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Thread: Looking for help with a Commodore Pet 2001

  1. #11

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    Quote Originally Posted by rglenn View Post
    The result of the PEEK command above was 21
    I do have the 2001 revision with 6550 RAMs
    The board has PET MAIN LOGIC ASSEMBLY 320008 printed on it.

    I am now getting some additional characters on the screen.
    Do you have a way to check that UD4 Pin 12 is maybe stuck to 0V or almost and instead UD4 Pin 13 is toggling? That should be checked during power on reset sequence or when running a simple basic program like:

    10 FOR A=32768 TO 33768
    20 B=PEEK(A):NEXT
    30 GOTO 10

    Anyway, symptoms indicates UD4 as bad, if BA9 signal was bad, it would not even boot to basic correctly probably.

    Frank IZ8DWF

    EDIT: could even be one of UC3 or UC4 that have a shorted A9 input, so since they're socketed, I would try to swap both of them with other 6550s and see what happens.

  2. #12

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    It could still be a shorted input lead on one of the RAMs A9, pin 11. It is not as likely.
    It is also possible that SA9 is stuck High as well. The symptoms just indicate that SA9 isn't changing and not which value it is stuck at.
    Dwight

  3. #13

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    Quote Originally Posted by Dwight Elvey View Post
    It could still be a shorted input lead on one of the RAMs A9, pin 11. It is not as likely.
    It is also possible that SA9 is stuck High as well. The symptoms just indicate that SA9 isn't changing and not which value it is stuck at.
    Dwight
    Correct, SA9 might be stuck even to +5V

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