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Markbass Dirty Pot??

Discussion in 'Amps [BG]' started by Joe Z, Jan 4, 2013.

  1. Joe Z

    Joe Z

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    Greetings:

    I had an unusual event at a gig a few weeks ago. I have a Markbass lmt 800 and an 8x10 Ampeg cabinet. I set up, powered up the amp and noticed I was not getting any gain. As I operated the volume knob, the volume would cut in and out AS IF I had a dirty potentiometer on the volume knob.

    Reason I say "as if" is because two things occurred that I think are uncharacteristic of a merely dirty pot.

    1. even with the volume knob all the way OFF or very low, I would sometimes get significant volume cutting in and out. With a dirty pot I've always seen it cut IN and OUT at the volume level the knob was hitting at that moment - not higher or lower.

    2. when the problem cleared, after maybe 2 minutes, it stopped completely, never to return that night. Each of the two gigs since then have had the same result. It appears to go away when things heat up.

    I never use the pullout mute, so I figured if it was a dirty pot I'd just articulate everything - volume up and down and mute in and out, in an effort to clear it.

    Still, for the first two minutes or so I continue to have the problem and then it goes away COMPLETELY for the rest of the night. I would have thought a dirty pot would have a more gradual change in it's character.

    The first time it happened I was sufficiently freaked out that I started bringing a spare amp with me.

    Anyone ever heard of this? So far, it doesn't seem to pose a clear and present danger of not having a functioning amp, so unless someone suggests otherwise I'll wait and see.

    Thanks for any feedback.

    Joe
  2. BassmanPaul

    BassmanPaul Gold Supporting Member

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    sounds like you have a failing coupling cap on your hands. The thing leaks for a while until the dielectric reforms partially and the leak lessens.
  3. Bassamatic

    Bassamatic keepin' the beat since the 60's Supporting Member

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    Yes. You are right that it is probably not the pot. In addition, the pots are not old enough in ANY Markbass to be dirty. It could be a cap or some other type of intermittent connection. It is probably not the Mute, as that would tend to fail OPEN and not mute.

    When it cuts out - does a whack on the side affect it? If it doesn't, it is probably a component failing.

    Unfortunately, trouble shooting intermittent parts is a huge PITA, but sometimes you can encourage the parts to fail by shooting them with heat or cold spray.
  4. Codger

    Codger Bradley Baker Supporting Member

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    My LM2 seems to have a dirty input gain pot (or the same problem as OP). I rarely have moved it since I have had it though. It seemed to clear up when I moved it several times through its range. Deoxit is on order.

    OTOH, I wouldn't have noticed it lately because I have been going in to the effect return from a preamp.
  5. Joe Z

    Joe Z

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    Interesting .... I haven't tried hitting or maybe tapping it hard yet. I'll try that tonite.

    If its a capacitor, is it doomed to fail completely? Does the fact that it seems to resolve after 2 minutes tend to point to a cap more than anything else?

    Thnx for the help!!
  6. figuredbass

    figuredbass

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    I have had this same symptom with several amps over the years. The problem may be that one (or more) of the solder joints connecting the volume pot to the printed circuit board is bad and the connection is now loose. As you turn the pot an intermittent contact is made/broken, accounting for the symptom you heard. You're right about heat - when the amp heats up internally the joint expands just enough to provide a temporary contact, and you get no symptoms throughout the course of the gig. (This is exactly the same thing that happens in a thermostat.) When the amp cools back down after being turned off, it contracts again and you're back to the intermittent connection. My guess is that the problem is right at the solder joint(s) of the volume control, but it is also possible that the bad joint is actually with another component or pot on the board, and the physical action of turning the pot stresses the board enough to make that component's connection intermittent while it is still cold, before the amp heats up.

    I think it's pretty unlikely that the problem is an actual component failure since components don't tend to toggle back and forth between failure and self-repair - usually when they fail it's for good.
  7. Joe Z

    Joe Z

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    That makes perfect sense and is consistent with what happened last nite. I kept the head in the house (warmer) during the day and kept it on the floor of the car (near floor heat) on the way to the gig. By the time I fired it up it had certainly warmed to room temp. The issue did not present itself at all.

    I guess I'll just wait and see?! I'm still gonna bring my backup regularly, however.

    Thanks again, guys for your help ....

    Best regards, Joe

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