Marshall JCM2000 DSL Channel Switching Not Working

Discussion in 'The Workbench' started by Nicky Fullstack, Mar 26, 2020 at 6:56 PM.

  1. Nicky Fullstack

    Nicky Fullstack New Member

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    Hey yall, I’ve recently acquired a Marshall JCM 2000 DSL that needed some work. The issue was the channel switching did not work and it was stuck on the clean channel. I opened up the amp to discover it had the classic biasing issue. C46 and R76 were totally fried. I ordered the stable bias mod from dr.Tube, a new set of Tesla power and preamp tubes and replaced c46 and r76. Fired the amp up to see where we were at and now the amp is stuck on the Ultra gain channel. The red led lights up faintly and goes completely dim when I attempt to switch to clean channel. The green led does not light up and the signal continues to be controlled through the ultra gain channel. I’ve replaced RL1, the channel footswitching jack and confirmed the tubes installed in the amp are working. All my work was clean with the install of the mod. I’m pretty stumped at this point looking for a needle in the haystack. Any help is greatly appreciated!
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Did you verify that the low voltage power supplies are working?
     
  3. Nicky Fullstack

    Nicky Fullstack New Member

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    Low voltage supplies are working from the PT and I can trace them on the face pcb (JCM2-61-00) up to r31 with 12v. After that (going towards the input jack) the voltage drops to about 2.1v measured from Lk9 which connects to the LED. The LED is very dim and lights up slow when the channel is selected.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Do you have the schematic?
    Are any of the ICs getting hot?
     
  5. Nicky Fullstack

    Nicky Fullstack New Member

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  6. Nicky Fullstack

    Nicky Fullstack New Member

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    Bump, still trying to figure this out
     

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