Peavey Tour 450 - Protection mode fault?

[Horse57]

Hi,

I have a Peavey Tour 450 on my bench which has me perplexed. Not something I'd usually take on (specialising in valve equipment) but this ones for a friend so I thought I'd take a look.

It came to me pretty lifeless and would go into protection mode on start up. All three PNP transistors were shorted. I replaced these and there is life, however...

When applying a signal (with dummy load attached) the amp will go into protection mode after a short period of time (relay 1 opens). There seems to be no rhyme or reason to this as the unit isn't running hot and there doesn't appear to be any over current conditions.

Removing input signal and then reapplying, the amp jumps back into life again (relay 1 closes) but this happens after a half second or so of the input signal being applied. This is repeatable.

After applying the input signal for a short period (10 seconds or so) and then removing the signal, relay 1 opens and closes every 3 seconds or so continuously... It's as if the fault detection circuit is closing the relay, perceiving a fault, and then opening the relay again. This loop goes on for some time, power cycling the amp usually stops it, or removing the dummy load.

All voltages appear okay. I've got +-86V on the output transistor collectors and the +-15V supply is present.

I'm just about ready to throw in the towel on this one so any thoughts would be greatly appreciated!

I've tried to upload a schematic but it isn't cooperating - link here: https://elektrotanya.com/peavey_tour.../download.html

Many thanks,
Andy

[The Dude]

Check, or temporarily remove triac TR3 and see if that fixes the problem. Check for DC before connecting a speaker and do not connect if there is DC on the output. If you had shorted outputs, it's likely the amp outputted DC, at least for a short period of time- possibly long enough to damage the triac.

[Horse57]

Thanks for your reply. This unit doesn't have the triac fitted, from what I've read, Peavey removed them after the first few hundred

No DC on output, although when a signal is output and I check for DC there does seem to be a few volts... This is only with one of my meters though and my Fluke doesn't show any DC so I'm putting that down to the meter for now.

A few additional findings:

- I've attached a photo of the waveform under moderate output, showing what I think are rather odd high frequency bursts at the peak of the waveform?

- The "post gain" pot has DC on it from the drain of Q3 - I first heard this when checking the amp with a speaker - could these faults be linked?

Thanks again for your thoughts

[Helmholtz]

Can't see the photo.

[Horse57]

Apologies, I thought it had uploaded. Please see attached.

Thanks!

[Helmholtz]

Certainly looks like HF bursts, i.e. an instability.