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Thread: Solid State Music RAM board problem (Altair 8800)

  1. #1

    Default Solid State Music RAM board problem (Altair 8800)

    So it appears that I got my Altair 8800 working (from a completely non-working state), but the problem I'm having is the D0-D7 LEDs are always in the on position. Hard reset works, stepping through the addresses works, but reading and writing to them do nothing to change their state.

    I've never repaired anything RAM related, so it's kind of new territory. I've re-seated every chip, swapped the ones that are identical to see if there's any change, but nothing's changed. I replaced a bad voltage regulator, but that's the only thing I've found wrong with the board. Bit 7 on the dip switch doesn't work, but from what I've read, the dip switches are only used for testing the RAM chips themselves.

    My next guess is to re-cap the entire thing, although I'm not quite certain on how to read the red capacitors. I'm guessing these are rated at 0.1 uF, but I'd like confirmation on that.

    Has anyone re-capped one of these boards and had it work afterwards?

    altairram1.jpgaltairram2.jpg

  2. #2
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    Are you sure you're at the right address for the board? Set up what you think is the right address, do an EXAMINE, and probe the output of the address decoder circuit with a logic probe (and, if you don't have a logic probe, get one!). I've got the same SSM 4K board and have been unable to find documentation on it, but it's a pretty straightforward board. Should be pretty easy to find the address compare output and trace it out.

    FYI, I generally start testing with either a ROM board or my debug board, which allows setting a known bit pattern on DIP switches. This will help you catch mis-wired front panel cables, bad bus drivers, etc.

  3. #3
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    I doubt that those bypass caps are giving you any problems, so I'd skip over that for now.

    I don't know where you got your information about the DIP switches, but I seem to recall that on these boards (and the 8K SRAM board), the switches set the base address as well as the "memory protect" state--i.e. when a block of memory is protected, you can't alter the contents--this was useful back in the days before affordable disk storage when most programs were toggled in through the front panel--you didn't want your hard work to go *poof* because of a coding or entry error. I think the remaining DIP switch sets the number of wait sates (memory, especially SRAM, could be pretty slow back in the day).

    I don't have the manual for your board, as it precedes the very popular (for SSM, anyway) 8K board.

    After verifying the DIP switch settings, i'd verify the output of each of the voltage regulators. I'd also re-seat the socketed interface chips.

  4. #4

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    Quote Originally Posted by glitch View Post
    Are you sure you're at the right address for the board? Set up what you think is the right address, do an EXAMINE, and probe the output of the address decoder circuit with a logic probe (and, if you don't have a logic probe, get one!). I've got the same SSM 4K board and have been unable to find documentation on it, but it's a pretty straightforward board. Should be pretty easy to find the address compare output and trace it out.
    I had no clue I had to do any of this! I don't even know where to begin! All I did was get the Altair functional (I'm a first time owner).

    I don't have the manual for your board, as it precedes the very popular (for SSM, anyway) 8K board.
    Although I have the assembly manual and test procedures that originally came with my Altair, I don't have any documentation for the RAM board. I tried looking for some online, but it doesn't seem to exist.

    I seem to recall that on these boards (and the 8K SRAM board), the switches set the base address as well as the "memory protect" state
    So I have to set up the address for the RAM board with the dip switches? My apologies if I'm not understanding clearly how this all works.

  5. #5

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    You should be able to get some documentation here.

  6. #6

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    The switches are likely in sets of 4.
    Try putting the sets of 4 in all 4 combinations.
    Use address 0 and write a 10101010 pattern.
    Another option is to trace the S100 address lines and the wires
    to the switches.
    Dwight

  7. #7
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    One configuration you could try which I've seen on a couple of older 4K boards is where the 8 switches are configured as two sets of 4. The first four switches selects one of four 16k banks within the 64k address space and then the second set of four switches selects the 4k bank within the selected 16k.

    Eg. If this is the only card in the system you would set it to 10001000 (First 4k in the first 16k bank)

  8. #8
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    A bit on the switches

    I don't see the 4K static RAM board in Herb's Retrotechnology list.

  9. #9
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    Quote Originally Posted by Bungo Pony View Post
    So it appears that I got my Altair 8800 working (from a completely non-working state), but the problem I'm having is the D0-D7 LEDs are always in the on position. Hard reset works, stepping through the addresses works, but reading and writing to them do nothing to change their state.

    I've never repaired anything RAM related, so it's kind of new territory. I've re-seated every chip, swapped the ones that are identical to see if there's any change, but nothing's changed. I replaced a bad voltage regulator, but that's the only thing I've found wrong with the board. Bit 7 on the dip switch doesn't work, but from what I've read, the dip switches are only used for testing the RAM chips themselves.

    My next guess is to re-cap the entire thing, although I'm not quite certain on how to read the red capacitors. I'm guessing these are rated at 0.1 uF, but I'd like confirmation on that.

    Has anyone re-capped one of these boards and had it work afterwards?
    this appears to be a SSM MB-2
    https://archive.org/stream/TNM_Solid..._20170922_0031
    MB-4 looks similar, and refers to it as an improved MB-2

  10. #10

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    Quote Originally Posted by Chuck(G) View Post
    A bit on the switches

    I don't see the 4K static RAM board in Herb's Retrotechnology list.
    I was thinking the MB4 would be the card. Maybe not.

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