Marshall Origin 20 Head PCB help needed, tech heads?

Discussion in 'The Workbench' started by Jason Fieldhouseespk, Mar 31, 2020.

  1. Jason Fieldhouseespk

    Jason Fieldhouseespk Well-Known Member

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    Ok.... Well, I need some help and tech advice now...

    So... further to some posts you may have seen regarding installing a PPIMV type 3, on my Origin 20 head.

    My tech has now closed for isolation, and retired for good (he is 80 yrs old), without ever doing any of the work I asked for, so I've taken it back and decided I may as well spend some of my isolation tinkering and try do it myself....

    So, I've opened it up, flipped the board, and now I have a predicament.....

    Those traces don't go where I had thought they would....

    Ref: Photo #1 + Photo #2
    If we take it from between the two power valve sockets, beneath the output tags that are at the top, there are 2 solder joints on their own, for the bias feed resistors, 100k, there's a gap and the other ends are the next 2 joints under those.
    Back up to the valve sockets, the smaller joint next to the bias resistors, and the one over at the far end of the board, on the other side of the valve socket are, I presume, for the grid leak resistors? 5.6k....

    Ref: Photo #3
    Now I expected those 2 to connect up going into pin 1 of each valve (from a rough sketch I made of the positioning from the top of the board)
    Ref:photo #4....
    .... but they now don't seem to.

    Ref:photo #5
    In fact if you follow the trace from the 5.6k resistors, the thin line goes along and down the board towards (I'ill get a fuller picture next time Edit: picture now at the bottom) the master volume insert... This would appear to be a PPIMV amp... :-O

    Would others tend to agree?

    This would mean that my prognosis of how the master volume knob is operating is all wrong (had initially thought that the reason this amp needs the master to be turned up past at least 7 was because it was possibly a pre-phase inverter master volume) and the problem isn't the result of it's place in the circuit, but I am now wondering about the type of pot it's self...

    What do we think of the idea that this pot could be linear taper instead of audio taper?

    Could it be that simple? What effect would that have on it?

    Any help with this is much appreciated, thank you, :)

    Reference Pictures below:



    Photo #1
    1 Back Board, V4, V5 sockets.gif

    Photo # 2
    2 Front of board.gif

    Photo #3
    3 Plexi6V6c.gif

    Photo #4 (ok, don't laugh too hard, I got further, but it's on my instagram)
    4 Schematic Drawing.gif

    Photo #5
    5 Back of board 2.gif
     
    Last edited: Mar 31, 2020
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Please post the pictures in the Marshall Forum.
    There have been numerous reports of malware from clicking 3rd party links posted in this forum.
    (the malware attacked my computer also)
    Therefore I do not click on links to outside photos.
    I cannot tell anything until we see lots of close up pictures of both sides of the board.
    There is no schematic so far.
    I am not sure that we can do anything without a schematic.
     
    Jason Fieldhouseespk likes this.
  3. Jason Fieldhouseespk

    Jason Fieldhouseespk Well-Known Member

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    Ok, yeah, no problems, but I've always had difficulty on here trying to do that?

    I have close ups of the board, both sides, and my sketchy schematic based on the locations of components, cross referencing various schematics for other Marshall amps of this style, including one 18 watt version running 6v6's.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Use a photo program such as microsoft paint to reduce the JPEG photos to approximately 25-30% of original size.
    Using MS paint, save the photos as GIF or PNG format.
    Posting JPEG or PDF in the forum: the files are probably too large.
     
    Jason Fieldhouseespk likes this.
  5. Jason Fieldhouseespk

    Jason Fieldhouseespk Well-Known Member

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    Photos all up and running, advice coming in from TGP, tbh, not very helpful so far.

    Just telling me PCB amps shouldn't be bothered with, and I should build another amp, and that the presence will be less effective with a cross line master...
     

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