Is this a mod?

Discussion in 'The Workbench' started by LPMarshall hack, Feb 16, 2020.

  1. LPMarshall hack

    LPMarshall hack Well-Known Member

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    So I have the chassis of my JCM 900 MKIII out of the box for some pot cleaning. I noticed the Sensitivity pot is different than the others. The pot is larger and has three wires (red, green, and black) going into it.

    Is this normal? If not, any ideas?

    I tried to load a pic but it says it’s too large. Story of my life!
     
  2. LPMarshall hack

    LPMarshall hack Well-Known Member

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    59B47ED1-D295-4F90-A0C8-B9B4D42DFC33.jpeg
    Finally got the pic to load...
     
  3. LPMarshall hack

    LPMarshall hack Well-Known Member

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    96B192B3-724F-473F-AAC2-6E58F129044F.jpeg 15690680-36AE-4DB2-B018-D146FF24F739.jpeg 9DCFA8E0-6CDD-4B6D-A0F9-13763CCCC187.jpeg
    here’s some more gut porn for you electronics pervs.
     
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  4. mickeydg5

    mickeydg5 Well-Known Member

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    Revised post. I see you got the pictures loaded.
     
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  5. mickeydg5

    mickeydg5 Well-Known Member

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    No not a modification. I have found the same in other JCM900 amplifiers.
     
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  6. LPMarshall hack

    LPMarshall hack Well-Known Member

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    Oh ok....thanks! What’s the reason one pot has these wires sticking out?
     
  7. MickeyJ

    MickeyJ Well-Known Member

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    i don't know, you amp dogs. this looks like the guy didn't have a stock pcb pot and just wired up a normal one.
     
  8. MickeyJ

    MickeyJ Well-Known Member

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    there is a mod. you can see a cap has been added to r5 just behind the pot.
     
  9. LPMarshall hack

    LPMarshall hack Well-Known Member

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    That’s what my initial thought was.
     
  10. MickeyJ

    MickeyJ Well-Known Member

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    while you have the thing apart, there is a cap that always goes in the bias section, a red poly one, unsolder it and put a new one in, higher rated.
    bad things happen when they fail.
     
  11. LPMarshall hack

    LPMarshall hack Well-Known Member

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    You think so? I love the internet...one says yes, one says no...lol

    What kind of mod do you think it is?
     
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  12. mickeydg5

    mickeydg5 Well-Known Member

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    That is the $10 question.
    Ask Marshal for their reason.
     
  13. LPMarshall hack

    LPMarshall hack Well-Known Member

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    That’s way above my pay grade!!!

    I was nervous just removing the head shell screws!!
     
  14. MickeyJ

    MickeyJ Well-Known Member

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    bro, how dodgy is it, see how they've just twirled the legs around the resistor leg like that?
    my vision is really shit, so I may be wrong.
    I'm not surprised you've seen this before, who wants to order a special pcb pot when a stock one will do the job.
     
  15. MickeyJ

    MickeyJ Well-Known Member

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    it is a mod, you can see they've used a radial cap, not axial. axial is like the one to the left of r5, a lead coming out of each end.
     
  16. MickeyJ

    MickeyJ Well-Known Member

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    adding a cap over a resistor like that can filter out certain things, highs, lows mids.
     
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  17. LPMarshall hack

    LPMarshall hack Well-Known Member

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    That particular pot was not the problem. It’s always seemed to work just fine. I always keep the sensitivity knob at zero anyway
     
  18. MickeyJ

    MickeyJ Well-Known Member

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    " R5 3k3 add .68uf bypass cap. (Do not decrease resistor value). found that online.
     
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  19. LPMarshall hack

    LPMarshall hack Well-Known Member

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    What does this mean?

    I am not an electronics guy at all
     
  20. MickeyJ

    MickeyJ Well-Known Member

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    its a post by new religion, that tone demon

    Free Easy Mods For Jcm 900 2500/2100
     
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