JMP 2203 problem

Discussion in 'Marshall Amps' started by tonycaster, Jan 24, 2021.

  1. tonycaster

    tonycaster Active Member

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    Hi guys, I have a problem with my '78 2203, no sound is coming from the amp after checking all the connections, the power tubes filaments don't become red and the very small amount of hum, typical after powering up the unit, does not appear. Could this be the OT?!?
    I've been using the amp until this morning without problems and suddenly, this afternoon it stopped working
     
  2. Travis398

    Travis398 Well-Known Member

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    It won't work if the heaters aren't working, I would check and make sure the heater circuit is at 6 volts

    I can't imagine all tubes going at the same time....
     
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  3. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Check the fuses. No power anywhere in the unit leads to the mains fuse.
     
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  4. tonycaster

    tonycaster Active Member

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    Am I wrong or if it was a fuse no power would go through? The unit powers up, apparently, I mean the power switch lightens up...
     
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  5. Travis398

    Travis398 Well-Known Member

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    Pull power tubes and check AC volts from pin 2 and pin 7
     
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  6. Old Punker

    Old Punker Well-Known Member

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    IMO it sounds like a heater circuit problem. Same thing happened in my attenuator recently. My power switch still lit up too, but the power amp was not working. There may also be a defective power tube associated with it.
     
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  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The first and easiest thing to do is to check both fuses if you can confirm that the fuses are good you can start to check voltage levels inside. I doubt that your OT went bad and now the heaters don’t work.
     
  8. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    The pilot light runs off the heater string:

    [​IMG]

    If the heaters are not lighting up you will need to check the heater voltage on all the tube sockets.
     
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  9. tonycaster

    tonycaster Active Member

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    Ok guys, thanks for your responses, you were right with the fuses, the HT is blown. To be honest, yesterday I was playing with preamp tubes, so maybe I did something I shouldn't... I'll check tomorrow with a new fuse and get back, thanks for your help anyway!
     

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