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JCM800 2205 effects loop

Discussion in 'The Workbench' started by StraightToTheBar, Oct 13, 2021 at 2:02 PM.

  1. StraightToTheBar

    StraightToTheBar Active Member

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    Another attempt at troubleshooting the effects loop of a 2205 ('89).

    It's very noisy, as in unsuable, no matter which effects (low/high impedance, rack/stomp, with/without linelevel convertor/DI, ...).

    I ran some tests and measured some things (and compared them with an fx loop of an 8100):

    Tests:

    • Guitar -> effects loop return jcm = extremely loud, no noise
    • Guitar -> Stompbox/rackeffects -> effects loop return jcm = extremely loud, little to no noise
    • Normal use of loop (effects between send and return) = MUCH noise
    • Effects loop send jcm -> effects loop return 8100 = MUCH noise and very low soundsignal even with everything on max.

    Measurements and comparisment:

    8100 SEND lvls:​
    • (level button on 0dB) U= 0.47V R= 10 KOhm
    • (level button on-20dB) U= 0.1V R= 1 KOhm
    JCM 800 SEND lvls:​
    • (Master volume on 1) U= 0.00V R= 10 KOhm
    • (Master volume on MAX) U= 0.004V
    So the 8100 sends a much hotter signal than the jcm800 (close to nothing).
    The 800 loop is tube buffered (I believe) and is located after the MV and before the PI. Aside from the fact that this bad design can introduce alot of unwanted hiss, I think there is more going on (those voltage readings seem way too low).

    Schematic:
    https://www.thetubestore.com/lib/th...arshall-JCM800-Splitch-50W-2205-Schematic.pdf
     
    Last edited: Oct 13, 2021 at 2:24 PM
  2. pedecamp

    pedecamp Well-Known Member

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    Have you scrubbed out your amp inputs and pedal inputs with Deoxit electronic cleaner? Are your cables in good shape? I had a noisy loop and after cleaning the inputs well it got rid of the noise.
     
  3. Pete Farrington

    Pete Farrington Well-Known Member

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    ‘Fraid not, sorry.
    The fx loop is a bit of a crap design, unfortunately.
    There’s no obvious fix for it that spring to mind.
    I’m somewhat incredulous that max send level is only 4mV though, something is winky there.
     
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  4. StraightToTheBar

    StraightToTheBar Active Member

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    I did, though I used another brand of cleaner. Cables are fairly new and tested.
     
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  5. StraightToTheBar

    StraightToTheBar Active Member

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    At first I tought it was because I didn't insert anything in the return jack (which I believe is needed to get a send signal, see schematic) but even with a dead lead insterted in the return, it remained 4mV (strumming very hard), which is indeed too low.
     
  6. Pete Farrington

    Pete Farrington Well-Known Member

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    The gain etc needs to be cranked, as well as the master vol.
     
  7. StraightToTheBar

    StraightToTheBar Active Member

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    MV & gain were on max, channel volume wasn't (3).
     
  8. johan.b

    johan.b Well-Known Member

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    Quoting myself from another thread on this exact problem from a few months back. The o.p in that thread never reported back so we don't know but o.p going silent usually means problem solved...?...

     
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  9. StraightToTheBar

    StraightToTheBar Active Member

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    I tried effects with different impedances (60KOhms rack effects to 1MOhms stompboxes, modern TC electronics to vintage Marshall fx).

    EDIT: you prolly meant a zero loss Metropoulos loop?
     
  10. johan.b

    johan.b Well-Known Member

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    Yes, replace the loop jacks with a metro zero loss loop or a tubetown loop if your in Europe and on a budget(a metro loop copy, basically) then you can set the levels to your liking and know it'll work with almost anything.
    35 years ago loops were still new and marshall didn't get this one right. It works for some people but not for all. i know John Norum use the loop on his 2205, but he runs the master on full, pretty much like a moded plexi, so the level will be fine at that point
     
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  11. StraightToTheBar

    StraightToTheBar Active Member

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    30 yrs ago they seem to have done it right (8100 Valvestate). 22€50 on the other hand doesn't seem too bad for an effects loop that is actually usuable. Will look into that tube town solution.
     
  12. hako

    hako New Member

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    The Tube Town loop is great, used it many times! Another attempt is moving the MV after the loop.... I unsoldered the MV pot, replaced it with a (1M) resistor and placed the pot at the "Y" point in the schematic (adding a .022 cap and the pot there).
     
  13. XTRXTR

    XTRXTR Well-Known Member

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    Try bypassing the reverb circuit with a temp jumper from MV to the send.
    I think its weird the way that reverb is mixed
     
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  14. South Park

    South Park Well-Known Member

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    You don’t have a tube buffed FX loop . Check r 29 the signal needs a ground
     
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  15. StraightToTheBar

    StraightToTheBar Active Member

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    Isn't this the same as maxing out the MV on the amp and using the volumecontrol of a stompbox/effectunit as MV? Cause I tried that, and noise is unbearable.
    Also, where's the Y point? :D

    I think right now I'm most concerned about the very low voltage reading. 0.4mV with MV on max sounds more like component bleed than an actual signal..
    I see you have a 2205 in your profile pic, unless it has the tubetown loop mind measuring the voltage on the SEND signal of yours? Cheers.
     
    Last edited: Oct 14, 2021 at 3:47 AM
  16. StraightToTheBar

    StraightToTheBar Active Member

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    Yes!
    You must be on to something. Going straight from R22 (easiest point to hook up) to SEND, so bypassing the reverb circuit, is giving me a much better signal: 0.3V

    All effects work much better with this jumper lead.

    Any idea of where I should look in the reverb circuit to find the problem?
    Could it be C24?

    Jumper lead:
    20211014_124702.jpg
     
    Last edited: Oct 14, 2021 at 6:51 AM
  17. Deftone

    Deftone Well-Known Member

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    FWIW: I have an '89 2205 and my FX loop is very noisy as well.
     
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  18. StraightToTheBar

    StraightToTheBar Active Member

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    Would you mind doing a voltage reading on the send jack out?
     
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  19. South Park

    South Park Well-Known Member

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    Change out the reverb pot . Good work on that
     
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  20. StraightToTheBar

    StraightToTheBar Active Member

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    Swapped C24 for a new one = no changes, signal way too low.
    Jumpering/shorting C24 = perfect signal.

    I don't get it. Need you on this one @Pete Farrington .
     

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