Peavey Triumph 120 channel switching problem

[J-75]

I'm starting a fresh thread on the Peavey Triumph, as recommended, since an earlier thread has grown too long. (thanks Enzo).

Symptoms and observations so far:

Although the foot switch LED's respond correctly, the crunch, ultra and reverb functions are not working.
The clean works OK when the bypass switch is on.
There is no sound when either the crunch or the ultra channel is engaged, and no reverb when the reverb switch and pot are turned on.
There is a discernible increase in background noise level when the post gain pot is turned up to max on either the crunch or ultra channel.
The feed to the relay coils is nominally 25v.
The wiper of the reverb pot stays at 0v. when the reverb pot is rotated, whether or not the reverb is switched on.

The schematic shows two types of ground references - plain and circled. What is this denoting?

What should I check out next?

[nashvillebill]

Is this the PAG, or the earlier (non PAG) Triumph?

[J-75]

Is this the PAG, or the earlier (non PAG) Triumph?

It's the non-PAG model.

[Jazz P Bass]

Wow.
I had to look that one up.
What does PAG stand for?
"the next generation of Peavey tube amps where the tubes were tucked away out of sight in the chassis. They were Parallel to the circuit board, hence the monicker Parallel Axis Geometry (PAG)."

[Jazz P Bass]

I'm starting a fresh thread on the Peavey Triumph, as recommended, since an earlier thread has grown too long. (thanks Enzo).

Symptoms and observations so far:

Although the foot switch LED's respond correctly, the crunch, ultra and reverb functions are not working.
The clean works OK when the bypass switch is on.
There is no sound when either the crunch or the ultra channel is engaged, and no reverb when the reverb switch and pot are turned on.
There is a discernible increase in background noise level when the post gain pot is turned up to max on either the crunch or ultra channel.
The feed to the relay coils is nominally 25v.
The wiper of the reverb pot stays at 0v. when the reverb pot is rotated, whether or not the reverb is switched on.

The schematic shows two types of ground references - plain and circled. What is this denoting?

What should I check out next?

Actually there are four different ground symbols.
They designate the different ground paths in the design.
PCB star grounding if you will.
As to the problem, are you getting a voltage drop on the specific relays when they are commanded to engage?
If you are, then you should look at the 'Clamp" circuits for each command.
They all involve a Jfet which normally has a low resistance & goes high with a gate signal.
They may be stuck at a low resistance.

[nashvillebill]

There's no jfet swiching in the earlier (non-PAG) Triumph, it's just relay switching.

OP, what's your skill level, and what tools do you have access to (and kinda know how to use)? I'd suggest plugging an input signal into the amp and then use a signal tracer to follow the signal through the Crunch/Ultra path. A scope works, but an audio signal tracer works too--even a second guitar amp can be used, anything to amplify the sound from the circuit. You do have a schematic I hope?

I suspect a broken wire where the molex connectors are plugged in but of course there could be another failure mode .

[J-75]

I'm formally trained & worked in electronics (years ago), have: DMM, cap meter, audio oscillator, scope, tube tester.
I've printed a schematic and the PC layouts, but the latter are hard to read component text labels, so it's kinda slow-going identifying particular component locations on the boards.
Anyway, today I finally have some time avail. and will follow your suggestion.
I'll report back later - thanks!

[nashvillebill]

Is this schematic and component layout any easier to read?

[nashvillebill]

Note, when you print those board layout, in Adobe Reader, after you open the Print dialog box, under the "Pages to Print" section there's an option for "More Options". When you click that, there's an option to "Print current view". So before you go to print a particluar PC board layout, zoom in on it on the screen, then use the "Print current view" to print that zoomed-in area.

Okay, so put a sine wave into the input of the amp. Adjust jour scope until you can see the sine wave on Pin 6 of V1B. Now switch to eiteher Crunch or Ultra. Now look for the sine wave on C20 or R12. Nothing on the input of R12, check the molex connectors that bring the signal back from the board that S1 is on.

[J-75]

Thanks for the good drawings and printing tips, nashvillebill. I printed up a set to use for my next round.

So, a recap on my previous effort:

I pulled the chassis and jumpered the speaker and reverb tank, hooked up my audio gen. and scope - ready for some signal tracing.
Fired it up and... everything was working correctly!
I poked around at all the Molex connectors, but could not induce a failure.

I then re-assembled the amp, fired it up, and first thing i noticed is the reverb wasn't working. Then, whenever I toggled the standby switch on, or off, a loud crackling came out of the speaker for several seconds, then stopped.

I took it all apart again, and the same symptoms prevailed, so I examined and prodded the filter caps, but nothing suspicious showed up.
I shut it down and tested all the preamp tubes - they were all OK.
I turned it on and, after a good warm-up (15 Min.), I could toggle the standby switch without any static.
This amp is haunted!
My hunch is with you - that it's a crappy connection - probably a Molex, but I have yet to prove it.
I'll be using the better drawings next opportunity, so things will progress more quickly.

More later...

[Jazz P Bass]

My advise?
Take the boards out & reflow all of the big connectors.
And while you are in there, look the boards over.
If any joints look thin, reflow them.