Peavey Mark IV bass head 400 BH.. Problems...

[wheatley]

I've got a 1982 Peavey mark 4 bass head which stopped working. i read some bad info on ehow.com and plugged speaker out into the power amp in jack on the front, and had the separate hi/lo outs on the front plugged into bass cabs. When i powered on the head there were bad loud noises, i turned it off immediately.
Now when i turn the amp on plugged into a speaker, Correctly, there is the usual, very subtle white noise that tells you its on but no signal registering from any of the inputs.

I took the voltages at the 5 space molex connector and they were correct, -15, 15, and 0, with the pre amp plugged in and UNplugged. also when i touched the white wire input to the power amp it hums like it should when you touch the tip of a instrument cable.

does this narrow the problem down to the pre amp?


any advice would be great...

[Enzo]

If the power amp amplifies, then it amplifies. Preamp sounds suspect. But make sure.

Plug a cord into the power amp on jack. At the other end measure for DC volts. Is there 15v sitting there?

Touching the input makes sound, but does plugging a signal into the power amp in jack itself make sound?

With signal at the regular input, is there good signal leaving the preamp out jack?

[wheatley]

i measured the voltage between the sleeve and the tip of the power amp in and it was -15 V. Plugging the 1/4" into the power amp in makes a little noise but no noise when i touch either the sleeve or the tip.

how should i check the signal leaving the preamp out?

[Enzo]

Plug a cord from that jack to the input of another amp.

But that 15v is a clue. And that is why I asked you to check it that way. Look on the preamp drawing. COnnected to that jack are CR21, CR20. They go to the two 15v rails. They protect the ICs from excess voltages. When you plugged the amp output into that input, the resulting feedback would have been peaking at +/-50v. Easily damaging a diode. I'll bet my lunch money CR21 is shorted. And since that input is shorted to -15v, any signal trying to enter there will be squashed. And if you unplug the cable from front to back, the shorted diode is on the front, so the power amp itself responds when you touch it.

Those are common, cheap 1N4148 diodes. If one is shorted, replace them both. See if that restores things. That has to be fixed anyway, regardless of any other problems.

[wheatley]

I just swapped out the old diodes, put it back together, plugged in and SUCCESS. thanks for the much needed help