Help: Volume jumps with 2:90

[elvis]

I have a blue light 2:90, and the volume on one of the channel jumps up intermittently. It gets quite a lot louder, regardless of the volume setting. If I toggle the amp into standby and back out, it goes back to where it was.

Anybody ever experience this sort of thing?

 

[TOOLGUY]

I had a very similar problem.
My amp would drop the A channel volume and I would loose all bass response (sounded tinny).
I went as far as probing with a meter and replacing a questionable ldr.
No luck.
I ended up buying a 50/50 cheap.
I still have the 290 and it works good for a mono amp.
Maybe someday I will get enough extra $$ to get it repaired.

One thing to check is the volume pots. They are known to spread apart after a bump and cause issues.
As alwayes...super high VOLTAGE!! Take care.

 

[Jackie]

Try replacing one of the inner 12AX7s with a known working one. Wouldn't be the first time a PI failure would cause volume issues.

 

[elvis]

GAH - why didn't I think of that? I replaced all 3 12AX7s when I got it, but one could still be bad.

Thanks!

 

[Jackie]

No problem, though I did re-read your original post and I noticed you said "it gets quite a bit louder".

Now I don't know about you but when I think "tube might be bad" I think "less of everything" so I'm not sure my advice applies :?

Anyway, if that turns out to be the case, then yippee.

 

[elvis]

Replaced all the preamp tubes, and swapped the power tubes between channels, and no change. I can play for 10-15 minutes, and then one channel (can't remember which, but it's the left channel when looking at the front of the unit) get significantly louder.

It is intermittent, but repeatable. It lasts from a few seconds to about a minute, then drops back to normal. Putting the amp in standby resets it to normal. It goes back to loud every few minutes.

I just got a RA from Mesa, so it looks like it's going home for a tune-up.

 

[elvis]

Sent my 2:90 to Mesa yesterday. Hopefully they can figure it out. 38lbs - yikes!

 

[elvis]

On its way back from Mesa. They couldn't reproduce it.

 

[ando]

On its way back from Mesa. They couldn't reproduce it.

Oh yeah, you gotta love the good old intermittent, unreproducible problem...

Hope you didn't lose too much money on a dead end.

 

[elvis]

Yeah. Though I bet I can reproduce it immediately when I get it back.

Mesa was very reasonable about it. Just a bench charge (I think $35) for cleaning the board and basic troubleshooting, plus shipping. They called me a couple times while checking it, and did really seem to try to help.

 

[TOOLGUY]

That stinks.
Maybe take a video of the problem when it happens?

Was hoping they would could solve the issue for ya.
My 2:90 is still mono-matic

 

[crane]

Are you sure it's not a preamp problem ?

 

[elvis]

Yes, I have tried it with two different preamps. I had no problem with my 50/50 in this setup, but had the problem immediately with the 2:90.

I got the 2:90 back today - will try it tonight if I have time.

 

[elvis]

Played it for almost an hour, and no volume jumps. Also, I used to have to turn one volume knob up a bit more than the other to get equal volume, and now they are pretty much exactly equal. Whatever happened (bumped during shipping, cleaning it, etc) seems to have fixed it.

 

[dregsman]

Check the line level switch on the back below the input jacks. Mine had the same problem so I had it bypassed because I never have a need to use the 2:90 alone without the preamp. When that switch goes bad you could see a +20db volume jump.

 

[elvis]

I actually wondered about that. When Mesa tested the amp, they switched it to INST and played a guitar thru it, then switched it to LINE and played an ipod thru it (at my request). It's possible the switch contacts had oxidized, and moving it back and forth helped.

 

[rjriley1]

Dust and dirt in the tube sockets can create problems. It is important to pull out the tubes periodically to clean tube sockets. Dust can be a conductor. Also, make sure the rest of the amp is dust free. A Mesa authorized repairman told me the tube sockets are a common problem. Just FYI. Your repair wasn't magic.

 

[elvis]

I prefer to think that it was magic.