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

Discussion in 'Amps and Cabs [BG]' started by MistaMarko, Aug 1, 2009.

  1. MistaMarko


    Feb 3, 2006
    Hey guys,

    Ok, hear me out. I'm posting about a MESA Dual Rectifier guitar amp. No this is not a bass amp-related inquiry, but some of the most helpful troubleshooting advice and very-gear savvy people post here and am needing urgent help. Please no flaming or gui**** stuff.

    My band's studio time is booked for next Wednesday. Our guitarist's head has a few issues. When you turn the gain/volume knobs on ANY of the three channels, it crackles and makes very strange popping sounds and in some parts of the knob it completely cuts out. Keep in mind it does this for all three channels:

    We have:

    1) Taken it to a technician who cleaned the pots (I'm not sure how thoroughly though...)

    2) Talked to MESA tech support. They suggested the pre-amp tubes were bad, so our guitarist ordered top of the line tubes and replaced them all. This is what MESA suggested to do, so we did.

    Neither of these solutions worked. If you strum a chord (leave it ringing) while slowly turning the gain OR volume, it sounds horrible. Crackling, buzzing, popping -- however, there are certain spots that will'll be like CRACKLEEEEEEEE *perfect tone* BUZZZZZZZZZ *sounds ok* POPPPPPCRACKLE *perfect tone* (<--- will do this will turning slowly. Don't laugh at my sound effects...)

    We are planning to take all the knobs out and the tubes and spraying electronics cleaner ALL in it. What would you guys speculate the issue to be, and what would you recommend trying now?

    Again, Im just looking for some help, as I know a lot of you are very gear-savvy. Please look past the guitar amp thing if it really bothers you that much. Thanks guys!

  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    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)
  3. Doodaddy


    Jan 11, 2005
    West Monroe, LA
    Have you talked to Mesa since the tubes were replaced?
  4. MistaMarko


    Feb 3, 2006
    Hmm. Well, if it is the first problem you mentioned, how would I go about resolving that issue?

    I do believe it was all three. The volume knob does it as'll be louder at 8:00 than it is at 10:00...then you'll go to 11:30 and the volume level will peak for about a millimeter of a turn on the knob, then at 12:00 will be about as loud as 10:00. Same for the gain and stuff.

    Basically, the five pre-amp tubes (12AX7's) all plug into the front of the chassis...all going to a board. Basically really all I can give you from memory.

    The Dual Rectifier Manual is here:

    I looked through it and don't see any schematic diagrams, but it might help if you (or anyone else) have/has time to look through it.

    -- This is such an odd issue...extremely frustrating too, haha.


    Not just yet, but we do plan too. They are closed on Sunday obviously, and we were going to spend the day toying with it, and were just looking to maybe get a few insights/pointers on some things to try, or some things to test out maybe to rule out a few problems...trial and error stuff, so that WHEN we call Mesa the day after, we'll be able to explain to problem more specifically, ideally.
  5. Selta


    Feb 6, 2002
    Pacific Northwet
  6. MistaMarko


    Feb 3, 2006
    Well, I'd unplug obviously :p
  7. Selta


    Feb 6, 2002
    Pacific Northwet
    Even unplugging isn't safe - at ALL.
  8. 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).



    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,


  9. BassmanPaul

    BassmanPaul Inactive

    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.

  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    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.
  11. MistaMarko


    Feb 3, 2006
    Well, again, we bought completely new tubes and put them all in, and it didn't fix the problem. We will call MESA tomorrow again and see. I'll post what they say.
  12. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    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
  13. MistaMarko


    Feb 3, 2006
    Thank God for electronics savvy people like you! I really appreciate all the help. I'll relay this to our guitarist to he can tell MESA and the tech, and I'll see what they both say. Thanks again...I'll post again soon with results I guess.
  14. MistaMarko


    Feb 3, 2006

    Haven't called MESA yet; doing this tomorrow.

    We messed with the amp a little more. Discovered that both the gain knobs and the volume knobs (excluding the gain knob for channel 3) for all three channels completely cut out (dead spot) at exactly 3:00 (same exact spot for each knob), and then resume getting louder when you move them past that point.

    Like I said, the ONLY knob that doesn't do this is the gain knob for Channel #3. Not sure if this helps any, but this is as much as I could narrow it down to be specific in the problem.

    5/6 knobs do have the same exact problem.