JCM2000 DSL 100w Head - Overheating plus vol loss

Discussion in 'Marshall Amps' started by Tim Stark, Jul 23, 2019.

  1. Tim Stark

    Tim Stark New Member

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    New to forum, jut had band practice with recently acquired JCM DSL, after 30 - 40 minutes use the volume dropped and the darn thing went nuclear Hot!
    I jnow this will be the 1000th time it's been asked but any knowledge or advice would be grateful, I'm supposed ti be gigging it on Saturday night!!
    I'm running a std 1960A at 16ohm, tc electronic Spark in the loop and Vox Wah, Polytune and Suhr Shiba Drive up front. Before over eating & volume drop it sounded gorgeous!
     
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  2. Kutt

    Kutt Well-Known Member

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    Search for "JCM 2000 Bias Drift" here and on the web. It's a known hot topic (no pun intended). I am no tech but it might be your issue.

    What year was the amp made? It should be printed as part of the S/N on the rear. The earlier the date of manufacture, the more prone it might be to the bias drift problem.
     
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  3. Tim Stark

    Tim Stark New Member

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    M-1998-11-1527-A
     
  4. Kutt

    Kutt Well-Known Member

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    OK. Like I said, I am no tech, but your amp is right smack in the cross hairs of the most prone years (1998 thru some point in 2001, and many say right into 2004 but it gets debatable the later you go).

    There are some real tech heads on here that will chime in soon I'm sure. To address your immediate concern you should probably find a backup amp for Saturday until this gets squared away.
     
  5. Kutt

    Kutt Well-Known Member

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    Also, you can go batshit reading everything on the web about the issue. You really just need to know that the earliest models were the most prone where one side, or pair, of tubes gets very very hot because the bias (voltage?) drifts due to defects on the main circuit board. It can eventually lead to component failure if you were to leave it powered up and running as such. There are at least a few debates on what exactly happens on the circuit board but what I wrote is a very brief summary of what you need to know. There are replacement OEM Marshall circuit boards available to buy online for around $230 that fixes the issue. There are other workarounds that you'll find when searching around too. All involve time and money.
     
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  6. Gunner64

    Gunner64 Well-Known Member

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    Oh boy..here we go again... Dr. Tube stable bias kit. About $80 give or take for the kit. Google it. I' d recomend changing the bias pots as well.

    You need to monitor your bias for drift to confirm if you have the issue.
     
  7. 67mike

    67mike Well-Known Member

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    Check bias
     
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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Are you able to use a volt meter / comfortable with taking readings inside the amp?

    Because if "no," you need a technician to work on it.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Oh dude,
    Welcome to the forum:cheers: Tim.
     
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  10. Tim Stark

    Tim Stark New Member

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    Yes I’m comfortable using a volt meter inside the amp
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Take out all the tubes.
    Turn the power on, but leave standby in "warm-up" position.
    Do not turn standby to: "operate position."
    Connect DC voltmeter
    Black lead to chassis ground.
    Red lead to pin 5 of output tube sockets.
    What is the DC voltage on pin 5 of all output sockets?
    Report all the readings.
     
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  12. Tim Stark

    Tim Stark New Member

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  13. Tim Stark

    Tim Stark New Member

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    31.7 31.9 31.8 31.9
     
  14. Tim Stark

    Tim Stark New Member

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    Sorry, I mean
    -31.7, -31.9, -31.8, -31.9
     
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  15. Tim Stark

    Tim Stark New Member

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    Replied
     
  16. Jon C

    Jon C Well-Known Member

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    AMS is PST ( UTC -8 )
     
  17. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    Welcome to the forum of Marshall
    Hope you get your amp together soon bro
    Cheers Mitch
     
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  18. Gunner64

    Gunner64 Well-Known Member

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    You need to let it run for a while while monitoring the bias..mine started to drift after about an hour..but it did drift. I put in new bias pots first thinking maybe it was just that, but the problem persisted. Installed the Dr. Tube kit, biased it to around 72 a side and it holds bias rock solid now. Sounds killer..the best one I have had out of 3 of them.
     
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  19. JCMDOUG

    JCMDOUG Well-Known Member

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    My 1998 did this Too Hot bias JJs.jpeg
     
  20. JCMDOUG

    JCMDOUG Well-Known Member

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    I did performed some mods to fix the problem for the most part it's fixed but I should replace the bias pots. I don't use the amp much so it has been at the bottom of a long list of things to do. If I had to do it again I would just replace the board.
     
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