Very fun day!

Sadley I had to give up on this repair. The customer wanted the amp back. Here's a recap of what I did that did not help.
1)Replaced IC1,2,3
Replaced input jack
3)Replaced reverb tank w new
4)hardwired new RCAs for reverb
5)replaced R66
7)replaced 6 2.2 small elecytrlytic
8)Jumped 4 .1 electrolytics
9)Replaced out transistors

See yiu all next time and thank you!
Mike

FYI Here's an old thread from 2011 on the exact same topic.  https://www.ssguitar.com/index.php?topic=2279.0

Yes the POST, the REVERB and the TREBLE have to be at 5. Bass, mid and volume no effect. The suturation has a minimul effect. Could C26 be shorted?? C24 and C25 which are the same value, show no continuity in cuircuit but the C26 shows dead short. 

Yes I do think that its a ground problem. So far the only thing I can do to make it worse is touch the outside of the RCA at the IN jack of the reverb tank.

I found your c26. In curcuit, there is no resitance between the 2 grounds. i measured across the cap. Is that normal?
Pulling IC1 makes no difference. The Bass and mid have no effect but the treble sure does. It also has to be at 5 to trigger the problem. So the POST, the REVERB and the TREBLE have to be at 5 or more to trigger the problem. i hadnt noticed that. 

The customer complaint is that the amp squeals or otherwise cuts out when the Post control AND the reverb are both past 1/2 way up.  I verified this and can make it happen like clockwork by turning up the Post AND Reverb past 1/2 way. The far left Volume seems to be irrelevant. The Saturation control has some effect but does not seem primary to the complaint.  "Cuts" out is an inaccurate description. More like the signal is blanked out or covered up. If there was a way to post sound clips at this discussion I will do it.

Enzo: Yes the RF appears at Q1 and Q2 emitters. They are tied together. The resister R49 takes these emitters to the -15v. The -15 voltage budges very slightly and NO RF when I emulate the symptom. At the Q1,Q2 side of R49 we have all the RF.

Footswitch control. No change when unplugged.  RF at pre-amp out-YES.

OK Enzo...logic has to solve this. The R66 22ohm resister was burnt looking at the outset of this project. I replaced it. Just now I accidently touched. Burnt my finger. There is a wierd looking .1/100v electrolytic in front of it. I have similar. Should I change that?

Here's the same scope settings normal reverb and just prior to the problem. Rev and post at .50

Here are the scope views on the Peavey. .50 mean at 50% or 5

Can't work on it till Wed, but I'm not explaining it clearly. Maybe I can post a video Wed.

There WAS originally a squeal. Around the time I re-soldered the 4 pin Reverb connector it went away. Enzo recommended tank 4EB2C1B but we ordered #8EB2C1B because of the small cabinet for this amp. Both the new and original reverb tanks sound great. In fact. the amp sounds great...until I turn the reverb control past 1/2 way with the post control up 1/2 way. The position of the volume control is irrelevant to the "cutting out".   This post by G1 seems logical. I'm going to post a short video on Wed so you can hear what I mean.

Posted by: g1
« on: December 31, 2017, 01:38:04 PM » Insert Quote
Have you looked at the output on a scope?
I wonder if you still have oscillation that has just moved up in frequency beyond what is audible.  And once it starts it overwhelms the signal so it sounds like the amp is just cutting out.  And the meter could mis-read it as DC.

Phatt: I wasn't clear. The "squeal" went away around the time I re-soldered the bass of the 4 pin reverb tank connector. But the remaining problem is that the signal at the speaker dies when the POST and REVERB controls are between 1/2 ans 3/4 up.  At the same time the signal dies, a small negative voltage appears at the speaker output (and various other places in that output circuit. I did purchase a 8EB2C1B tank and tried it.  Same results as with the stock reverb tank. The irony is that the amp sounds great and the reverb effect is good.

I'm getting so close I can taste it!

@Phatt The wire trick did squelch squelch the problem but it reduced the reverb effect to almost nothing. Soldered the 2 resisters. Same.

@katoda There was a slight change in voltage at the center and top of the Reverb control as the hum kicked in. Like negative .05 but very consistant. So I went to "reverb send" pin on the board. There it changed from .23v to NEG .2. Same at pin 7 of U3 4558. .23 to neg .2v as I tyrned up the reverb and the hum kicked in.

So I went to R66 which is a 22ohm 1watt resister from the speaker output. (I had replaced this R66 early in the process because it appeared badley over heated). It fluxuates from 0 to neg .68 as the reverb control is turned up and the hum kicks in.

So them I checked voltages at the speaker output yellow. It goes from .001 to Neg 3.4 volts as the problem kicks in with the reverb control

12/26 update. I reordered the exact reverb tank I needed. It's a little better.  This may be a clue: The channel 2 input is fine. The channel 1 input (the louder one) is where the problems comes in. If I had to describe it, I'd say its overloads and shuts the signal off. The symptom is only when the "saturation" and the "reverb" controls are past 3/4. Maybe its ok?  Any opinions.