Troubleshooting a MESA...dead spots in volume knob??

Generally, the issue of knobs causing noises etc is due to leakage of DC onto the pot. Then turning the pot can shift the bias of the following tube, and cause all manner of funny noises, plus the scratching typical of DC on the control.

With it on three channels, that makes it seem less obviously that sort of problem, but it still could be. Mind I do NOT know the innards of the dual rectifier, I have no schematic handy.... BUT if the gain controls all feed into the same point, that is if the pots are at the mix point, which is certainly possible, then DC leaking out of the input of the mix stage could do that.

With three doing the same thing, anything relating to just one of them is unlikely. It is much more likely to be something common to all three. Particularly since you don't mention any otehr controls being noisy.

If this is it, cleaner obviously is unlikely to make a difference, unless it cleans some sort of conductive goo off the innards, which is unlikely but not completely impossible.

just my 2 cents without seeing or knowing much of anything specific about it (but I HAVE worked on other Mesa products)

 

Have you talked to Mesa since the tubes were replaced?

 

Might help a bit.
http://www.schematicx.com/schematic/mesa-boogie-dual-rectifier-schematic/

Lots of M/B schematics there.

Note: Don't just go poking around in there unless you really know what you're doing. Tons of lethal voltages in there!

 

Even unplugging isn't safe - at ALL.

 

Hi.

+1 on Jerrold's post.

A few additional questions:

How did the problem start?

How old is the head? ROHS perhaps?


While cleaning potentiometers can work on some instances, not once in the 25 or so years I've done it, have I been able to resurrect a scratchy pot for any significant lenght of time. It does work for a while if the innards are still in good shape (which they usually aren't).

+100

Do bleed the PS caps too, othewise You might be in for the surprize of your life.

If You don't know how to do that,

PLEASE DO NOT OPEN THE AMP.

Regards
Sam

 

I agree fully with Jerrold's synopsis. While it could be a bad batch of pots it's unlikely that all three would go at the same time. Take it back to the tech and get him to actually fix the amp this time.

The user manuals are very seldom of use, it's the Service manual with the schematic that's needed. See if Mesa will send you out a copy.

Paul

 

Looking at the schematic as posted (which may be current, or not), it all goes into V2A......

AND, V2A has NO input capacitor. The grid just goes to the pots without even a resistor to ground other than the LDR units connecting to the pots.

OK, so as for the scratchy pots, there is a definite way for DC to get from the grid onto the 1 megohm pots.

A tube with excess leakage onto the grid is certainly not unknown. It could be as simple as a tube in position V2 that is not very good. Try swapping them around and see if any will behave OK.

I am assuming this amp did work OK in the past. Mesa has had some 'funny circuits" in their past that had issues. But I have seen tis done elsewhere with satisfactory results, even if I probably wouldn't do it like that.

BTW, the schematic in the link shows TWO channels, "red" and "orange"....... But if they added a third, it probably would be similar to the connections made for the two....... As is typical for mesa, they use all the tubes in all channels, with gain adjustments etc.

 

Capacitor C 21 on the posted schematic is another possible..... because of the design, it is used in all "channels". It's a "DC blocking capacitor", and if it is leaky, you will have noise from DC on the control.

The unit will also act strangely, because positive DC will get fed to the grid of the next tube. That will mess up its bias, and potentially cause distortion, temporary dropout of sound, etc..

As for the dead spots, I am not quite sure. Possibly it isn't a "permanent" dropout, but one related mostly to the fact that the control has (by design) areas where the output changes faster as you turn it.
If there is DC from a leaky capacitor, and you turn the control pot through one of those areas, you may have a temporary "DC clipping" that will stop the sound as various parts charge up to a new voltage. When they are done charging, it may come back, but that might take long enough that you hear it as a 'dropout" associated with that position.

The tech should be able to find it, if you give him/her a full description