That's why they didn't save your expensive power transistors.
They have to be rated so that they don't blow on the inrush when the capacitors charge, so they are too large,
They would have done a little better using a lower value of Time-delay fuse, or much better if they were after the capacitors.
The counter-argument is that they wouldn't fail in the event of a rectifier or capacitor short, but almost certainly the transformer primary fuse would.

I see this as odd myself. But I'm not an audio engineer, and this was a highly rated mixer/amp back in the day.

 

Is that an inrush current limiter on the line? Could that be the secret?

 

Is that an inrush current limiter on the line? Could that be the secret?

View attachment 344558

Yes it is. And still intact after all these years!

 

Yes it is. And still intact after all these years!

It reads 1.1 ohms cold. Sound about right?

Two thermal switches mounted on the TO-3s read closed. One on opposite side of board mounted to middle of heat sink reads closed.

Correction: the central thermal switch is open. It connects to the power entry board, and I believe it is a crowbar that blows the mains fuse upon closure.

 

It reads 1.1 ohms cold. Sound about right?



Correction: the central thermal switch is open. It connects to the power entry board, and I believe it is a crowbar that blows the mains fuse upon closure.

That would seem odd, if not a bit dangerous (especially for those people who use tinfoil fuses)

 

Ya.. I forgot....!
These peavey has funny kinda crowbar in the output path to blow the fuse.

 

What is the number on the black circled ones ?

I think those are the crowbar protections ....!

 

What is the number on the black circled ones ?

I think those are the crowbar protections ....!

 

They're not shorted, but I can't get any other good readings on them in-circuit.

 

I do not replace all power transistors after fixing.

Are you saying you just replace the blown ones? Even though the original good ones are nearly 30 years old and a completely different part than what's available today?

 

That would seem odd, if not a bit dangerous (especially for those people who use tinfoil fuses)

This is a feature. It allows you to turn the amp up to 11.

 

View attachment 344560

Yup...Crowbar protection triac.

I have seen these creating issues.

Do you have the schema...
I believe these are wired across the output or something. It was log time ago when I fixed peavey's

 

Are you saying you just replace the blown ones? Even though the original good ones are nearly 30 years old and a completely different part than what's available today?

No no..
I am saying to test the amp I use just one pair and power up.
to do tests. After I am satisfied I install all pairs

 

Yup...Crowbar protection triac.

I have seen these creating issues.

Do you have the schema...
I believe these are wired across the output or something. It was log time ago when I fixed peavey's

If I had the schematic, this would be much easier. There is this, though, which has many similarities:

https://forum.allaboutcircuits.com/threads/peavey-xrd-680s-plus.205749/post-1971924

No no..
I am saying to test the amp I use just one pair and power up.
to do tests. After I am satisfied I install all pairs

Yes! Makes much more sense. Thanks.

 

Not the same one but close.
No crowbar I can see ...bummer

 

Not the same one but close.
No crowbar I can see ...bummer

?

 

Repairs complete. It powers on tonight.

Hopefully.

 

It's working. Sounds great.

Thanks everyone who chimed in.

 

Many of the front panel pots are noisy.

Do they just need cleaning, or replacement?

 

Many of the front panel pots are noisy.

Do they just need cleaning, or replacement?

Clean first. Nothing to lose.