Nyqusit5]

Hi,

I am currently repairing a Peavey Bass version B wityh the 400 Power module. The amp has 500mV on its output and produces a slightly fuzzy sound when played. It puts out its rated output but the sound has a kind of low fuzz tone on top of it. I can't see it on the scope but it can easily be heard.
I removed all power transistors and associated drivers (all 60142s) and the output still shows 500mV. Any suggestion as to what could be the cause of this? The last 430C maybe?

Here is the schematic (its the version A's but the power module is the identical)
Peavey Bass 400 A Series-1.pdf

[Enzo]

How did we decide it was the power module being fuzzy? I wouldn't worry about the 500mv just now. The schematic has voltages on it, how do your stack up in the diffy pair?

Have you played this amp through a different speaker cab? Your description also matches a rubbing voice coil in a speaker.

here:
http://bmamps.com/Schematics/Peavey/...Schematics.pdf

Use a signal tracer, is the distortion on the preamp output?

[Nyqusit5]

Thanks Enzo,

Yes, the amp has been played through several known and verified good speakers. It actually does sound like a rubbing coil but it isn't.
The fuzz comes from the power amp since I have disconnected the preamp and tested the amp with a different preamp (it is quite simple to do so on this model).
The differential pair is slightly off: one reads .750V accross the 470Ohm resistor, the other 1V. I don't have thez transistors to replace them yet (I only have 2n3906).

[Nyqusit5]

Update

I finally got the transistors I ordered.
I replaced the phase splitter transistors and now I have 250mV on the output.
Anyway, the problem is still there. Without a load, the amplifier has a clean sine wave on its output (when fed with one), but as soon as a load is connected, there is a small amount of crossover distortion and the top of the sine wave seems to drop or sink slightly. The negative part doesn't move.
So I changed pretty much all transistors while trying to identify the problem. Most voltages read fin, except for the .6V accross the 22Ohm resistor on the collector of the 431 (TIP30). There is no voltage there (and neither on the other 22Ohm on the collector of the opposite 430). Also, I don't have 100mV accross the 1Ohm/10W resistor as indicated on the schematic.

It seems the transistors aren't biased correctly but I don't really know where to look to next...

[Jazz P Bass]

Have you tried removing (or where they replaced also) the current limit transistors? (3642 & 3638)

[Nyqusit5]

Yes, I replaced these too but with no success.

[Jazz P Bass]

I would (using a scope) start at the beginning.
Is the signal good at the input of the LTP?
The first 430C base?
Next, the predriver 430C & 431C bases?
Next, the 60142 driver bases?
And finally, the output transistor bases?

I have a feeling that you have a bad connection or a low value resistor is open.