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Altair 8800 Troubleshooting Help

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  • Rick Bensene
    Hello, all, I have a nice original Altair 8800 system that includes the MITS CPU, later 18-slot motherboard, MITS floppy controller, and two 8 MITS floppy
    Message 1 of 6 , Apr 2, 2014
      Hello, all,

      I have a nice original Altair 8800 system that includes the MITS CPU, later 18-slot motherboard, MITS floppy controller, and two 8" MITS floppy drives.
      It also as a MITS 2SIO, a trio of MITS 16K Dynamic RAM boards, and a MITS 2K ROM board. It was built from a kit (K serial #), by someone who was clearly skilled, as the soldering work looks very nice.

      The system ran beautifully up until yesterday, when a malfunction started. It started off intermittent, meaning that sometimes the system would work and other times, it wouldn't, and progressed to the point where it is malfunctioning continuously. When it is malfunctioning, the address lines count through the entire address space. RESET stops the operation as long as it is held on(as expected), but once released, it immediately resumes. Early on, as quickly as it started, the malfunction would stop, and the machine would run fine for a time, then, it'd go back to "counting" without the machine being touched at all. It is as if the processor is going into RUN state and the front panel is not able to bring it out of RUN state.

      When it is in malfunction mode, activating the STOP switch has no effect. Holding STOP, then pressing and releasing RESET doesn't stop the behavior. I've checked the STOP/RUN switch, and it is working properly. When the malfunction is occurring, the RUN/STOP flip flop doesn't toggle when the RUN/STOP switch is activated (alternately RUN, then STOP). However, during times that the machine is behaving (which is less and less frequently), it does toggle properly.

      I have removed all boards except one 16K memory board and the CPU, and the behavior persists. I have cleaned the edge connectors on the boards using De-Ox-It, and that didn't make any difference. I've reseated all of the IC's that are socketed, including the 8080A, and all fit firmly in their sockets. I've tried different memory boards, and even removed the memory completely, and it makes no difference.

      I've checked the +8V to the front panel board, and it's running at +10, but the regulator on the front panel board is giving the ICs +5.00 V.
      The main +8V to the motherboard is running at about +10V. The other motherboard voltages also check out within spec.
      I checked both the front panel and motherboard voltages with a scope, and ripple is minimal, so the caps in the power supply seem to be OK.
      I checked +5V at the regulator on the CPU board, and it too is good.

      The malfunction does not appear to be shock related -- I can wiggle the front panel and CPU boards around, and even lift the machine slightly and jar it and it doesn't seem to create the malfunction, nor make it go away. It also does not appear to be temperature related - when it was intermittent, it would do it when cold, and warm. Now that the behavior has become permanent, it will sit there counting through memory from cold start for an hour, with no difference in the behavior.

      Does anyone out there have any idea what might be going on with the machine? I'm wondering if anyone has had a similar experience, and may be able to provide some pointers as to where to look. I have all of the original MITS documentation for the machine, including schematics and board layouts. The front panel logic design with all the one-shots acting as de-bouncers and triggers for the EXAM/EXAM NEXT, DEPOSIT/DEPOSIT NEXT and SINGLE STEP functions makes me crazy. I'm wondering if one of the 74123's might have gotten flakey, then completely failed. If so, it could be messing with the PRDY signal, making the CPU think it's supposed to be running all the time. Are 74123's prone to failure?

      Working on the front panel is almost impossible with the hard-wiring to the backplane on the right side, and the AC Power and 8V supply wiring on the left side, pretty much locking the panel in place. I don't want to mess with undoing the front-panel to backplane wiring, but have thought of putting a connector in on the 8V and AC Power wires so that at least the left-hand side of the front panel board will be free, and then panel can then be tipped up and over to get to the reverse side of the board. Would installing such a modification be a "bad thing"?

      What do people think about using sockets when replacing chips? The original instructions have all of the ICs soldered to the board on the front panel. If something in the front panel logic has failed, would it be verboten to cut out the failed chip, clean out the holes, and put a high-quality socket in, so that in the event of issues in the future, IC's can be replaced more easily.

      I am not really concerned about the collector value of the machine, as I never intend to sell it. I just want to have this machine running again, and will do what is necessary to facilitate its repair, but am interested in opinions about making modifications to make the machine more serviceable. Clearly, MITS didn't design the front panel to be "service friendly".

      Any thoughts will be greatly appreciated.

      Thanks,

      Rick Bensene
    • John Crane
      Personally, I think a restored Altair with new clean machined sockets for the IC s and that is WORKING and MAINTAINABLE is far more valuable than an original
      Message 2 of 6 , Apr 3, 2014
        Personally, I think a restored Altair with new clean machined sockets
        for the IC's and that is WORKING and MAINTAINABLE is far more valuable
        than an original spec machine that is NONWORKING and/or UNMAINTAINABLE.

        Looks like you've eliminated the obvious systemic issues related to
        age. To save yourself grey hairs over troubleshooting the logic, use
        the age-old technician's method of "shotgunning". Those chips are cheap
        - much cheaper than your time. And eventually more will fail. So
        replace all of them with newer ones.

        That's how I resurrected my Dazzler ;)

        -John

        On 04/02/2014 11:35 AM, Rick Bensene wrote:
        > Hello, all,
        >
        > I have a nice original Altair 8800 system that includes the MITS CPU, later 18-slot motherboard, MITS floppy controller, and two 8" MITS floppy drives.
        > It also as a MITS 2SIO, a trio of MITS 16K Dynamic RAM boards, and a MITS 2K ROM board. It was built from a kit (K serial #), by someone who was clearly skilled, as the soldering work looks very nice.
        >
        > The system ran beautifully up until yesterday, when a malfunction started. It started off intermittent, meaning that sometimes the system would work and other times, it wouldn't, and progressed to the point where it is malfunctioning continuously. When it is malfunctioning, the address lines count through the entire address space. RESET stops the operation as long as it is held on(as expected), but once released, it immediately resumes. Early on, as quickly as it started, the malfunction would stop, and the machine would run fine for a time, then, it'd go back to "counting" without the machine being touched at all. It is as if the processor is going into RUN state and the front panel is not able to bring it out of RUN state.
        >
        > When it is in malfunction mode, activating the STOP switch has no effect. Holding STOP, then pressing and releasing RESET doesn't stop the behavior. I've checked the STOP/RUN switch, and it is working properly. When the malfunction is occurring, the RUN/STOP flip flop doesn't toggle when the RUN/STOP switch is activated (alternately RUN, then STOP). However, during times that the machine is behaving (which is less and less frequently), it does toggle properly.
        >
        > I have removed all boards except one 16K memory board and the CPU, and the behavior persists. I have cleaned the edge connectors on the boards using De-Ox-It, and that didn't make any difference. I've reseated all of the IC's that are socketed, including the 8080A, and all fit firmly in their sockets. I've tried different memory boards, and even removed the memory completely, and it makes no difference.
        >
        > I've checked the +8V to the front panel board, and it's running at +10, but the regulator on the front panel board is giving the ICs +5.00 V.
        > The main +8V to the motherboard is running at about +10V. The other motherboard voltages also check out within spec.
        > I checked both the front panel and motherboard voltages with a scope, and ripple is minimal, so the caps in the power supply seem to be OK.
        > I checked +5V at the regulator on the CPU board, and it too is good.
        >
        > The malfunction does not appear to be shock related -- I can wiggle the front panel and CPU boards around, and even lift the machine slightly and jar it and it doesn't seem to create the malfunction, nor make it go away. It also does not appear to be temperature related - when it was intermittent, it would do it when cold, and warm. Now that the behavior has become permanent, it will sit there counting through memory from cold start for an hour, with no difference in the behavior.
        >
        > Does anyone out there have any idea what might be going on with the machine? I'm wondering if anyone has had a similar experience, and may be able to provide some pointers as to where to look. I have all of the original MITS documentation for the machine, including schematics and board layouts. The front panel logic design with all the one-shots acting as de-bouncers and triggers for the EXAM/EXAM NEXT, DEPOSIT/DEPOSIT NEXT and SINGLE STEP functions makes me crazy. I'm wondering if one of the 74123's might have gotten flakey, then completely failed. If so, it could be messing with the PRDY signal, making the CPU think it's supposed to be running all the time. Are 74123's prone to failure?
        >
        > Working on the front panel is almost impossible with the hard-wiring to the backplane on the right side, and the AC Power and 8V supply wiring on the left side, pretty much locking the panel in place. I don't want to mess with undoing the front-panel to backplane wiring, but have thought of putting a connector in on the 8V and AC Power wires so that at least the left-hand side of the front panel board will be free, and then panel can then be tipped up and over to get to the reverse side of the board. Would installing such a modification be a "bad thing"?
        >
        > What do people think about using sockets when replacing chips? The original instructions have all of the ICs soldered to the board on the front panel. If something in the front panel logic has failed, would it be verboten to cut out the failed chip, clean out the holes, and put a high-quality socket in, so that in the event of issues in the future, IC's can be replaced more easily.
        >
        > I am not really concerned about the collector value of the machine, as I never intend to sell it. I just want to have this machine running again, and will do what is necessary to facilitate its repair, but am interested in opinions about making modifications to make the machine more serviceable. Clearly, MITS didn't design the front panel to be "service friendly".
        >
        > Any thoughts will be greatly appreciated.
        >
        > Thanks,
        >
        > Rick Bensene
        >
        >
        >
      • Rick Bensene
        Here s a (good news) follow-up to the woes with my Altair 8800-- After digging around with a scope, I found that the RUN/STOP flip flop, made up of a 7400 Quad
        Message 3 of 6 , Apr 5, 2014
          Here's a (good news) follow-up to the woes with my Altair 8800--

          After digging around with a scope, I found that the RUN/STOP flip flop, made up of a 7400 Quad 2-in NAND IC, wasn't changing state when the RUN/STOP switch was moved to the RUN or STOP positions. I tracked it down to one gate that wasn't working, which was causing the RUN state to be active all the time. I carefully removed the faulty 7400, put a machine-turned socket in its place, and put a date-correct (1975) replacement part in, and fired the machine up.

          Things were better, in that the RUN/STOP flip flop would properly change state when the RUN/STOP switch was used, but the machine would still stay running all the time.

          Again, more scoping, and this time, I found that the PRDY signal going into an 8T97 open collector bus driver IC was moving when RUN/STOP was used, but the output on the S100 bus was not. The 8T97 appeared to have gone bad.
          So, again, the chip was removed, and a socket put in its place, and a replacement chip put in (again, date-correct), and the machine fired up, and this time, everything works as it should.

          I find it odd that two chips went away at the same time, but perhaps the 8T97 went bad first, and somehow caused problems with the 7400.

          I did put a connector in on the left side of the front panel to provide an easy way to disconnect the AC power and 8V power supply lines from the front panel. It's not "stock", but it makes it possible to get to the back side of the front panel board for the work that I had to do to it. I used a 4-pin Molex socket pair commonly used for providing power to older disk drives. Works great.

          While I was messing with things, the machine has an old MITS PROM board that uses 2708 EPROMS. For some reason, the socket containing the Floppy Disk boot EPROM at 0xFF00 wasn't working. I had read the EPROM with a ROM programmer, and it was fine. However, it would always read back as all zeroes when in the Altair. To use the ROM, I had moved it to a different socket (0xFC00) on the EPROM board, and it would read fine there. I'd just toggle in the relocate program that copies the boot code to RAM at 0x4C00, and would boot the system that way, but that was a hassle.

          I traced the problem to the -9V switching circuit for the two EPROMs at 0xFE00 and 0xFF00 (the ROMs have their Vgg voltage switched in pairs of two on this board to minimize power consumption on the -9V supply). One of the transistors in the Vgg switching circuit for this bank was bad, so the Vgg was always at +5V, the "low power" mode for the 2708. I replaced the transistor that was bad, and the ROM then worked at 0xFF00, so now I just examine 0xFF00, set the sense switches to 0x10, and I can boot up Disk Extended BASIC or MITS DOS from floppy quickly and easily.

          Rick Bensene
        • deramp5113
          Great work! It is frustrating when seemingly random failures take down a machine that you have fixed up and working. I have to keep reminding myself lots of
          Message 4 of 6 , Apr 5, 2014
            Great work! It is frustrating when seemingly random failures take down a machine that you have fixed up and working. I have to keep reminding myself lots of the parts are nearing 40 years old and that finding and fixing the problems is "fun!"

            It's good to find out there is one more collector out there with a working Altair floppy on their Altair. They're becoming extremely rare!

            Mike

          • Michael George Hart
            I have a working Altair 8800 system but zero software Sent from my iPad
            Message 5 of 6 , Apr 5, 2014
              I have a working Altair 8800  system but zero software


              Sent from my iPad

              On Apr 5, 2014, at 12:31 PM, <deramp5113@...> wrote:

               

              Great work! It is frustrating when seemingly random failures take down a machine that you have fixed up and working. I have to keep reminding myself lots of the parts are nearing 40 years old and that finding and fixing the problems is "fun!"


              It's good to find out there is one more collector out there with a working Altair floppy on their Altair. They're becoming extremely rare!

              Mike

            • Rick Bensene
              What hardware configuration does your machine have? Do you have any kind of disk drive on your system? Without any mass storage, the only practical way to put
              Message 6 of 6 , Apr 5, 2014
                What hardware configuration does your machine have? Do you have any kind of disk drive on your system? Without any mass storage, the only practical way to put software on the machine is to use a loader that transfers software over a serial port, or using a paper tape reader. There are loader ROMs that can be used to receive software from a serial port, if your system has ROM board. With a PC and terminal emulation software hooked up to the serial port, you can stimulate loading paper tape images at 9600 baud, which is time consuming, but works well. There are many sources for software on the Internet that can be downloaded, including most of the various BASICs that were made for the machine. If you do have disk drives, then you need to find someone who can write some disks that are compatible with your disk system, with software on them. I can't make copies right now, as my second 8" drive is currently down. ...That is the next thing on my project list.

                Rick Bensene


                -----Original Message-----
                From: Michael George Hart [michael.george.hart@...]
                Received: Saturday, 05 Apr 2014, 1:39PM
                To: altaircomputerclub@yahoogroups.com [altaircomputerclub@yahoogroups.com]
                Subject: Re: [Altair Computer Club] Re: Altair 8800 Troubleshooting Help

                I have a working Altair 8800 system but zero software


                Sent from my iPad

                > On Apr 5, 2014, at 12:31 PM, <deramp5113@...> wrote:
                >
                > Great work! It is frustrating when seemingly random failures take down a machine that you have fixed up and working. I have to keep reminding myself lots of the parts are nearing 40 years old and that finding and fixing the problems is "fun!"
                >
                >
                > It's good to find out there is one more collector out there with a working Altair floppy on their Altair. They're becoming extremely rare!
                >
                > Mike
                >
                >
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