Is this a mod?

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

  1. NewReligion

    NewReligion Well-Known Member VIP Member

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    Perhaps I am misunderstanding you. This is not my thread I simply posted exactly what I did to a JCM900-2500 series with a video and the exact changes I made to it.

    As to the OP and the inverted master volume as it appears at VR2, I have no idea.

    No I did not & I still do not recognize the original mod on the opening thread.


    Of course I remember all of my mods even those that take 14 hours to achieve. They are all archived in schematic and layout format.

    David
     
    Last edited: Feb 17, 2020
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  2. LPMarshall hack

    LPMarshall hack Well-Known Member

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    Hi Dave

    so what you’re saying is, the mod done to my amp, it’s not the same as what you did with your sons 2500? Am I understanding you correct?
     
  3. NewReligion

    NewReligion Well-Known Member VIP Member

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    Correct.

    1.) I do not know the values that were used on your R5 & R8. Should have been R5 3.3k / R8 1.5k

    2.) D2 appears to be the stock single 1N4007 instead of a set of 1N4733

    3.) R4 is 220k instead of 330k

    4.) R7 appears to be 100k instead of 330k

    David
     
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  4. MickeyJ

    MickeyJ Well-Known Member

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    Hi David, I wasn't directing your post at you, I was directing it at the person who took credit for your mod.
    Your're not the type of person who does that shit.
    I knew who designed the mod from the start, see I read the link before I posted it.
    Regards
    Mickey.
     
    Last edited: Feb 17, 2020
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  5. NewReligion

    NewReligion Well-Known Member VIP Member

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    Try changing out D2.

    David
     
  6. MickeyJ

    MickeyJ Well-Known Member

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    What he's saying you beer guzzling amp pirate is that the mod was done on a jcm900 2500 with possibly different resistor placement to yours.
    I think you'll find in relation to r5, it's the same.
    different types of jcm900, different boards, different printing.
    It's david mod on another jcm900.
     
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  7. LPMarshall hack

    LPMarshall hack Well-Known Member

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    Oh I’m not looking to mod anything David. I just posted this thread because I discovered the odd looking pot while cleaning the pots. Was just curious if it was a mod, so I asked here.

    turns out my amp had a couple of mods that I didn’t know about! Glad I posted this thread. Thanks guys!
     
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  8. BanditPanda

    BanditPanda Well-Known Member

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    10-4. Over & out.
    BP
     
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  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Man, I have had my MK III stored away for awhile. I really want to hear it now . . . . thinking I need to start selling some of these amps I have man lol.

    It was cool to look at the schem for these now that I know a bit more these days.

    Be careful as there are two versions it seems with some different component callout locations between the two.

    I can not remember which pot is the Sensitivity? Is it VR1 pot? If you are able to put it at 0 and sound still comes through, then by the looks of the schem it must be VR1.

    I am trippin on the Presence circuit in this amp - very different for sure. Just adds a .22uF cathode bypass cap to 3rd stage.

    https://drtube.com/schematics/marshall/cd0109-iss4.pdf

    https://drtube.com/schematics/marshall/cd0109-iss9.pdf
     
    Last edited: Feb 18, 2020
  10. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Geebus, How many g damn amps do you actually have Ricky?
     
  11. RickyLee

    RickyLee Well-Known Member VIP Member

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    I do need to get a count some day lol.

    Though the steady gigging has all but slowed down, I am holding onto the guitar amps in hopes that this will finally be the year when I get my recording studio built. Then, this massive collection will start making practical sense again. I will then be able to start seeing which amps will be the ideal recording situation.

    I was considering starting up an amp leasing/rental business as well.

    :D

    I am already thinking I need to offload the generic speaker cabs that I truly do not need. The one that I might want to save the speakers from is the old Randall 4X12 slant that is loaded with 8 ohm G12-65's. I rarely see 8 ohm 65's out there.
     
  12. LPMarshall hack

    LPMarshall hack Well-Known Member

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    Hey ampmad...I think you only commented on the one mod that creates some more lower frequencies.

    but you also mentioned the board has been modded in several other places. I dont believe you mentioned what the other mods were. Can you elaborate where the other mods are on the board and what they do?

    Thanks in advance.
     
  13. 6550

    6550 Member

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    Mine is the same as the other pots..



    20191227_160513.jpg
     
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  14. Jaska9

    Jaska9 New Member

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    Hi, hard to say just by the picture, is this a mod. I mean, is the pot function different than original one. But it's not an original pot.

    The original broken pot has been replaced with this bigger one, so it had to be wired with separate wires, because it doesn't fit straight into printed circuit holes.
    Regards, Jaska


    Is this normal? If not, any ideas?

    I tried to load a pic but it says it’s too large. Story of my life![/QUOTE]
     
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  15. Tobaccosun

    Tobaccosun New Member

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    It’s normal. It’s a clipper circuit similar to what you find in Friedman be-100 (sat switch) but in this case it’s placed in the middle of the preamp instead of after it and before tone stack. It uses clipping diodes for solid state clipping. If turned up too much it will sound like total crap, it would’ve been much better placed later in the circuit.
     
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  16. LPMarshall hack

    LPMarshall hack Well-Known Member

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    Not sure if you read the thread, but most seem to think it’s just a replacement for the stock pot...as if someone didn’t have a stock pot to put in there.

    also pics of other mkiii’s dont have this kind of pot.
     

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