Marshall JCM-800 master volume/pre-amp issue

Discussion in 'Amps and Cabs' started by más_fuerte, Mar 19, 2015.

  1. más_fuerte

    más_fuerte Supporting Member

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    Hey guys,

    I'm having an issue with the master volume/pre-amp knobs on my 1986 Marshall JCM-800. Basically what's happening is, regardless of where the pre-amp knob is set, the volume coming out of the amp is deafeningly loud when the master volume knob is set all the way in the counter-clockwise position at "0". However, if I turn the master volume knob clockwise towards "1", it actually gets quieter and will function somewhat normally as the knob is turned up. Meaning, the volume from the amp gradually increases as the master volume knob increases, as long as the master volume knob is not set to "0".

    Not sure if that makes any sense... anyone know what could be going on here? There's a tech here in town that I can take it to, but I figured I'd ask around before I make the trip.

    Thanks!
    Matt
     
  2. broken_sound

    broken_sound Silver Supporting Member

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    I would say that the pot is busted and showing wide open at home, but the fact that the volume doesn't decrease with the preamp dialed back makes me wonder. With your pre-amp at 0 you should have no output.
     
  3. más_fuerte

    más_fuerte Supporting Member

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    I'll try it again and see what the output is when the pre-amp is at "0". It might actually be silent in that situation. But when the pre-amp is dialed in past 1, the master volume is wide open when set to "0".
     
  4. broken_sound

    broken_sound Silver Supporting Member

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    My guess, the pot was over rotated a few times and now its wide open in the zero position.
     
  5. demea26

    demea26 Member

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    This is most likely the case
     
  6. más_fuerte

    más_fuerte Supporting Member

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    Is this something I can fix on my own, or should I take it to a tech? I don't have much experience working with amps.
     
  7. demea26

    demea26 Member

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    I'd say if you can solder and somewhat familiar with electronics then this is definitely a do it yourself job. Me, I'd just take it to my amp tech
     
  8. más_fuerte

    más_fuerte Supporting Member

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    Great, thanks everyone!
     

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