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Thread: Altair 8800 restoration question

  1. #11
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    The L00 part on the front panel can't use an unvarnished LS00 or --00 in place of the original L00; MITS used a wired-OR (or wired-NAND if you will). That one stumped me. I solved it by running the wired-OR outputs through a couple of 1N914 diodes and using a standard 7400. There are extra diode drops involved, but it got me out of a jam.

    That's my memory from 40+ years ago. Doubtless MITS got rid of that in their later versions. I thought it pretty sneaky. It would have been worse had MITS slipped in an L86--that tripped me up on another piece of gear. The L part has a different pinout from the --,S, LS, etc. parts.

  2. #12

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    A simple check of the data lines to the front panel display would be to remove the RAM boards and ground the DI pins on the bus, one at a time.
    Dwight

  3. #13

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    The resistors that parallel the regulator were chosen for a different IC family. They may be too much or too little.
    Dwight

  4. #14

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    It is not clear, do the address lines also light now when you increment the examine?
    Dwight

  5. #15

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    So quick update: I will be getting my logic probe out of storage this weekend, and I just bought a new reproduction MITS 16k SRAM board on ebay that I will be buying components for and assembling soon. Apparently there is a guy selling everything from reproduction display boards, to 18 slot backplane boards complete with Sullins connectors on ebay. LOL

  6. #16

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    Quote Originally Posted by Dwight Elvey View Post
    It is not clear, do the address lines also light now when you increment the examine?
    Dwight
    When you reset, your video showed the address lights all lighting because of the buffers are going 3state. My question is that do all the addresses light in sequence when you increment the address. This is controlled by the value that the CPU sends out to the buffers and then to the buses address lines. Since this is a different control path, that is why I asked. In a previous video, I could see that the address that the CPU had internally was correct but was not making it to the bus, or the RAM card was shorting out the address lines. You don't need to do another video but do the address lines sequence as expected with the RAM card removed?
    Dwight

  7. #17

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    Just checked, and yes they do. They increment in sequence perfectly with the RAM card removed.

  8. #18

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    That is good and bad to hear. ( bad for the memory card )
    With the boards removed, you can do a quick check data in lines by grounding them on the bus. It is a simple test to make sure when you have a working board, you are not seeing an issue with the bus or the processor. Before running such a test, put the address lines all high. It will help to look for any possible shorts on the mother boards. Unless you've made a pass through the board with an ohm meter you can't be sure there isn't a hidden short someplace. You have to measure across and also diagonally across ( I'd have to have a board in my hand to tell you which diagonal ). This all depends on the direction the wires zig to get around the pin on the opposite side. I helped a friend bring one back to life that was said, by the seller, to have worked at one time. I found 4 shorts on the mother board from the original soldering. Just having it work as good as you have now is not an assurance that there are no shorts.
    There must be someone else in your area that you could borrow a memory board from.
    It is a good thing you replaced the power supply parts. Another machine, I'd worked on, had a number of bad diodes.
    Dwight

  9. #19

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    I'll check for any shorts on the motherboard a bit later today or possibly tomorrow, but as for borrowing a memory board from someone, I'll have to do some digging. The old MITS building where the Altair 8800s were built and shipped from is just down the street from where I live here in Albuquerque, but it seems I'm the only one I know of so far in ABQ that still has one. So I may have to wait and assemble the Static ram board I have being shipped to me. I CAN tell you that I noticed some corrosion on various pins of several IC's, and ceramic caps on the DRAM board I was using which I cleaned off as best I could, I also replaced the ceramic caps on the DRAM board, but none of that seemed to help. I also noticed that various jumper wires on my DRAM board don't seem to be going to the right spots. I dunno if it was rewired for something else or what but J10 for example is wired to J2 instead of the other J10, the second J10 is wired to pin 49 on the board ect. This board was bought off ebay a while back. I can also confirm that voltage is going to the right pins on the DRAM board at least. The + and minus 15 and the +5 seems a little high at +19 -19, and +9 respectively, but the onboard regulators seem to be doing their jobs the 7805 shows +5.0, and the 7812 shows +11v. They don't seem to be getting too warm either considering the higher voltage on the bus. I think the higher voltage is due to a combination of me not having hardly any load on the bus. I check it with only the RAM card in, and the new transformers with a slightly higher voltage on the winding than the originals. However they were recommended on here as the suitable replacements if I didn't want to go the switching power supply route.

    IMG_20190815_185158564.jpgIMG_20190815_185213465.jpgIMG_20190815_185220305.jpgIMG_20190815_185329741.jpg
    Last edited by k1atn; August 15th, 2019 at 06:16 PM.

  10. #20

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    Here is a manual for your board.
    http://www.s100computers.com/Hardwar...S/88_4kdrb.pdf

    Be careful about that extra regulator. It may be for the negative rails of the RAMs. You'll have to look at the manual to see if the jumper wires make sense.
    It wasn't a good idea to replace the ceramics. Besides the fact that I've only seen one bad ceramic in my entire life time, some are turned for timing. Those that are near the 26L123 are just such capacitors. The electrolytic caps are more likely to go bad.
    Anyway the manual should help with most of the jumper and the address selects.
    I believe this is the schematic for it:
    http://www.s100computers.com/Hardwar...0Schematic.pdf
    Dwight

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