Mesa DC5 Problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Staff, Dec 29, 2005.


  1. Staff

    Staff Member

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    May 28, 2005
    Location:
    Dallas
    My DC5 has a problem when first powered up. Start the amp with Standby "on" for 30 secs, switch "off" the standby, and the "lead" channel works fine, but no "rhythm" channel. After playing the guitar for appoximately 10 minutes, bang, the "rhythm" channel comes back on and works fine the rest of the night. Turn it off for an hour and you have to go through the 10 minute wait again before the rhythm channel wants to come in. I have opened up the amp and there is no obvious "trauma" or anything that looks burnt/bad etc. I've poked around with a chop stick and no snaps, crackles or pops. I swapped out every tube in the thing with no change. And it's no a gradual "come back" it's all at once, bang it works again. I'm at a loss. Any ideas, suggestions, advice appreciated. Thanks
     
  2. John Phillips

    John Phillips Member

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    Location:
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    I'd take a fair bet on a sticking or faulty channel-switch relay.

    Mine did something similar, although less dramatic - the rhythm channel would crackle and change in tone when the amp was first turned on; switching back to the lead channel then to the rhythm again would usually cure it immediately for the rest of the night and I didn't bother doing anything about it, but it got progressively worse and eventually started crackling with no provocation so I had to fix it. It was the relay that switches the presence circuit; yours sounds like it will be the main channel relay (RV3b).

    If it is this, the bad news is that the relays are one of the only components that can't be changed from the top side of the board - they're larger than their pin connection footprint, which means you have to lift the board. The best way to do this is to remove the preamp tubes and undo all the pots and jacks on the front panel, then you can lift the board just far enough to prop it up and work on the underside - it's still not easy though.

    FWIW, this is the only problem I've ever had with mine, and these amps are very reliable in general. If you need a diagram for it, I can send you one - although I think the component numbers are printed on the board and it should be fairly easy to find.
     
  3. Slick51

    Slick51 Colonel Curmudgeon

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    Location:
    Northwest Corner of NC, 5 mi from MerleFest
    +1 on the channel-switching relay. I had the same problem with a Carvin Belair. This one wouldn't switch, and when it did it often wouldn't stay switched. Got the replacement relay from Carvin for a buck or two, pulled the board, and repaired it in an hour or so.

    The DC-5s I've owned (and Mesas in general) were very reliable. But parts are parts, and inevitably something goes. This one is a fairly straight forward fix, though.

    Slick51
     
  4. Staff

    Staff Member

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    48
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    May 28, 2005
    Location:
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    I must say I'm very impressed, but not surprised. I've been lurking around this board for 6 months and never made a post. Now I have a problem, tried a post, and have an answer (and backup vote) in just a few hours. You guys are the best, and this is something I've noticed for the last 6 months. This sounds like my problem with the amp to a "T". I first started noticing it occasionally with the footswitch and thought the pedal was going out. It would change the light on the amp, but not the channel, and since the switch on the amp worked, I didn't worry about it. Last gig, the amp switch stopped working too (until I had waited the required 10 minutes or so). Is there any way to check this by testing equipment etc., or do I just try to order another one from Mesa and try it out? Thanks again for the replies so far, I really think you nailed it.
     
  5. John Phillips

    John Phillips Member

    Messages:
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    Joined:
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    Location:
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    Hmmm... if the footswitch didn't work but the panel switch did, that's quite puzzling as they do the same thing. The only difference is that the current to the footswitch goes via the LED (which drops the voltage slightly), so it's possible that just this very small voltage difference would be enough to prevent the relay unsticking. If the relay is getting progressively worse it's possible that for a while it would work with the panel switch but not the footswitch.

    Or, it could be a different problem related to the supply for the switching circuit - maybe a failing cap or something that's stopping the switching voltage building up enough.

    One thing you could do to try to identify it is to run the amp with the chassis out, get it to stick (no sound) and then tap on the relays with something like a small screwdriver handle and see if you can get it to come on. This was how I finally nailed the problem with mine... which had been difficult since it would generally run OK when it was in bits :).
     

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