Peavey Mark III 400BH problem

[10pizza]

hi All,

new to this forum which I discovered after searching for similar problems with my Peavey amp.
Hope somebody can help.

My Peavey Mark III 400BH give a loud hum on the speakercab. I disconnected the cab and measured 1.1VDC on the speakerconnection. Switching my meter to AC, I measure around 30V~

I connected another amp to both Preamp and Line-out outputs, this gives no problems, clean signal, so I think the pre-amp is ok.

I read some other posts with similar problems, but I'm not really sure where to start. I found the schematics of the amp already on this forum. I'm not experienced in amp repair, but I did have electronics education 20 yrs ago, so I should be able to understand ;-)

Hope you can point me in the right direction to start troubleshooting.

thanks Age

[10pizza]

some more info:
I've checked the voltage on the molex connector to the front panel: just below 15VDC and -15VDC where it should be.

[Enzo]

There are two main power supplies for the output stages - one positive and one negative. I forget, but without looking it up, I think around 45-50 volts on each. measure both for DC volts. Are both up to about the same level? Or is one about 45-50v and the other about 28v? Now flip your meter to AC volts and measure the same places. You should get very low AC (we are measuring ripple on the power supply now), but if one of them shows about 30VAC, then the filter cap for that supply has either failed or broken free of its solder.

[10pizza]

I take it you mean the two big capacitors?
they are +36V and -34V DC.

Looking at the printboard however I see that some solder/printing has come of off the printboard just where the molex connector from the transformator connects to the board. I'll first get that fixed and try again after.

thanks for your help sofar, I'll keep posting on progress here.

[10pizza]

Problem solved!

I fixed 2 broken printboard connections. Turned it on without a cab and measured on the output. No more DC or AC volt on the speaker outputs!

found an old 2 ohm speaker and connected that for testing and it works again!

only issue left is a *pop* sound when switching off the amp. I'll search the forum to see if that can be fixed.

glad to have this monster going again!

thanks for your help here. Nice to know about this forum.

[Enzo]

Power on/off popping is quite normal in many many solid state amps. Nothing there to "fix."