JCM 2000 DSL 100 effects loop treble increase

Discussion in 'The Workbench' started by StraightToTheBar, Jun 6, 2020.

  1. StraightToTheBar

    StraightToTheBar New Member

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    Hi guys,

    I'm having some trouble with the fxloop of my dsl 100 (made in uk 1998). No matter what I plug into the loop, even just a cable to bridge it, it results in a noticeable loud high frequency trebly increase in sound. In a bad way, it's not something I can eq out with the on board tone controls.

    I must admit I use the amp at bedroom volumes for recording and when I occasionally turn the volume up, the issue is less noticeable (but still there).

    I've found some info on here that describes the same issue back from 2018:
    JCM 2000 DSL 50 Loop Issue (Treble Increase)
    But I didn't want to resurrect that thread.

    In that thread they refer to this website: http://gsharp.co.uk where they even offer a mod for it, however the guy doesn't produce them anymore and, perhaps more importantly, not many seem to have the issue in the first place. So perhaps the problem lays elsewhere?

    Cheers
     
    Last edited by a moderator: Jun 6, 2020
  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    fixed your reference links...

    :welcome:to the forum...

    :cheers:
     
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  3. RickyLee

    RickyLee Well-Known Member VIP Member

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    I installed Graham's loop mod kit in my DSL many years ago, back when I was quite green to the electronics subject. I remember him saying he came of with the fix to cure some bleedover or cross talk type interference in that loop area on the PCB.

    There is a chip on there, IC2. Look to see if it is socketed in there. If so, you can try an easy swap to see if that cures your problem. There is also a couple caps in there, but I do not see them having the issues you are describing.
     
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  4. StraightToTheBar

    StraightToTheBar New Member

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    Thanks, looks like it's indeed a bleedover/cross talk problem.

    I unmounted the fx loop pcb leaving the connectors in place, so I'm able to move the pcb further away from the tube sockets. The problem disappears. I'm going to try to make some kind of (copper) shielding plate to put in between the main pcb and the fx loop pcb, perhaps that'll do.

    Not sure if I should mention here myself for others who find this thread in the future that these kind of repairs involve opening the amp, giving access to electronic parts carrying lethal voltages.
     
    Last edited: Jun 7, 2020
  5. South Park

    South Park Well-Known Member

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    Does the signal wires have a shielded ground you may have a grounding problem
     
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  6. ampmadscientist

    ampmadscientist Well-Known Member

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    What's been said many times, the schematic is not the whole circuit.
    Welcome Straight THB, :cheers:.
     
  7. dslman

    dslman Well-Known Member

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    From what I understand, the 2K DSL heads bleed off the caps within 20 seconds of shutting the amp down, unlike other amps.
     
  8. StraightToTheBar

    StraightToTheBar New Member

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    I spoke too fast it seems.. After putting the amp back together the treble increase issue reoccurred.

    I'm just using normal, cheap instrument cables, I guess those aren't shielded properly?
    How could I find out if there's a grounding problem? Earthing an effect pedal in the loop?

    Thx! Not sure what you mean by that though :lol:
     
  9. StraightToTheBar

    StraightToTheBar New Member

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    So I'd like to try a new IC2 chip, see if that makes things better. Would anyone have any idea exactly what opamp I would have to order? Thx
     
  10. StraightToTheBar

    StraightToTheBar New Member

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    Follow up: Installed a new IC2 chip, caps c25 and c26, but things didn't get any better. Another thing I've noticed is that there is sound even with volume on 0. Much more quiet when nothing is in the fx loop, but still sound nonetheless. Is this a case of a cap going bad? Dc-voltage in the circuit? And could it be related to the loop problem? Ah I'm starting to give up on this thing. So sad, cause it sounded so great..
     

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