MarkV 90w CH3 Presence issues

[Poltergeist059]

Hello, I've had issues with the Presence control on Channel 3 for some time now and it's just gotten worse. For the past year or so the presence pot has been a little scratchy, creating popping noises when turned. I was ok with this, I just didn't turn the pot very much. Well now in addition to being scratchy it completely cuts off all volume around the halfway mark. At first I thought it was just a matter of "excercising" the pot, which did help at first. Now it seems like the higher range of the presence pot of CH3 is inaccessible. Any suggestions on what it could be? Could it be a tube related issue?

 

[rarebitusa]

The member who knows this circuit well is @MarkV User there are many related Ch 3 presence posts in this thread WRT to the mods that were done. This perhaps may provide some insight.

The V 90W thread

That said it may just be a failing pot, unlikely it is a tube, but you could roll them to see if anything changes.

 

[MarkV User]

Wow, that is surprising. If the pot fails open or shorted you should still get sound out. So maybe it has shorted to ground.

Let's say you will replace the pot no matter what and that is likely all that is wrong. Get a new pot and put it in and see if the problem goes away (before putting the amp back together). Or if you want to then remove both wires and test the amp that way. With no feedback you should have a presence boost. Mesa will send you the pot and they are on the web site. The part number is on the back of the pot. Or if you make sure to match the shaft style and body diameter and max resistance then you can use a different one.

 

[MarkV User]

Wow, that is surprising. If the pot fails open or shorted you should still get sound out. So maybe it has shorted to ground.

Let's say you will replace the pot no matter what and that is likely all that is wrong. Get a new pot and put it in and see if the problem goes away (before putting the amp back together). Or if you want to then remove both wires and test the amp that way. With no feedback you should have a presence boost. Mesa will send you the pot and they are on the web site. The part number is on the back of the pot. Or if you make sure to match the shaft style and body diameter and max resistance then you can use a different one.

Is there any chance the pot has gotten loose, allowing the body to rotate inside the amp? I'm not sure how the pot shorts the signal otherwise.

Maybe the no sound out problem is something else, like the possibility of a tube issue that you mentioned. Things always seem more interesting when there is more than one independent problem.

 

[Poltergeist059]

Is there any chance the pot has gotten loose, allowing the body to rotate inside the amp? I'm not sure how the pot shorts the signal otherwise.

Maybe the no sound out problem is something else, like the possibility of a tube issue that you mentioned. Things always seem more interesting when there is more than one independent problem.

I guess I should clarify. The issue has actually gotten worse after "exercising" the pot again. Now there's only one spot on the presence control around halfway where I get full volume and any other position drastically reduces the volume. It's not completely silent, but significantly quieter. I do have a spare preamp tube, I'll try rolling with that first.

 

[MarkV User]

If the sound cut is responsive to the knob like that then it is hard for me to imagine this has to do with a preamp tube. Doesn't mean you shouldn't try it. Doing experiments and seeing what happens is the only way to really know what is happening. But also take the knob off and make sure the collar isn't loose. That would be a giveaway that the contacts on top of the pot can rotate and short the signal against the adjacent pot. Since the pot is in bad shape I would replace it anyway. 85% that is all you need to do.