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Thread: Model 4 Font

  1. #21
    Join Date
    Mar 2016
    Pikesville, Maryland


    Don't worry --- replacement flex cables ARE avaiible from either Ian Mavric or me

  2. #22


    @Larry. Hey thanks for the write up. Super helpful! I'll troubleshoot the model 4 this weekend. I have a multimeter.

    Good to know about the pencil and those flex cables. I bought a "beat up" Model 4 on eBay just to replace one of those :/

  3. #23
    Join Date
    Mar 2013
    Chaffee, MO


    Just in case you are wondering how the MCM68A316E gets the address for each Character (8 Rows of 8 Bits in each Character) here is a bit
    of information on the wiring of the MCM68A316E.
    A7  Pin 1         Pin 24  Vcc
    A6  Pin 2    M    Pin 23  A8
    A5  Pin 3    C    Pin 22  A9
    A4  Pin 4    M    Pin 21  S3
    A3  Pin 5    6    Pin 20  S1
    A2  Pin 6    8    Pin 19  A10
    A1  Pin 7    A    Pin 18  S2
    A0  Pin 8    3    Pin 17  Q7
    Q0  Pin 9    1    Pin 16  Q6
    Q1  Pin 10   6    Pin 15  Q5
    Q2  Pin 11   E    Pin 14  Q4
    Vss Pin 12        Pin 13  Q3

    D0 thru D7 come from Data Lines or U21 Video Ram - 4016
     D4  Pin 1         Pin 24  Vcc
     D3  Pin 2    M    Pin 23  D5
     D2  Pin 3    C    Pin 22  D6
     D1  Pin 4    M    Pin 21  S3-|
     D0  Pin 5    6    Pin 20  S1-| Tied to OE*
    RA2  Pin 6    8    Pin 19  D7
    RA1  Pin 7    A    Pin 18  S2 - Tied LOW all time.
    RA0  Pin 8    3    Pin 17  Q7
     Q0  Pin 9    1    Pin 16  Q6
     Q1  Pin 10   6    Pin 15  Q5
     Q2  Pin 11   E    Pin 14  Q4
     Vss Pin 12        Pin 13  Q3
    So, for every 8 Rows in each Character, the Address will increment one Character Block (Next Data) that is
    eight Rows of 8-Bit words, as far as the MCM68A316E is concerned. This is done by using the RA0, RA1,
    and RA2 Signals from the CRT Controller to step through the Characters.

    Does anyone have a HEX Dump of the Character ROM? Hopefully someone has already dumped a copy
    and has it saved. If anyone has a HEX Dump I'd like a copy. Maybe MESS has a copy available.



  4. #24


    No luck so far. I've re-soldered dozens of joints from the PDF including around chips U23, U10, and U11. Some of the joints looked like they needed additional solder. I'm assuming RA0-RA2 are okay or else other characters would be affected (column / rows).

  5. #25
    Join Date
    Jun 2018
    Pacific NW in the islands


    i've seen buffer devices, such that there are multiple buffers/inverters in a package, short inputs or outputs internally. it's an odd failure, at least to me, but i saw quite a few of these failures at RS. my suggestion is to get a logic probe from somewhere, get a decent one, and start looking at data/address line activity. a scope is better, but if you don't have one it's an expense and logic probes are comparatively inexpensive and handy if you intend to keep restoring old gear.

    you can write loops in BASIC or debug to exercise targeted parts of the machine, output characters, and such so that you can look for specific activity. this is much the same as what's already been suggested and is particularly useful in port decoding problems, just as a side note...

    sounds to me like it's time to start tracing bits and signals...

  6. #26


    Guys I decided to swap the motherboard with a similar newer motherboard it works perfectly now. All pixels are there finally AAHHHH. I know, it's cheating. I moved the 128K RAM over. I'll keep this "bad pixel" motherboard as a backup. If somebody wants to look at it let me know and I'll send it to you.

  7. #27


    Here it is restored and operational.
    128K Model 4 with Green Screen:


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