Peavey Transtube Bandit (1994) Intermittent Sound, Nasty Distortion

[fhtiek]

So I've got the first issue transtube bandit I use as my gigging amp. I recently moved and apparently the amp didn't survive the move.

When the amp is buttoned up and running it will only work if I run directly into the power amp input on the back. This is just after the FX loop and the loop buffers in the circuit.

I opened it up today, touched up some shoddy-looking solder joints and got the amp working for the most part. Running direct into the FX return worked, and so did the gain channel from the common input. The clean channel sounded like it had a bias issue. It's gated silent until I hit it hard, at which point it stutters open and clips like mad.

I've been referencing this service manual: Supreme Bandit transtube schematic.pdf

My VB+ and reference voltages all measure at or very close to the mark, with the exception of the +3.5V at CR4/Q1's base which measures 2.9V.

So I buttoned it back up after awhile and noticed that all the progress I had made disappeared. The amp is back to not working unless the guitar runs straight into the power amp. I'm stumped. Any suggestions?

[52 Bill]

Welcome to the place.

My VB+ and reference voltages all measure at or very close to the mark, with the exception of the +3.5V at CR4/Q1's base which measures 2.9V.

I wouldn't think that this is a concern at this point.

You've done a good job of localizing the problem to the preamp. Looking at the schematic, there are so many places for the signal to get cut off or squashed, that you need to try and localize it some more.

I think I saw 2 jfets that are used to mute the signal at different points and the usual fx loop jacks, but the fx loop also has a relay that is in the signal path here.

The fact that the problem comes and goes, leads the first thought to be a bad connection or solder joint. I know that you said that you had resoldered a few cold looking connections, but have you tried poking and prodding components to see if there is anything loose? Have you cleaned and checked all of the plug on connectors for loose contacts or unsoldered pins?

[fhtiek]

I wouldn't think that this is a concern at this point.

I just figured I would check, since the clean channel was gating and stuttering like a fuzz face with a dead battery. It does seem redundant at this point though.

Continuity seems to check out on most of the joints. I'm also getting continuity from the the signal/ground solder pads on the jacks to the tip and sleeve of a plugged-in cable. The switched connections on the jacks also seem to operate as they should, shorting together when the cable is removed.

The only off-board connections are to power, the spring tank, and the speaker which all work fine running into the power amp. Continuity checks out here as well.

I'll keep poking at joints, I certainly haven't been able to check each one individually yet. That clamp JFET and the relays are good suggestions too. I'm just hoping that removing and replacing the board isn't breaking the joints I've just fixed.

Thanks for your time!