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285RE: [Altair Computer Club] Re: New Vintage Computer Forum and a question

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  • Erik S. Klein
    May 6 8:28 PM
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      > You're lucky to have two machines to compare, even if they aren't the
      > same models. Troubleshooting is a lot tougher with just one computer.



      I'm certainly having an easier time troubleshooting now that I have a
      known-good configuration. I've had my 8800 and 8800a with their drives
      for about a year and a half but just got the BT and its drives a few
      months ago. When that machine booted up out of the box I was ecstatic
      because I knew I'd have my other machines figured out eventually.

      > I think you're examining the correct starting address of the DBL boot
      > ROM, but you mistyped it in your answer. The correct address to
      > examine then run is 177400 or hex FF00. You said 177000, which is
      > the address of the MBL cassette loader. You were correct, though,
      > when you said you raised the 8 switches for A15 thru A8.



      Yeah, I was trying to do Octal in my head to get the email out before a
      meeting. . . :-)



      If the 177400 address is for the MBL then I can have both mounted at the
      same time. I figured the MBL was set up for another address but I
      hadn't gone so far as to figure out what that was. . Thanks!


      > Are you using the same good I/O cable (the ribbon cable that goes
      > from the computer to the drives) with both computers? It's not
      > unheard of to have bad connections in them.



      I am using the same ribbon cable. I've got a bunch around, of course,
      with one for each drive. The cable within the machine from the disk
      boards to the connector in the back might be in question, but it seemed
      okay on physical examination. I may have to pull out the manual and
      meter and make sure it's pinned right and that there is connectivity
      where there should be.


      > You know the CPU board, PROM board, and DBL are all probably good or
      > you wouldn't be getting the head to load on the floppy. On the other
      > hand, the problem you're having can indicate bad RAM.



      Yeah. Swapping the 8800BT RAM in is a good idea. I'll have to do that
      soon. I should also find or write a quick RAM test program (I'm pretty
      sure there are some small ones in the older magazines like Byte or Dr.
      Dobb's. I have plugged in some reasonable code via the front panel, but
      never more then a few hundred bytes.

      > I would try to eliminate as many variables as possible. That is, use
      > the same known good RAM boards, controllers, cables, drives, drive
      > interfaces, diskettes, etc. in both machines. You can't use the BT's
      > Turnkey board, serial port (you're using the one on the turnkey
      > board, I assume), or CPU board, but the other boards should be OK in
      > either machine.



      Yep. I've done it all but the RAM including the boot PROM from the BT.


      > Have you checked the 3 system voltages on the 8800? Sometimes it was
      > easy to overload the 8800's -12V supply (the raw DC should be
      > about -16V on the buss and where it enters boards' -12V regulation
      > circuits). The supplies on the model B were relatively bulletproof.



      I've done that periodically while diagnosing other problems. I don't
      know if I've checked with the system fully loaded in the current
      configuration. When I swap in the RAM that's an easy test to perform so
      I'll do it then.


      > Be sure none of the fragile wires going from the mother board to the
      > front panel have broken loose at either end.



      I am quite certain that the internal wiring is 100%. I've tested every
      wire to the front panel, power supply, processor and motherboard and
      I've tested every pin on the buss for continuity. I'm confident that
      the problem wouldn't lie there. By the way, what type (solid or
      braided) and gauge of wire is used for those front panel lines? I need
      to rewire another Altair and I want to use something as close to
      authentic as possible.


      > PS to akessel- You should be able to leave or put the address
      > switches in any position after you have examined the correct starting
      > address of the boot ROM (177400). You shouldn't need to flip them
      > all down before hitting the RUN switch. Andy, are you sure your
      > computer really require all switches to be down before running?



      Actually, I think this depends on WHAT you're running. Disk BASIC
      requires "Sense Switches" to be set to indicate what communications
      device and parameters are to be used. These are the upper 8 address
      switches (as indicated by the silk screening on the front panel) and the
      configurations are part of the BASIC manual. I've tried all of the
      variations. The BT has a dip switch block dedicated to the sense
      switches.


      > By the way, Erik's method of raising STOP and RESET together is a
      > good one. Sometimes, but not always, the 8800/8800A would
      > get "stuck" and you couldn't stop or reset it to 0000 with just one
      > switch. The 2-switch method was the recommended action.



      I do want to thank you guys for all of your help and all of your
      insightful, knowledgeable responses. Like I said in my first email, I
      stumbled across this list by accident and I've gotten more information
      about this problem here then I have on any other forums or lists in the
      past year. This is the type of exchange I am hoping to get out of my
      web forum someday soon. . .



      Best regards,


      Erik

      www.vintage-computer.com <http://www.vintage-computer.com/>





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