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Thread: 4D / 4P CRT card in M3/M4

  1. #1
    Join Date
    May 2019
    San Diego CA

    Default 4D / 4P CRT card in M3/M4

    A little information, buried in my Model 4 thread.

    We know there are 3 CRT card designed used in the M3 / M4 / 4D /4P.

    First is the analog card used in the M3 and maybe early M4s. This has the edge connector on the "end" of the card.
    Second is the large digital board used in M4s, and probably late M3s. This has the edge connector on the "end" of the card.

    These two cards can essentially directly replace each other, taking into account the yoke is different. The wiring from computer to CRT card is the same.

    The 3rd card is the small digital board, designed for the 4P, but ended up used in most 4Ds as well. This card has the edge connector on the "side" of the card.

    The schematic of the small digital board and large digital board are very slightly different, and the interface to the computer is different, although it uses the same connector.

    2 pins for Brightness are swapped and there is a new signal for "DC CONTROL" where the earlier boards had an NC.

    In short, the pots for brightness and contrast are wired differently, and instead of the video signal passing through the pots, the video signal goes directly onto the board and the pots drive the new DC CONTROL line. The DC CONTROL voltage is used to adjust the video signal. The end effect is the same, I can only guess this change was made to some constraint on the 4P design or perhaps this gives a slightly better video signal needed for the smaller 9" display in the 4P.

    The board is NOT a direct replacement for M3/M4 machines, but it's not too far away.

    To use a 4D/4P small digital card in an earlier machine, simply swap the wire harness, brightness/contrast knobs and CRT card/yoke as a unit.

    The 12V connector on the donor harness will likely need changed if you have different power supplies than the donor computer. I was able to use the 12V connector from the original wiring harness and spliced it into the new harness.

    If you are using a CRT controller from a 4P, you will have to re-create the 4D's wiring harness. The 4P is a different form factor so the knobs won't directly swap into a M3/M4 case.

    The end result -- a Model 3 or 4 with the "best" CRT controller card. This is a little more complicated than using a Model 4 CRT card and green tube to swap into a White phosphor machine, but gives you more options when looking for donors/parts.

    Big thanks to Ian who got me started on this upgrade with his Super Micro build article, and more importantly the portion outlining the different cards and CRTs.
    Last edited by TheDrip; January 16th, 2021 at 08:50 PM.

  2. #2
    Join Date
    May 2019
    San Diego CA


    Here is the link to "TRS8BIT - volume 06 Issue 02 - June 2012" which has Ian's article on TRS-80 CRTs and controllers.


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