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Thread: Weird error in my 8M

  1. #1

    Cool Weird error in my 8M

    My lovely 8M let me down while testing my home made PCB's...

    When I started the BIN loader for example, it did run but it would stop for whatever reason.
    And while it was running I could deposit stuff into memory... How weird!

    So back to a basic setup.. But the problem stays the same. Swapped the CPU boards, memory etc...
    But nothing changed. But then I swapped the timing generator. And because I was a bit lazy I put in my
    timing board from my 8E into a different slot which I could easy reach. And the 8M worked again!

    Okay then it must be my timing generator. So I put the original back into the machine. Also in a
    slot somewhere in the middle of my machine. And guess what... My 8M runs fine. How weird is that...
    Oh well, lets put it back into the original slot and enjoy the system. Probably some dirt...

    Well no! The machine refused to work! So put it into another slot and it runs fine again.
    So can a backplane go wrong?

    Well, the backplane has never been right! Look at that little pin. It is inserted backwards!

    Backwards 1.jpg Backwards 2.jpg

    Thanks to Lou I still had my spare part of a H803. These have exactly the same pins.
    So I replaced the pin because it didn't had the normal shape...

    part H803 Lou.jpg

    It has made contact for many years at the bottom of the backplane. But this was a very tiny contact point.
    So I guess my heavy usage of this 8M did pop up this 47 year old production fault

    And now I had the change to give the backplane a really good cleaning with compressed air.

    8M back online!

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #2
    Join Date
    Jul 2014
    Rhode Island


    I have found a few unsoldered component leads in DEC boards that worked fine for many years and then decided to stop working.
    Member of the Rhode Island Computer Museum

  3. #3
    Join Date
    Jun 2012
    UK - Worcester


    A warranty issue then !

    Nice detective work...


  4. #4


    That's a weird issue, all right! I don't think I've seen one quite this odd before. Thanks for sharing.


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