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Thread: Apple III Troubleshooting and Repair

  1. #11
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    Thanks for finding that PDF. I tried numerous web searches to find useful information and came up with nothing. Hopefully somewhere in that monstrous file are complete schematics.

    The CPU is probably running, I haven't much of an opportunity to dig in too deep. But the reset circuit is not working so the CPU probably isn't doing anything useful. I ran out of patience trying to figure out the reset circuit so I've been trying to find schematics. I did run into some power problems, possibly due my disassembly, but I seem to have that sorted out. I also found a couple chips running far hotter than I think they should.


  2. #12
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    This should help
    http://www.apple3.org/iiischematics.html

    I've never worked on one of these, but a quick look over the schematics and it looks like the reset is generated by the keyboard circuitry. Try it with the keyboard connected.
    Last edited by David_M; May 17th, 2018 at 09:39 AM.

  3. #13
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    I have seen those schematics, and they are incomplete. Unless I missed it, I couldn't find the timer circuit for /RESET.

    The reset circuit is partially working, with or without the keyboard. It goes high too quickly. I haven't had a chance yet to try to force it low for longer to see if the system does something different.

  4. #14

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    On the Mac II and IIx, one of the Sony audio chips is involved with timing /RESET. I'm not mentioning it because I expect it to be identical on the III, but in case it gives you a new idea of where to look.

  5. #15
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    Thank you. This thing probably doesn't have any hardware like that. But it is helpful to understand the thought process of the designers. This /// is an odd duck.

  6. #16
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    Quote Originally Posted by KC9UDX View Post
    I have seen those schematics, and they are incomplete. Unless I missed it, I couldn't find the timer circuit for /RESET.

    The reset circuit is partially working, with or without the keyboard. It goes high too quickly. I haven't had a chance yet to try to force it low for longer to see if the system does something different.
    If you look closely at the keyboard schematic, reset is indeed generated on the keyboard circuitry schematic.
    The IORESET line is buffered by a 74ls05 using (open collector) pull up resistor because it is designed to work with other reset sources (from cards).
    RESET is different, it is buffered with a 74ls14 which is not open collector, so it must be the only output connected to the line.
    The length of the reset pulse based on the components in the schematic calculates to about .25 seconds. To force a longer reset just use the reset key on the keyboard.

    Bottom left
    http://www.apple3.org/Documents/Sche...rd%20Logic.jpg
    Last edited by David_M; May 17th, 2018 at 04:11 PM.

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