Peavey Triple X 120W head acting up

[Fred G.]

Greetings,

I'm repairing a Triple X head that came in with a complaint of "volume loss".

When I fired it up to test, I found it was behaving in one of two ways, which I can actually toggle by turning the amp on and off:

The amp will power up, have severely reduced volume, will be very noise-free, and have a raging, thrash meta-type of distortion;
OR - it will power up, be extremely loud (as you would expect for a 120W head), but have horrendous hiss, as well as faint motorboating along with a very clock-like ticking. The tone also seems kind of boxy when it's in this "behavior mode". And finally, V1, when physically tapped with a chopstick in this mode, seems to be very microphonic and sensitive. It's not the tube itself, I've swapped out several and this behavior is common to all.

The Power and Standby switches also pop extremely loudly, and the amp throws out a pretty strong surge when turned off.

I've tested and replaced all tubes (the customer wanted full tube replacement as well).

I'm suspecting decoupling caps might be shot? I mostly work on vintage amps and Marshalls and Fenders, this is my first venture into a newer Peavey, and I'm wondering if there are some common faults here to look for.

Thanks!

[Jon Snell]

I would firstly check the effects loop sockets, connectors on the board and the +-17v supply for the relays and the many contacts they have.
Keep it simple and look for a bad contact somewhere.

[Enzo]

And check operation/condition of the mute JFET.

[Fred G.]

And check operation/condition of the mute JFET.

OK, a couple of questions:

Would a failing JFET cause the motorboating? I'm not really knowledgeable in solid-state tech; I'm self-taught and the fancier amps with all the switching, etc. never turned me on as a player, so I never really learned the technology, other than when I was self-instructing in electronics basics decades ago.

I don't know how to test a JFET...I'm certainly interested and willing to learn, as I'm of retirement age and am dabbling in amp repair as an income source, so the more experience I can get the better!

Thanks!

Fred G.

[Enzo]

Page 1 right center. The output of the preamps is selected by relay contacts K4B and right below that is Q2, the muting JFET. It mutes the signal very briefly when you change channels to prevent noises. Unless it doesn't work. Test it by simply removing it. If the symptom changes or goes away, then it was bad. If nothing changes, then it was OK.

Isolate the problem. Plug guitar into FX return to bypass preamp. Is the output clean and strong or does it still have the issue? We want to know if the problem lives in the power amp or the preamp.

Could JFET be involved in boating? WHo knows? It certainly can cause the failing level issue, so test it. It either cures the motorboating at the same time or it doesn't. You can have more than one problem at a time, but we work on them one at a time.

You have ribbon cables connecting the boards together. Pull each one most of the way off and push it back down. This refreshes the contacts.

The input tube is microphonic on many many amps.

[Fred G.]

Page 1 right center. The output of the preamps is selected by relay contacts K4B and right below that is Q2, the muting JFET. It mutes the signal very briefly when you change channels to prevent noises. Unless it doesn't work. Test it by simply removing it. If the symptom changes or goes away, then it was bad. If nothing changes, then it was OK.

Isolate the problem. Plug guitar into FX return to bypass preamp. Is the output clean and strong or does it still have the issue? We want to know if the problem lives in the power amp or the preamp.

Could JFET be involved in boating? WHo knows? It certainly can cause the failing level issue, so test it. It either cures the motorboating at the same time or it doesn't. You can have more than one problem at a time, but we work on them one at a time.

You have ribbon cables connecting the boards together. Pull each one most of the way off and push it back down. This refreshes the contacts.

The input tube is microphonic on many many amps.

Thanks, Enzo! Duly noted.

I'll try what you've suggested, and report back.

Fred G.

[Fred G.]

Welp, it turns out that the FX return jack was the problem, I cleaned it with Deoxit and all the issues cleared right up.