Peavey MP-400

[soundinvst]

Greetings,

An MP-400 that I'm checking has distorted audio which increases as the volume is increased anything above a low setting. It appears to be happening in the power section, although there seems to be plenty of power. No indication of shorted components; voltages that I have checked have been normal; DC voltage on the output is low (0.6 mV). I have injected the signal directly to the board so as to eliminate the pre as a source. I don't have a scope to see what is happening here. On the power board I found the 5W resistors needed the solder joints touched up, and I tested the caps at both ends of the signal path. Where do you suppose I should look for the problem?


Thank you for taking the time,

R.

[The Dude]

What are the voltages on your output transistors Q3 & Q4?

[soundinvst]

What are the voltages on your output transistors Q3 & Q4?

Sorry it took so long...

~+/-35VDC

[Jazz P Bass]

Sorry it took so long...

~+/-35VDC

Please supply the voltages on the base, emitter & collector pins.

[J M Fahey]

Agree and add: we need 4 voltage measurements for each transistor:
* From ground (black test probe) to each emitter/base/collector , that makes 3 per transistor.
* from emitter (black probe) to base (red probe) , thatīs the fourth.
Thanks.
Always amplifier on, no signal applied, all controls on 0.

[soundinvst]

Agree and add: we need 4 voltage measurements for each transistor:
* From ground (black test probe) to each emitter/base/collector , that makes 3 per transistor.
* from emitter (black probe) to base (red probe) , thatīs the fourth.
Thanks.
Always amplifier on, no signal applied, all controls on 0.

Q3 (A1941)

1. base 34.9
2. collector -0.1
3. emitter 34.9
base to emitter 0.0

Q4 (C5198)

1. base -34.9
2. collector -0.1
3. emitter -34.9
base to emitter 0.0

Both collectors have neg. voltage. Measurements taken as instructed with no load.

Thanks!

RM