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Thread: Just bought an Altair 8800 - Need some help!

  1. #1

    Default Just bought an Altair 8800 - Need some help!

    Greetings,

    I purchased an Altair 8800 for my collection today. The guy I bought it from got it for $1 at an auction! When I turn the power on, everything seems to power up just fine, but the 16 address bus lights do not turn off when I do a hard reset. Does anybody have suggestions on what I might need to do? I'd really like to get this computer working so I can take it to my daughter's school and do a demonstration. I can post pictures tomorrow of what is happening. Any advice would be greatly appreciated!

  2. #2

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    There are several 10's of things that can cause that. First thing to do is get something to trace the logic. Check online locations for schematics.
    If you have an oscilloscope that would be good but even a simple home made logic probe can do a lot.
    The front panel can be run with only the CPU card and front panel.
    Check to see if the CPU card is an original or some other. Some times people have upgraded with a newer processor board that doesn't support the front panel, other than the reset button.
    Post pictures.
    Dwight

  3. #3
    Join Date
    Jun 2014
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    + What Dwight said...

    Also:

    What cards do you have in the system? Without that we really have no idea what you are working with.

    I restored both and Altair and an IMSAI this year. Out of habit I recap the P.S. And anything that looks questionable on cards. On Both of those machines I completely rewired the power. The card edges are also a problem spot. You can clean them with a rubber eraser. Start with as simple of configuration as possible. CPU and 1 memory card.

    In the Altair I had a breakout card and tested all of the signal paths from the F.P. first before testing the processor and memory.

    Divide and conquer!

  4. #4

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    If the processor card used supports the front panel, there will be a separate ribbon cable between the front panel and the CPU board. This usually connect to the CPU card with a 16 pin dip. It only uses 8 wires as I recall but it can be connected upside down, causing data operations to fail, although the address looks OK.
    That is because C3 in hex is still C3 if the bits are reversed. The front panel uses C3, the jump instruction, to get the processor to put the address switches to the address buss.
    Dwight

  5. #5
    Join Date
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    When did MITS go to ribbon cable on the 8800? Mine is a bunch of cheap white stranded wire soldered to a molex connector.

    Also, if the LEDs are at about half-brightness, you should probably check the capacitors in the power supply. Most original Altairs have cooked their filter caps dry.
    Last edited by Chuck(G); October 20th, 2017 at 02:39 PM.

  6. #6

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    Quote Originally Posted by Chuck(G) View Post
    When did MITS go to ribbon cable on the 8800? Mine is a bunch of cheap white stranded wire soldered to a molex connector.

    Also, if the LEDs are at about half-brightness, you should probably check the capacitors in the power supply. Most original Altairs have cooked their filter caps dry.
    Oops, you're right, the Altair just used two connectors and 8 wires.
    The wires can still get swapped, with a similar problem as the IMSAI that used the ribbon cable.
    The circuits are almost the same.
    Many boards were sold for the IMSAI with the 16 pin dip. They could still be connected to the Altair front panel with a change in the connector.
    Many later processor boards did not support the front panel operation on either machine.
    The diodes use in the power supply of the Altairs were really cheap. I've replaced them in two machines for friends.
    After replacing capacitors, it is a good idea to look at the evenness of the ripple and note the drop across the diodes.
    Dwight
    Last edited by Dwight Elvey; October 20th, 2017 at 10:58 PM.

  7. #7
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    Yes, I have a Z80 board that I made an adapter for to go from the DIP to the Molex header. I might even still have the adapter, somewhere.

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