Amp Techs: Gain channel volume jumping up and down! Help!

[jman7272]

Hi folks,

My Mesa DC-3 just arrived fia FedEx. When it arrived, the carrier plate that covers the 12ax7 tubes was dangling and 3 of the 12ax7 tubes were smashed.

I opened it up and removed the tube nubs, cleaned out all of the glass and retubed with brand new Mesa 12ax7 tubes.

I fired it up and it sounds great except on the gain channel the volume will go slightly higher at times and then get slightly muted.

I checked for hot looking tubes and all of that and everything else seems to be functioning properly.

Any ideas as to what this can be? Any help is appreciated.

 

[disassembled]

Maybe one of the preamp tubes is bad.

Greg

 

[Boogiebabies]

DC's are prone to pot damage, especially when shipped. If they get hit, the four tabs that hold the back on get pushed out and the wiper looses contact with the resistor element. Wiggle the gain and volume pots to see if they are really loose, or make the problem better or worse. Try the rest of them as well.

 

[jman7272]

Interestingly enough, I didn't notice that the effects loop mix was set to 100% with nothing plugged in.

When i set it to 0%, the problym seems to have instantly gone away.

If I turn the mix back up from 0% with nothing plugged in, it comes back.

I would think with nothing plugged in that it would be irrelevant what the mix was turned to, no?

Do I just not get effects loops?

 

[Fxrs]

Hmm...,deja vu all over again!

 

[Boogiebabies]

Interestingly enough, I didn't notice that the effects loop mix was set to 100% with nothing plugged in.

When i set it to 0%, the problym seems to have instantly gone away.

If I turn the mix back up from 0% with nothing plugged in, it comes back.

I would think with nothing plugged in that it would be irrelevant what the mix was turned to, no?

Do I just not get effects loops?

No, it's similar to the MK series loops. but when there is dirt, crud or somthing causing a dead short your entire preamp signal wil go dead.

 

[disassembled]

Try plugging a patch cord into the send and return jacks and then turn the mix to 100%. If it works fine you probably have a bit of dust or dirt in the return jack.

Greg