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Thread: Apple ][+ white screen issues

  1. Default Apple ][+ white screen issues

    I recently got two Apple ]['s the older one works fine, but the ][+ shows a white screen, but otherwise boots normally. The cursor shows as a blinking black square on white and entering characters on the keyboard advances the cursor on the white screen and system beeps, resets, etc seem to function normally, except the screen is always white.

    The Apple ][+ has an 80 col card in slot #3 (a viewmax 80) and typing PR#3 (blind 'cause the screen shows white) changes the screen to 80 char normal black&white. From there, everything appears normal There is a "]" prompt, until I press reset, which sends the screen to white ( PR#3 again sends it back to 80 chars, black&white).

    I have reseated most of the chips, and all of those referenced in the repair docs that I have found. Does anyone know what the likely cause is, so I can switch/replace the appropriate chip(s)?
    I did find a single trace that looked iffy, but checking for continuity confirmed that it was fine.

    A2 service notes:
    Blank white screen
    A3-74166
    B5,8-74LS174
    F14,9334
    6502

    a2 troubleshooting
    Chip Swapping
    A2 74LS00
    A8 74LS257
    A9 74LS151
    A10 74LS194
    B2 74S86
    B10 74LS74
    B13 74LS02
    C2 74LS195
    C11 74LS04
    D11,12,13,14 74LS161
    Last edited by Juror22; July 28th, 2017 at 02:28 PM.

  2. #2
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    Not sure how many cross-posters there are, but I would ask on applefritter.com

  3. #3
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    The all white screen can be caused by a few things.

    To eliminate some of the potential causes, boot the machine to basic (no floppy).
    If you hear the speaker beep type gr and press enter.
    Let me know if the screen clears.
    If most of the screen clears then the problem is in the character generation circuit, if it stays white then look to the video source multiplexing.

    edit: Given that you have a flashing cursor and stable sync, D11,D12,D13,D14, B2,B10,B11,B13 are likely to be fine.
    I'm leaning towards the problem being character generation, that would indicate that the likely culprits are A3 or A5 in that order.
    A quick test is to take the ROM from F11 and fit it into the A5 socket. If you see random dots then you have a faulty A5 chip.

    Try those things and let me know the results, I'll be able to narrow it down further.
    Last edited by David_M; July 29th, 2017 at 03:25 AM.

  4. #4
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    I thought about that at first, but I didn't know for sure if the chargen ROM was used for 80 column mode or not. Is it?

  5. #5
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    Quote Originally Posted by rittwage View Post
    I thought about that at first, but I didn't know for sure if the chargen ROM was used for 80 column mode or not. Is it?
    The 80 col card in a 2+ is a completely separate video subsystem.

  6. Default

    Thanks for the responses, I thought that I had a login at applefritter, but I don't, so I applied for one - I am now awaiting for them to determine that I'm not a robot.

    Anyway, moving on to the practical next steps. typing in gr <enter> yields a mostly black screen, so that confirms the chargen circuit is the issue.
    The chip swap produced no change - booting with the F11 ROM in the F5 holder still yielded a white screen. Reseating F3 likewise did not help.

    Thanks for the assistance.

  7. #7
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    Quote Originally Posted by Juror22 View Post
    Thanks for the responses, I thought that I had a login at applefritter, but I don't, so I applied for one - I am now awaiting for them to determine that I'm not a robot.

    Anyway, moving on to the practical next steps. typing in gr <enter> yields a mostly black screen, so that confirms the chargen circuit is the issue.
    The chip swap produced no change - booting with the F11 ROM in the F5 holder still yielded a white screen. Reseating F3 likewise did not help.

    Thanks for the assistance.
    Then its 90% likely to be the shift register (74ls166) at A3.

  8. Default

    The current chip is a National DM74166N, is that likely to be the original chip? I have seen where it is a replacement for the ls166, but I'm curious what would be original to the machine, since both are available. I'm looking around to find if I have a spare about, without disassembling my working ][.

    Finding no other spare, I took the donor chip out of my ][ and placed it in the 2+, but it still produces the white screen. Putting the donor chip back and booting the ][, it still works fine. Where should I look next?
    Last edited by Juror22; July 30th, 2017 at 08:52 PM.

  9. #9

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    From Winston Gayler's wonderful book, the path for text video is A3,A5,B3,A9. You've eliminated A3 as a possibility.
    I'm not so sure you've eliminated A5 but maybe B3 or A9 ?

    video.jpg

    Larry G

    PS - my gut still tells me bad character ROM A5. A white screen is all bits on as if it was read out that way from ROM.
    Last edited by retrogear; July 31st, 2017 at 11:29 AM.

  10. #10
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    Its less common but possible that the shift register is not getting the load signal.
    There could be a problem with the 74ls00 at A2 or the 74ls02 at B13.

    Keep in mind that chip swapping won't always find a problem, with these old boards the problem can be open circuit tracks or sockets faulty beyond the help of reseating chips.

    What test gear do you have available to you?

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