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Thread: Model 4 Font

  1. #1

    Default Model 4 Font

    Does anybody know why the letters "w" and "m" are truncated on this Model 4? See attached screen shot eg. "View Formatted Buffers?"

    I have tried replacing the following from a known working Model 4 without any luck:
    -Video board and CRT
    -All removable chips, large single piece motherboard


  2. #2
    Join Date
    Mar 2016
    Pikesville, Maryland


    Could it be a defective character generator chip (just a wild guess) .... contact Ian -- he may have a better answer

  3. #3
    Join Date
    Jun 2018
    Pacific NW in the islands


    I don't have a schematic handy, but looking at your screen shot... If i'm counting right, it appears that the character generator only outputs a 7th character column of dots (counting left to right) on the lower case w and m characters (in your screen shot--but other characters may be affected, like graphic characters). That's your clue, I think. Like Jay suggested, the problem could be right around the character generator... data line, buffer... something like that... might be the character generator.

    If you don't have a scope, an inexpensive logic probe will help you see activity on data and address lines. you can probably build one for less than a cup of coffee at your favorite emporium. Amazon has them, even! I kept one of the RS versions in my tool kit for years. Fast when I didn't want to drag out a scope and just needed to see activity through buffers/inverters and like that. So, active data on one side of a buffer but zero activity on the output: bad buffer, or a component shorted to ground just after perhaps... A simplification, but you get the idea.

    The good thing about video problems is that you can actually see the issue.

  4. #4


    @shank Graphics characters are affected as well, the right most pixels are missing probably, like you said.

  5. #5
    Join Date
    Mar 2013
    Chaffee, MO


    The problem is with the column section for the dots. The character is printed by a Row & Column Matrix.
    Here is a Photo of the two characters m & w showing R0 thru R7 and C0 thru C4.


    I've blown up the photo 150% ans it's still not large enough.


  6. #6


    I was going to guess the problem was in the line from pin 17 of U19 (the font ROM) to pin 33 of U17 ("Vidarray", the video support gate-array chip).

    But if you're getting the same missing-column-7 behavior in graphics characters, then that sounds like a problem in the Vidarray. The Vidarray generates the graphics characters internally, and sends all 8 columns of them out the same pin (pin 19), so I don't see how the fault can be in the lines external to the chip. Are you sure you're seeing this behavior with a chip in U17 that works fine in another system?

    (We are talking about a gate-array motherboard, right? With a big socketed 40-pin DIP at U17?)

    Can you post a pic showing what happens with graphics characters, and with reverse video?

  7. #7


    Btw, here's a picture of the font in the standard Model 4 character generator (part 8049007). You can see column 7 is pretty sparsely used, and appears in only 4 ASCII characters (_ m w ~). Leaving a blank column on the left AND the right in most characters avoids illegibility in the first or last character of a section of text in reverse-video.

  8. #8


    Chip U17 I haven't replaced because it's a soldered chip. Ahh maybe there's a patched wire on that chip that's loose or broken off so this is helpful. I'll check around U17. Attaching photo showing the graphics characters.


  9. #9
    Join Date
    Sep 2011
    Melbourne, Australia


    Is U23 a socketed 24-pin chip on your motherboard?
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  10. #10


    U23 is socketed. I can try replacing it again--I have two other U23 chips available.

    Attaching photos of motherboard.



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