Marshall DSL 401 red plating

Discussion in 'The Workbench' started by Holt2010, Nov 25, 2020.

  1. Holt2010

    Holt2010 New Member

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    So my DSL 401 has been giving me fits for a while now and i cant get it figured out. The amp has had the br changed and is raised off the board with a heat sink. When it died it was blowing F4 so i replaced it and the amp would work for 30 minutes or so then nothing. However the fuse was still good, so i took the board out and did some investigating for bad solder joints. I found several and touched them up and tried again. The same thing happened, it worked for about 30 minutes and then nothing. I checked voltages, touched up solders again and fired it up. I got all the tubes back but had very faint sound and the tubes were very hot almost instantly. The tube furthest from the power transformer started to red plate and smoke started coming from around the transformer area. Im almost ready to give up, can someone please help.
     
  2. StingRay85

    StingRay85 Well-Known Member

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    Do you have a bias tool at hand? Are the tubes drawing too much current?
     
  3. Holt2010

    Holt2010 New Member

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    I had my Meter hook up to the bias test points and the bias was holding until it started red light plating and had to shut it off although I only had it hooked to one test point and common didn’t have time to switch them
     
  4. StingRay85

    StingRay85 Well-Known Member

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    Drifting bias issue maybe?
     
  5. Holt2010

    Holt2010 New Member

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    Maybe but I’ll have to go back in and resolder whatever has a unsoldered itself before I can test again .
     
  6. South Park

    South Park Well-Known Member

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    The bias drift is caused by a conductive circuit board . I believe thar is a bias kit that fix’s this
     
  7. tmingle

    tmingle Well-Known Member

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    The master volume pot may have failed. The bias for these amps feeds thru the MV pot. It failed in mine. I bought a replacement for about $10 shipped.
     
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  8. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    IIRC, the DSL40 doesn't have the bias drift problem...

    Are you able to work on amp's?
     
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  9. StingRay85

    StingRay85 Well-Known Member

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    Bias feeding through mv pot? :scratch:

    Any good reason for that?
     
  10. tmingle

    tmingle Well-Known Member

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  11. tmingle

    tmingle Well-Known Member

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    The pot in question is VR12A & VR12B. The condition in mine was Red Plating on 2 of the 4 EL-84 tubes. VR12a & b is a dual trace pot. If 1 of the traces fails, no bias voltage is applied to the grids & there is no throttle on current flow. I sprayed mine 1st & it worked, but was not stable. I tried to find a link for the part I used but, it seems to be obsolete. The part is available though.
    https://www.etronic-parts.com/Poten...-2x-A200K-audio-log-PC-mount-14-mm::1224.html
     
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  12. Holt2010

    Holt2010 New Member

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    The master volume pot was replaced about 3 years ago.
     
  13. tmingle

    tmingle Well-Known Member

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    Are 2 tubes red plating or all 4? If only 2(1 inner & 1 outer I believe), switch them to see if the problem follows the tubes. Are they the original tubes?
     
  14. Holt2010

    Holt2010 New Member

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    It was just one tube V8 i think, and the tubes are not the original tubes.
     
  15. tmingle

    tmingle Well-Known Member

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    Switch V8 & V5 to see if it follows the tube. You can also remove V8 & V5, plug the speaker into the 8 ohm jack & play test it to see if the redplating continues. . Output will be 20W instead of 40W.
     

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