Okay, so the wife means well, but sometimes she gets expensive.

My Peavey amp is not putting out sound since my daughter's bday party Friday. The mixer section is working, all the way up to the power section input.

Upon cursory inspection, I noticed that a strand of decorative garland -- hung by my wife behind the amp -- was sucked in by and wrapped up around the cooling fan preventing it from spinning. Removed the garland, fan runs fine.

Question for the peanut gallery: how much damage was done to the power output board, and what should I look at first?

 

BTW, the amp could have been running for hours with no cooling -- it runs in parallel to another amp driving another set of speakers.

All anyone would have noticed during the party was a slight reduction in overall volume, assuming they were even paying attention.

 

Output transistors, failed short and blown the DC rail fuses. At a guess, they would be the only casualties.

 

Output transistors, failed short and blown the DC rail fuses. At a guess, they would be the only casualties.

Is it possible the fuses saved the transistors (I think there's 6 per channel)? Or would a fuse blow only in the case of a damaged transistor?

 

Is it possible the fuses saved the transistors (I think there's 6 per channel)? Or would a fuse blow only in the case of a damaged transistor?

The fuse would only blow if the transistor failed.
Otherwise, fuses would blow if run at full output for any length of time.

 

Confirmed: both fuses blown.

@Ian0, is it possible to determine which transistors are failed without desoldering all of them?

 

Confirmed: both fuses blown.

@Ian0, is it possible to determine which transistors are failed without desoldering all of them?

Just look for emitter-collector shorts.
Probably best to replace them all.
If there are several in parallel, then it's better if you have almost identical devices in parallel.
If only one has blown, e.g. the one on the positive side, then only the fuse on the top side would have blown.
It's not common for them to blow the driver stage, but check the base-emitter resistors on the output transistors.

 

The amp is circa 1994. Should I change out the electrolytics while I'm at it?

BTW, power audio is absolutely not my forte.

 

No, unless one or more appears to be leaking or deformed

 

The amp is circa 1994. Should I change out the electrolytics while I'm at it?

I don‘t usually bother. It‘s generally a case of “Will you fix my amp?”
”When do you need it?”
”We’re on the way to the gig, and we’d forgotten that we blew it up at the last gig”
It won’t do any harm to replace them, but more important is to check that the heatsink compound hasn‘t dried out, and if it has, clean it off and apply some more, then apply some to your fingers and transfer it to anything you touch for the rest of the day.

I‘d fix it for you if you weren’t so far away. It would probably take me less time to fix it that taking it apart and putting it back together.

 

I‘d fix it for you if you weren’t so far away.

Likewise, I'd pay you to do it.

...then apply some to your fingers and transfer it to anything you touch for the rest of the day.

Noted.

 

I would also search for the service manual. A wholesale replacement of the output transistors operating in class-AB may mean that you very likely require to adjust the bias.
The manual will tell you what that bias level is and where you should measure it.

 

A wholesale replacement of the output transistors operating in class-AB may mean that you very likely require to adjust the bias.

Seriously? This is why I don't do analog anymore.

Guess I gotta find a service manual.

 

Or 50mA in the absence of any instructions to the contrary.

 

Or 50mA in the absence of any instructions to the contrary.

Where do I measure it?

 

You **may** be lucky and the amp will run fine without ant adjustment.
But too little bias and the sound will distort or too much bias and the transistors will run hot.

 

It’s not exactly hi-fi. If there is some bias it will be OK. Anything between 25mA and 100mA Will work. Above that and it will start to run warm.

If there is no preset there is no adjustment!

 

If there is no preset there is no adjustment!

What is "preset"? Where do I find one?

I'm not able to find a service manual. Where would I measure the bias?

 

The preset will be located close to a transistor called Vbe multiplier. It will be a small signal transistor in close thermal contact to the main transistors. The measurement will be done by measuring the voltage drop across the main emitter transistors.

Of course, I am assuming that this is an ALL BIPOLAR OUTPUT stage.
If it has Mosfets, it won’t have bias adjustment but may require to match the transistors for Vgs(th). Again, you may be lucky and this step is not required.

FIRST THINGS FIRST. Open the unit and determine the transistor part number.
While you are at it, take photos (clear and focused) of the board area: one of the whole board and several close ups in the vicinity of the output transistors.

 

Hello,

Elektrotanya seems to have the manual:
https://elektrotanya.com/peavey_xrd-680s.pdf/download.html

Bertus