MP2 Instant Volume Increase

[Hanneman213]

This problem repeated. I dont think that its a tube related problem. I changed both with 12ax7 Tung sol's in 04/2018 and I did not play too much since that date. I replaced them each other. But The problem continues.

When I am playing, the sound Instantly Increases like you turn up the volume or you press a tube screamer. Then it turns back normal levels and gain.

What the problem can be ?

[vansinn]

Uh oh..  a very careful guess is one the vactrols controlling how various levels gets defined has gone bad.
No doubt MJMP will chime in on this.

[MarshallJMP]

Does this happen on all voicings?

[Hanneman213]

Good question. It did not come to my mind. I will check it at home. I use Warm Hi gain mostly. I will check it also in cleans.

[Hanneman213]

I spent sometime to get problem again. Because it has an irregular timing. The problem is happening on crunch voicings. Sounds increases and decreases with a wavy motion.
And one more thing. The display screen can not be visible with a straight front angle. I can see it with like 45 degree angle from a side.

[rnolan]

The display screen is an easy fix, it uses the same one as Quadverb and a few other gadgets, even comes in various colours, just be careful around the ribbon cable.  There's some posts about it from a while ago.In recent discussions regarding using the loop returns for other preamps to pick up the cab sims (and I thought the tremolo as well as that's what the signal flow diagram in the manual indicates) MJMP said the volume vactrol was also used for/by the tremolo, maybe an/the issue ?  Just a guess based on it being a wavy motion.

[Dante]

I have been battling this for a while now. I keep thinking it's a bad jack or cord, or whatever. Now, I'm convinced it's not.

Honestly, it's one of the reasons I don't use my MP2 live anymore. That, and the fact that I've beaten the crap out of it so much that it's now delicate.

Let's see if this gets worked out...I'm much better with a soldering iron now.

[vansinn]

Generally, I'm thinking:
If signal goes up'n'down non-rhythmically, it's likely related to bad connections, soldering..
If signal does it rhythmically, it's likely caused by an oscillating circuitry, involving a capacitor.
If [dialed-in]distortion/gain remains constant, it likely isn't in the tube gain-staging section.

I have a problem seeing this problem located with the main volume vactrol; even though this one also does the tremolo effect, this is controlled by an algorithm in the software and I cannot see how such an code snippet can become unstable.

Over time, we've seen several issues with the compressor circuitry.
When this signal gain wobbling problems is present, is the compressor set active?

Another potential: When touching the various inst/line-level switches on the back panel, does gain stay constant, or does is result in flickering gain + scratching noises?

[MarshallJMP]

If it's the vactrol it should happen on all voicings. To be clear you hear volume changes not gain changes?

As for the lcd, try to connect disconnect the grey flat cable a few times on both ends.

[Hanneman213]

Volume increases and my classic tight ''metallica sound''  becomes like a Marshall MG 10

[Kim]

I have what appears to be the same problem with both my MP-2s.
In my case, it seems that the Loop jacks are the culprit, as wiggling around/plugging/unplugging seems to "fix" the issue.  I may need to source new jacks if a good contact cleaning doesn't help.

Btw, it's exactly what you described; suddenly the volume jumps tremendously with a complete wash of ugly harsh fuzz and mess....

[MarshallJMP]

Kim, this could also be the loop gain switches.

Hanneman, so the sound also changes?

[vansinn]

One of my MP2's has known bad gain switches (the push-type ones on the backside).
While it doesn't sporadically change gains, it nevertheless doesn't have constant gain.
Massaging those switches cures the problem - for a while..

Corrosion in switches, jacks and potmeters can manifest in weird ways ,ile gain changes.
A corroded contact point can behave as both a resistance and a diode stretch, causing gain drops, noise and distortion.

Clean and/or replace - remember, we're dealing with fairly old electronics that does need maintenance.

[rnolan]

I've had issues in the past with my studio MP2 where I lost a channel (Cab sim outs), the culprit seemed to be one of the push switches, seems they need a clean from time to time.

[Hanneman213]

I changed the patch cable ( which I bought new) with my another old one and as yet, I hear no problem. I think, I'm gonna wind around the seller with it.