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Thread: PDP-11/04 Console Repair - Part 1

  1. #1
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    Default PDP-11/04 Console Repair - Part 1

    Continuing with my PDP11/04 and 11/34 repairs, now that the power supplies are sorted,
    next comes the console and its interface, the M7859 board (together termed KY11-LB)
    These work properly on the 11/34, but there are multiple issues with this section of the 11/04.

    Having a working set of interchangeable parts from the 11/34 was a huge help in identifying
    where the problems lay in the 11/04. (all three PCBs involved had faults)

    I've invested quite some time coming up with a detailed block diagram of the M7859, and then
    more time was spent in making an input protection board for a simple logic analyzer.

    Block Diagram:-


    In this console repair part 1 video I find and repair two problems, both broken conductors,
    but realize that the remaining problem(s), even with the aid of the logic analyzer will
    need a more rigorous and "clever" approach.

    Since recording this, I'm well on the way with my new problem determination design, and I'm
    extremely confident that it will locate the fault(s) on the failing M7859 board. I'm still
    working on it, and will present it soon in part 2 of the video.

    https://www.youtube.com/watch?v=RByPWcqJUrQ

  2. #2
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    Very interesting video of troubleshooting. Good luck with M7859 board. I look forward to the video on that repair.
    -Dave

  3. #3

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    Very nice work on the block diagram! I've recently acquired a PDP-11/34A, and I saved your diagram for future reference. I wasn't aware of your YouTube channel before, and I'm going to work through your PDP-11 videos in order. I've just watched the first one so far. And your old dog video, of course!

  4. #4
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    Thanks for comments above.
    I'm working on the next stage and part 2 video atm.
    I believe I've found the cause of the (hopefully only) problem, which is that the Address Register Clear FF [E6-6] is being set when it shouldn't.
    But have not not yet found what causes that...

  5. #5
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    What software did you use to create that detailed block diagram?

  6. #6
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    Done with a very old version of "RFFlow".
    I don't like it very much, because it has many shortcomings in my opinion.
    But one thing I do like about it is that diagrams can be embedded in a word document, and dbl-clicking on one opens RFflow to edit it (OLE).

  7. #7
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    Quote Originally Posted by dave_m View Post
    What software did you use to create that detailed block diagram?
    I've used http://blockdiag.com/en/ for simple diagrams and found it pretty easy to work with.

    It's open source on github.

  8. #8
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    Default PDP-11/04 Console Repair - Part 1 and 2 of ?...

    This is becoming quite a saga!

    In this video, I find and fix a fault in the M7859 board (a 74175 Quad D flip-flop), plus uncover and fix a bug in my testing hardware.

    But after that, the board is still faulty!
    I'm not sure why, as I believe I've tested every part of the board involved with its basic operation when in the computer.

    But as the video is already more than long enough, I've ended it at this stage of the troubleshooting, and will have to make a part 3.

    And to continue further , my test system needs to be rebuilt, which will take a bit of time.
    So when that's done, and I've been able to delve further into the problem, I'll present part 3...

    https://www.youtube.com/watch?v=q9FLhAlfLZs

  9. #9
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    Quote Originally Posted by intabits View Post
    But after that, the board is still faulty!
    I'm not sure why, as I believe I've tested every part of the board involved with its basic operation when in the computer.
    Your test set checked out a lot of circuitry except the 8008 CPU. Shouldn't you replace the 8008 or have you?

  10. #10
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    Yes, that was the first thing I tried, as the MCUs are the only interchangeable part.
    Both of them worked in the good M7859. (I think I mentioned that in the previous video?)
    But in case of sensitivity or tolerance issues, I just tried the other 8008 in the "repaired" faulty board - it was no better.

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