Well for starters, its not my amp, nor do I play with gain on "10". It IS unrealistic to play this amp cranked like that. I went through and cleaned any pots contributing to any noise. I did get the amp back to a controllable state. Again it only feedsback with that squeal if the switch on the Ultra channel  is set to Ultra or the Lead channel is set to High Gain. Both functions put it into high gain territory which normally is associated with feedback. After cleaning the pots I was able to play with the gain around 3-4 oclock, channel volume around noon and using the master for overall volume before I got the nasty squeal.  I did have to stand at least 5-6 feet away to be controllable. Cleaning the pots was a major improvement. Perfectly quiet on any other setting. No results from the chopstick test. I put it back together and sending this one on its way.

With a shorted plug inserted it the same high frequency squeal. Only does it with the voicing switch set to high gain. I went through and cleaned all the jacks, and all connections inside are solid. No obvious component failure. I cleaned all the ultra channel pots. Testing afterward the ultra channel is no longer squeling. But when I turn the LEAD channel gain to 10 - while on the ultra channel there it is squeel. I'm going to clean that pot and see if it makes a difference. Also no difference with effects loop jumpered.

I have a Peavey XXL (solid state) amp on the bench. Sounds great until you flip it to the Ultra channel then its squeal mania. It happens across all volume, gain, and eq levels and with different guitars, cables , etc. The issue is in the amp, in the Ultra channel only. The squeal goes away when your actually playing but as soon as you stop its there piercing your ears. Where should I start? I plan on getting it hooked up to my oscope today to see if I can see it.

That sounds feasible to me. It does have a 24v fan in it, so a simple circuit may be the right answer. I will call Fender today and see if thet tranny is still available (I figured it was an outdated part). I do like the idea of planting a smaller second transformer inside as there is plenty of room.

Transformer nonrepairable. I didn't have any luck yet finding a replacement either with those specs.

So the brown wire should be at 22 volts correct? If so I'll get a look at the transformer connections inside to see if anything came disconnected. Should I still disable the fan circuit for further testing or are we at a standstill until I fix/get a transformer?

Yes, wiring hanging in the air. P#'s only added for reference. Thanks for the help so far, always a pleasure here!

I have a Sunn Stage Lead, best sounding SS amp I have!

Ok, relative to the red/yellow wire (P6)
red (P5) - 53.6VAC
red/wh (P7) - 53.5 VAC
brn (P8) - 0 VAC
brn/wh (P9) - 22.4 VAC

Well whatever shorted did cause quite a show, I am not surprised. I'll take a look at that tranny. The overvoltages with an open winding doesn't make much sense to me either.

Voltages between P8 (brn) and P9 (brn/wh) is 0VAC.  P8 to ground is 0VAC.
P9 to ground is 22.4VAC.
A = red wire
B = red/yellow
C = red/white
D = brown
E = brown/white

A has continuity with all but D
B has continuity with all but D
C has continuity with all but D
D has no continuity with all
E has continuity with all but D

Transformer wires are correct. No voltage on P8 or P9 to ground unplugged.
No doubled wires on posts.
Only 5 secondary wires.
I have to take board back out to disable fan rectifier, will try to get to later tonight. Fan does come on as soon as amp is turned on.
The floating piece of metal was found on D112 of the fan circuit just a FYI.

P5 (TP1) - 54VAC    52.9VAC
P6 - 0V                    0VAC
P7 (TP2) - 54VAC     51.9VAC

TP3 - +72VDC         +70VDC
TP4 - -72VDC           -70VDC

P8 (TP5) - 23VAC       43.2VAC measured this 3 times
P9 (TP6) - 23VAC        22.4VAC

TP7 - +30VDC            +56.2VDC
TP8 - -30VDC              -55.9VDC  measured also 3 times

0VDC between P8 and P9 unplugged.

R161 is 11.2 ohms

QuoteDoes this mean +70V at TP4 and -70V at TP5?
How can you have +70V at TP4 which is a negative point?
Please confirm.

Sorry I mixed those up...-70 at TP4 and +70 at TP5 like it should be. I'll recheck what you have requested.

Quotewhat is the mains voltage where you are?

110 volts USA

QuoteWhere are you placing the negative/blackmultimeter probe?

To ground at the chassis AC ground

Bad transformer?