Having Issues With My Jcm 900 Mk Iii Model 2100 Head

Discussion in 'The Workbench' started by Jared Strickland, May 10, 2019.

  1. Jared Strickland

    Jared Strickland New Member

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    I have a 1990 JCM 900 Model 2100. When i first turn the amp on sound is coming out, but after about 5-10 seconds the sound fades out until it’s completely inaudible. The tubes are glowing, and the head is on but there’s no sound. When i unplug the cable from my guitar and leave it in the amp, it doesn’t even make a sound. It sounds as if the cable isn’t plugged in at all.

    Any sort of help is appreciated!
     
  2. Jon C

    Jon C Well-Known Member

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    Caps !!
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Welcome to the Marshall Amp Forum, Jared. :cheers:

    Plug a guitar into "return." turn up the master volume. can you get sound?
    Plug a guitar cable between send and return, re-test the amp, any sound now?
    Buy 1 new 12AX7 and change the preamp tubes one at a time,
    retest each time you swap one out w/ the new one.
    If there is a bad tube you can find it.
    If Not: Low voltage power supply or bad IC chip in preamp. This should be worked on by somebody who is qualified to repair PC boards, and has test equipment..
    Soldering can damage the circuit board, don't use this amp to learn on.

    Tip: a bad IC in the preamp will usually get hot. This will also drag down the plus minus 15 volt power supplies.
    The 15 volt power supplies can also be lost from a bad zener diode....
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    could be the low voltage caps...probably right.
    The caps are rated 16 volt, in a 15 volt power supply. So they have a relatively short life expectancy.
    Should have been 35 volt caps for reliability sake. Too late to change that.
     

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