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Thread: CBM 8032 Character Display Problem

  1. #1

    Question CBM 8032 Character Display Problem

    Dear Community,

    I got a Commodore CBM 8032 from ebay and I would like to restore it.
    I've already restored the keyboard, so each key works again now.

    20170912_235416.jpg

    But unfortunately the characters aren't display correctly as a 8x8 pixel matrix. There are just cryptic points and a line on the bottom of the display:
    This is the start-up screen:

    20170913_203020.jpg
    20170913_203045.jpg
    20170831_120926.jpg

    The CBM 8032 make a beep sound on boot and the BASIC also works.
    This program just prints the word "Hello" (but the characters are just some cryptic points).

    20170912_235402.jpg

    Would be very happy if someone could help me.

    Here are some pictures of the board:

    20170913_131414.jpg
    20170830_181622.jpg
    20170913_132325.jpg

    Best Regards,
    Andreas
    Last edited by Andreas Zock; September 13th, 2017 at 11:43 AM.

  2. #2

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    Hello, could you post a close up picture around the character ROM - from what I can see from the photo 4 (upper left near the Orange Capacitor) it has a blue adapter socket .... that is strange and wonder why it is there or part of the cause of what you are seeing. A photo of the chip that is inserted into the adapter would be good, showing the numbers.

    Cheers
    Andy

  3. #3

    Default

    Hello, thank you for your help.
    I've made a picture of the character rom

    20170913_194030.jpg

    It has the label 901447-10.
    I've tested the ROM without the blue socket, but nothing changed.
    Only the cursor is displayed correctly.

    Cheers
    Andreas

  4. #4
    Join Date
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    As this is your first post, welcome to the forum...

    I agree with Andy - let's have a look at the 'modification' or 'fix' first.

    A close-up of the characters on the screen immediately after power-up (but before poking any keys) would also be extremely useful. I think I can see the lower half of the characters missing (or is that my eyesight...).

    EDIT: Now I've had another look I can just make out what should be "*** commodore basic 4.0 ***". So you are correct - the computer part of the machine is fine: we just need to work on the video section (but it would still be good to have that photograph of the start-up screen in detail).

    Dave
    Last edited by daver2; September 13th, 2017 at 11:22 AM.

  5. #5

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    Thank you daver2 for your help.
    I took two new photos of the start-up screen (added in the first post).
    The ROM IC on the blue adapter socket has the label 901447-10.
    I removed the blue socket but nothing changed.

    Andreas

  6. #6
    Join Date
    Feb 2009
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    The part number on the Character Generator ROM (UA3) should be 901447-10. It may be a problem there (or with its socket/solder splashes), or the serial shift register part 74166 (UA2). Less likely it may be a raster scan addressing issue in the CRT Controller (UB13). The video board is analog and not likely to exhibit symptoms like this such as the gap in the middle of the characters.

    I notice that the blinking cursor seems fine. Curious.

  7. #7
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    My thought exactly Dave...

    One possibility - some sort of problem with the "NO ROW" signal?

    Another is a partial character generator ROM fault - taking out one 'line' of each character. Just wondering if the block cursor is in a working section of the ROM.

    I had a really 'weird' fault with a 64K DRAM on an Apple IIe - it exhibited a really strange fault which was most likely down to an internal addressing fault within the IC logic. Perhaps a similar thing has happened here to the character generator?

    If the CHARGEN is in a socket - removal and forcing the 8 data lines HIGH into the shift register should give us a solid screen.

    Dave

  8. #8
    Join Date
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    Quote Originally Posted by daver2 View Post

    If the CHARGEN is in a socket - removal and forcing the 8 data lines HIGH into the shift register should give us a solid screen.
    Dave,
    Excellent quick test! With the character rom removed from its socket, if there are still gaps in the raster lines, problem should be down stream such as a fault in the shift register, etc. If there is a solid green screen, then problem is the ROM or in socket and circuit traces around the socket.

    Andreas, do you have a voltmeter or logic probe if needed?

  9. #9

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    When I removed the CharGen ROM, I got a block filled screen.

    Quote Originally Posted by dave_m View Post
    Andreas, do you have a voltmeter or logic probe if needed?
    Yes, I have a voltmeter. Let's see if I have some shift registers...

  10. #10

    Default

    Guys,

    Great idea, removing the character ROM should give a solid green screen........ the only reason I can think of why that adapter socket is present is that there has been an issue with the socket or hairline crack in a track around the socket (the previous owner could have been swapping out the chip regularly and didn't want to damage the original socket but not sure why).

    Andy

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