XR686F Amplifier blowing output transistors

[WTHIESSEN]

Hi,

I currently have this PA on my bench. I'm using a light bulb limiter and variac.

Original Problem: no monitor output, monitor EQ lights stay on
During power amp troubleshooting: Both SCR's blew up, a bunch of the of the NPN/PNP Q200/208 etc (driver?) transistors shorted.

I replaced the Q100,Q200,etc transistors and need to replace all the power transistors. Both side don't work now. When I turn on the power amp with the preamp connected, there is a loud buzz, so I turned it off right away.

Also, while troubleshooting the preamp EQ issue, I seemed to have caused more problems. It powers up for 10 seconds then loses power.

This amp has gone from bad to worse. I would appreciate any help you could give me. I'm considering finding another used one to replace it, or just replacing both boards.

Thanks,
Will

[Enzo]

First off, NO SPEAKER until the amp is not making DC on the output.

We'll need to get into it to see what might be going wrong, but let me offer another option.

Instead of replacing it or replacing all the boards, consider taking/sending the thing to a qualified repair shop. Peavey does do repairs for customers, you can call the repair department there and ask about rates. They also have a network of authorized repair shops all around the country - I was one for 30 years.

[WTHIESSEN]

I’m the one fixing it for someone else, but I’m thinking that might be a good option.

Is there a possibility that the 15V ics are providing power to the preamp for a short time then failing bc of overheating?

[Enzo]

Not a likely scenario.

You had a power amp blow up? You had to replace a bunch of burnt parts there? Then now is not the time to suspect preamp op amps heating up.

[WTHIESSEN]

I forgot to put the heatsink back on the 7815 regulators. Preamp is back to where I started with the LED's staying on, on the one channel.

[WTHIESSEN]

I ended up ordering the output transistors. Any suggestions for testing it before replacing all of them? Is there any danger in replacing half of one side at a time and running the voltage low?

[WTHIESSEN]

on the one side of the power rail I'm getting too much voltage, and the the other has zero voltage. (should be 50V on each side at this point. I'm using a variac).
I haven't been able to locate the fault.

Do you know of any reason that this could be happening?

[J M Fahey]

on the one side of the power rail I'm getting too much voltage, and the the other has zero voltage. (should be 50V on each side at this point. I'm using a variac).
I haven't been able to locate the fault.

Do you know of any reason that this could be happening?

Short and incomplete answer: Peavey on some power amps (this one could be one of them) uses what is called "flying rails" topology.
FWIW this was pioneered by QSC power amps.

Cut and paste Enzo´s answer at DIY Audio:

Enzo 17th July 2006 01:01 AM
Aroound here we call that "flying rails" and many amps are made that way. Even small Peavey solid state amps are often wired that way. Or at least grounded emitter.

If you would ground the emitter in a conventional PP output stage, why not reverse things and ground the collectors and run the rails to the emitters? Then the output xstrs can be grounded removing the need for insulators etc.

The first post included a link to the QSC schematic collection.

The EX, MX, and USA series are not all that old and are typical. the Powerlite and PLX series are switchers.

In their flying rail circuit, the speaker is between the main filter cap common and ground. The "output" of the power stage is also grounded.

To the uninitiated, this arrangement looks counterintuitive at first, but once you look at it a bit, the amp topology is not all that strange, and it is just a change of frame of reference.

When output transistors short, typically one rail gets grounded and the opposite one doubles

So what you see is normal (for a dead amplifier) and repairing takes the same steps as on a conventional amplifier, only it takes time to "see the World upside down" .... which won´t exactly help you.

Imagine driving someplace but you are only allowed to look at a mirror

With due respect, I think you´d be better off by sending it to a specialized Peavey Tech, or maybe even Peavey itself, which might repair it for a fixed (quite acceptable) price.
Don´t think replacement modules are available for it.

[WTHIESSEN]

Thanks for taking the time to respond. I've had another Peavey guitar amp on my bench that also seemed to work like this.
When back to it with fresh eyes this morning and have found the short on the Q200/208.
Unfortunately there's no local Peavey service center around here anymore, and I'd rather not ship it away.

[g1]

Please post the schematic.

[WTHIESSEN]

See http://music-electronics-forum.com/t38079/