jmp-1 rack mount preamp control knob issue

Discussion in 'The Workbench' started by cjr71244, Aug 4, 2014.

  1. cjr71244

    cjr71244 New Member

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    I just bought a jmp-1 and the knob that controls the digital channel selection doesn't work very well the numbers jump all over the place when turning. Is this easy to replace? Where do I get the part?? Thank
     
  2. blues_n_cues

    blues_n_cues Well-Known Member VIP Member

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    that was basically a factory defect from day 1.
    I'm not sure where to find a replacement part other than Voodoo Amps who also does replacements & mods.

    FAQ's - Marshall JMP-1

    Data Control Question - I would like to get the data knob (the component itself - not the plastic knob-cap) replaced – it turns rough and it skips numbers when turning so that it’s difficult to land on the patch you’re trying to select. Can this be fixed / replaced and how much would it cost?
    ◾In most cases the Data Control can be fixed / repaired.
    ◾We also routinely stock all proprietary Marshall parts for the JMP-1 preamps and can replace the control if need be.
    ◾Parts Cost - $49.95.
    ◾Labor Cost:
    1.) $15.00 - If we are Modding the unit at the same time as we will already have the unit disassembled.
    2.) $50.00 - As we have to disassemble the unit as well as reassemble.
    ◾Note - More times than not when the rotary Data control is damaged there are other things that are damaged on the front panel (IE: input jack, etc) so it may be wise to plan on an additional buffer of approximately $25.00. (See below)
     
  3. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    Also note, at the bottom of that page:

     
  4. gjohnson

    gjohnson Well-Known Member

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    Mines the same.If I turn it slow it works fine.I think it just very touchy.
     
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  5. a2dconverter

    a2dconverter New Member

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  6. MKB

    MKB Well-Known Member

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    I just went through this exercise awhile back with my JMP-1, here's what I found.

    The encoder from Mouser does not have a shaft that works with the Marshall knob (I know as I ordered one). And IIRC the Mouser part had a shaft too short to work in the JMP-1. So you might have an issue fitting a knob that works with this encoder.

    I ordered and installed the new Mouser part after rigging a shaft extension, and it had the same or worse jitter as the original encoder! Not happy. But this got me to thinking. This encoder, Bourns ECW series, is a very high quality part (Bourns makes great stuff), and I found it very difficult to accept that the original part had worn out and the new one was defective.

    So I pulled up the data sheet on the encoder. Turns out that Bourns recommends using a debounce IC with the encoder (MC 14490), and Marshall did not put that part in the JMP-1. So I built up a small circuit board and added a MC 14490 between the original jittery encoder and the JMP-1, and this completely fixed the problem with the factory encoder. The original encoder works perfectly now. (I later installed the new Mouser encoder into a Roland drum brain, and it showed no jitter at all in that unit, so the Mouser encoder was obviously OK too).

    So IMHO this encoder issue is due to an omission in the JMP-1 design (no debounce IC as recommended by the encoder manufacturer). Adding the MC 14490 is not simple as you have to build up a little PCB with a surface mount IC and solder it into the circuit, but a reasonably competent tech should be able to do it. The circuit I used was just the standard circuit in the MC 14490 data sheet, and I attached the PCB to the back of the encoder with foam tape.

    BTW jittery encoders are not limited to Marshall preamps, the Roland VS-1680 recorder also is notorious for having a jittery encoder. There are aftermarket PCB's available to add a debounce circuit to the VS-1680, but AFAIK none exist to add the circuit to the JMP-1.
     
  7. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    Interesting info...
     
  8. ricksteruk

    ricksteruk Well-Known Member

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    MKB - you really know your onions! :)
     
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  9. blues_n_cues

    blues_n_cues Well-Known Member VIP Member

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    or....
    you can just buy a midi foot controller,program in up to 128 presets,& not have to touch the knob again.:naughty: :lol::yesway:
     
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  10. MKB

    MKB Well-Known Member

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    True indeed, but if you want to program patches from the JMP-1's front panel, you have to use the encoder to enter parameters. You may be able to figure out how to use an external midi controller to adjust the individual parameters (bass, gain, patch level, etc) but that might be more difficult that it would be worth. There is probably a PC/Mac patch editor available, and you could edit patches that way as well with a computer. But it is usually much easier to edit with the onboard encoder (especially during a gig).
     
  11. blues_n_cues

    blues_n_cues Well-Known Member VIP Member

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    :squint:
    it was a freakin' joke......lol
    actually,I get mine set & then can run an eq in the loop for any fine tuning I may need live. but no, you don't need the controller wheel to edit bass,mids,etc., just to get to the preset #.
     
  12. blues_n_cues

    blues_n_cues Well-Known Member VIP Member

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    :squint:
    it was a freakin' joke......lol
     
  13. Nebulozny

    Nebulozny New Member

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    I have JMP and the sister JFX-1 effects unit, funny thing...on the JFX the control knobs are by default very fast.

    At first its very annoying but then I realized I could change settings much quicker, go from 0 to 100, and then slowly adjust when I am near desired value. Takes some time to get used to, but now I am flying when adjusting or renaming patches.
     
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  14. cjr71244

    cjr71244 New Member

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    MKB, any chance you can post a pic of the circuit board you made up with the mouser dial? We have A local tech here in Orlando that would probably be able to do it.
     
  15. a2dconverter

    a2dconverter New Member

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    From the prompting by MKB to check the spec sheet for the Bourns encoder, I added the de-bounce IC between the encoder and the PCB and it is rock solid!

    I designed it to be plug & play...(beside one solder connection to +5V)


    [​IMG]

    [​IMG]


    If you have this problem with your JMP-1 it is probably not the encoder!

    Thanks MKB for the extra info!

    seeya

    Joe
     
    Last edited: Oct 7, 2014
  16. adamf1989

    adamf1989 New Member

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    Just wondering if anyone can help a newbie here. My JMP-1 Preamp has started doing this weird thing where it keeps skipping between random patches when I'm playing. Its already been sent back to Marshall who couldn't replicate the issue but it keeps happening. Any ideas?
     
  17. gorgamin

    gorgamin New Member

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    Can I ask for an example of the debouncing circuit. The Motorola, and ON Semiconductor Datasheets both don't really give an example, only pin out, with no mention of a 3n3 capacitor as shown in the pic above. Would really appreciate the help.
     
  18. gloriousbrick

    gloriousbrick New Member

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    This might be a late reply, but I thougt I should share.

    You could also go with the analoge route.
    When you look at the datasheet of the encoder, bourns also placed a debouncing circuit that uses 2 smal capacitors.
    If you look at that schematic and the schematic of the jmp-1, these are practically the same with the exepcion of the 2 capacitors.

    I had the same issue, but after I soldered those caps in the bouncing was gone.
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes, this type control tends to wear out fast.
    Contact Marshall service for the correct part #, order it from a Marshall dealer.
    Kevin (Kdog) Drury
    Marshall USA Service
    1000 Corporate Grove Dr.
    Buffalo Grove, IL 60089
    800-877-6863



    Have an experienced tech do the soldering, I recommend. (if it needs soldering...)
    Board is very easily damaged by inexperienced soldering.
     

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