Jmp-1 Fault

Discussion in 'The Workbench' started by adb86, Feb 18, 2017.

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  1. adb86

    adb86 New Member

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    Hi everyone,

    I'm new to this forum, but I'm hoping someone out there may have some ideas I can try to help me figure out what's wrong with my JMP-1.

    I bought it about 12 years ago off eBay and it worked fine for a while, but it suddenly started playing up. It's hard to describe what it does, but I'll give it ago.

    It switches on fine and everything powers up ok, visually at least. All the buttons work and the display changes as it should.

    The problem occurs when I plug it in and play though it.
    I haven't got a power amp as of yet, I use it with a practice amp. I intended to get one but as it stopped working it seemed pointless.

    The two clean channels have absolutely no sound coming out of them. The OD channels however have sound, but it sounds as if the amp is in another room behind a door, is the best description I can think of. There is little volume control and it will peak and then drop. At the loudest volume, it's barely audible. Have to turn up the practice amp volume at least three quarters full to get any volume.

    As I said, it's hard to explain in words what's going on with it. I've had a look at the board for any obvious signs of damage, but I can't identify anything. The transformer does however get very hot and start to smell.

    Any advice at this point would be greatly appreciated.

    Cheers
     
  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    :welcome:to the forum...

    1st thing to try is to replace the tubes...

    :cheers:
     
  3. adb86

    adb86 New Member

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    Forgot to mention that I've tried it. Of course it's possible the new valves were faulty when I bough them. But thanks though.
     
  4. johan.b

    johan.b Well-Known Member

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    Do the filaments in the tubes glow softly when is powered up? If not it could be an internal fuse issiue
    j
     
  5. adb86

    adb86 New Member

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    Yes they do. Far as I can tell they seem fine. But honestly I'm not sure. Internal fuse looks fine as well. Any advice on how to further check and of these invade I've missed something?
    I've got some knowledge of electronics, but I'm very limited.
    Thanks
     
  6. johan.b

    johan.b Well-Known Member

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    Are you comfortable working on a live circuit.?....i assume you are since you're here in the workshop. .
    If so. Check to see that you got high voltage to the tubes. If you don't. Power down and measure R86. Its a10 ohm flameproof resistor acting as a fuse for the high voltage..
    j
     

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