Peavey Mark3 head problem

[hostile1]

I just picked this Peavey mk3 400bh head up for cheap, along with a Cerwin Vega cab. When I turn the head on with no load / not connected to a speaker, it seems fine. When I plug the speaker into it, I get a very loud hum/buzz out of it. If I leave the speaker plugged in for more than few seconds, the head will eventually start to smoke a little and/or blow a fuse.

Anyone have a clue what the problem could be, and if it's worth keeping/fixing?

Thanks!

[hostile1]

Well I just checked the thread here: http://music-electronics-forum.com/s...ead.php?t=4262

That person has a similar problem, but I'm still not sure. Most other problems, the amp will either be distorting, supplying low power, or no noise at all. This one stays on fine with no load, but seems to overload and go full wattage on any cab thats plugged in.

It's not worth it to me to spend $100+ for repairs since I have a Peavey combo 300 that's basically the same power section. If anyone has any ideas that point away from costly repairs, I'd love to hear them.

Thanks!

[Amp Kat]

My guess is one of the output transistors is faulty or perhaps a driver that is bad or maybe both. Most of the cost of repairing amps is the time it takes to take these things apart. $100 really isn't very much when considering the time it takes to take it apart,troubleshoot the problem,change the bad component and perhaps fix something else along the way noticed in the mix so it may be $200 if all the caps are old enough. Maybe you can try changing the output trasistors yourself. You can ohm them out with a meter and if they are shorted you can almost guarantee it to be the culprit. If your not going to spend the cash to get it repaired what do you have to loose ?

[Enzo]

Seems fine without a speaker? That means what? power light comes on?

I'd bet my lunch money you have DC on the output, and KB is right about a shorted output transistor or two. For reference, my shop would want about $60 labor and $0-40 parts. if the outputs are shorted, likely $15 for them, or if the filters are bad, maybe $20 for them. Not likely both. Since heat and smoke result, I'd bet on the output transistors instead of the filters. SO about $80 here. Check your local shop rates.

[hostile1]

Thanks Enzo

I asked a few guys at the local Guitar Center, their answers varied from the transformer to the power capacitors.

What happens is it will turn on fine with no speaker connected to either of the outputs.. I can hear a quiet hum inside and the power light turns on. As soon as I connect a speaker, I get a VERY loud hum/buzz, no matter the volume or eq setting. I haven't done this with an instrument plugged into any of the inputs, but I think that wouldn't matter much.

I have the combo 300 version with a similar head/power section, different eq. I've connected the external speaker out on that to the same cabs used with the mark 3 and have no problems.

I'm not too worried about it if this head is bad, I wouldn't spend the money to get it fixed unless it were under $50 walkout.

[Enzo]

I'll bet my lunch money and throw in dessert that you have DC on the output. When your speaker makes the loud hum, if you looked at the cone, when power is turned on, the cone woul dmove one direction and stay there. Instead of a speaker, if you plugged a speaker cord into the amp and then measured voltage at the far end of the cord, there would be 40-50 volts of DC.

If that is the case, there is at least a blown transistor. ANy shop will have to charge at least an hour. Sight unseen, I'd guess it would cost $60-80 on my shop. Of course the decision comes out of your wallet, not mine. But dead it ain't worth much. Alive, well look on ebay to see what they are worth these days.