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Thread: The RICM Just Received a second MITS Altair 8800 Donation

  1. #1
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    Default The RICM Just Received a second MITS Altair 8800 Donation

    We recently received a second Altair 8800 donation, Serial Number 223482K. The front panel has one toggle-switch that won't toggle, and won't stop the processor. Altair #1 came with two spare toggle switches so we can fix the front panel in Altair #2. The unregulated 8VDC output in Altair #2 is about 13.9VDC, so we need to fix that. It makes the onboard regulators run really warm! We are not sure what is causing the high voltage. We will connect a 'scope and see if one of the diodes in the bridge is bad. It came with a Tanner Computers 64k Static RAM, Micropolis 1071 Floppy Disk Controller, 88-2 Dual SIO, MITS Revision 1 CPU, and a Micropolis 1023-II diskette. The SIO is configured for RS-232@1200 baud. That will be helpful for testing MS BASIC in the first Altair instead of waiting forever for it to load through the Teletype. Altair #2 came with a Godbout Econoram VII, PSS RAM8, Quantronics MM8, and a MITS 4k Static RAM board. After we fix the Econoram VII we will put the RAM boards in the Altair #1 to replace the misbehaving early design MITS 4K RAM boards.
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    The unregulated +8VDC voltage is really high, but we don't see any way to reduce it other than using a Variac to reduce the line voltage.

    We replaced IC J, an SN7473, in the front panel and now the RUN/STOP works. Since we can now stop the CPU the EXAMINE and DEPOSIT are starting to work. EXAMINE isn't loading the address switches and is actually doing an EXAMINE NEXT, so that will be the next project. The first Altair came with a spare toggle switch and a spare momentary contact switch. We replaced the toggle switch for bit-12 with a new switch, and will try to fix the broken one.

    We replaced the really early design 4k DRAM boards in Altair #1 with the static RAM boards that came with Altair #2. It works much better now. The next project is to program MS BASIC into EPROMs and install them into the Cromemco Bytesaver board.
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    We found a bad 7400 on the front panel that was letting EXAMINE NEXT signals leak into the EXAMINE function. We were able to toggle in a little program and run it and single step it. Time for more testing.

    Another Altair enthusiast write a program like dumprest that will let us send a MDOS diskette image through the serial console to a floppy. Then maybe it will boot and run.
    Member of the Rhode Island Computer Museum
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    I'd have to look around. But there is a MITS modification that uses a pass transistor and zener to preregulate the 8v rail. I made the mod to mine.



    Quote Originally Posted by m_thompson View Post
    The unregulated +8VDC voltage is really high, but we don't see any way to reduce it other than using a Variac to reduce the line voltage.

    We replaced IC J, an SN7473, in the front panel and now the RUN/STOP works. Since we can now stop the CPU the EXAMINE and DEPOSIT are starting to work. EXAMINE isn't loading the address switches and is actually doing an EXAMINE NEXT, so that will be the next project. The first Altair came with a spare toggle switch and a spare momentary contact switch. We replaced the toggle switch for bit-12 with a new switch, and will try to fix the broken one.

    We replaced the really early design 4k DRAM boards in Altair #1 with the static RAM boards that came with Altair #2. It works much better now. The next project is to program MS BASIC into EPROMs and install them into the Cromemco Bytesaver board.
    Spread the joy of Vintage Addiction

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    Spread the joy of Vintage Addiction

    -->www.chronworks.com/<--->www.i8008.net/<--

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    I asked the power supply designer at work for some help. It will be interesting to see what he recommends.

    I also looked at the LT1038. This part is like a giant LM7905 and can handle 10A. It would be easy to mount it on a heat sink and perfboard, configure it for 8VDC output, and wire it between the existing 13.9VDC and the backplane. It would also be easy to remove this modification in the future.
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    We worked on the SIO2 serial I/O board in the Altair #2 today. We know that the I/O address is being decoded correctly, and that a CPU wait state is being generated for an I/O Input instruction. We also know that the 6850 ACIA is not driving data onto the S-100 bus for an I/O Input instruction. Discovering why will have to wait for the next debugging session.
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    We spent the day checking all of the 2SIO logic that generates the control signals for the ACIA chip. Everything looked OK. When you read the ACIA Status Register the data signals from the ACIA were all about 2.0V, and never went to ground, so the register contents were all 1s. I bought an ACIA chip a few weeks ago to expand the 2SIO on the first Altair, so we removed both ACIAs to install a new one. We found that the Vcc pin on the ceramic Motorola ACIA was missing, and some of the other pins were about to fall off. With the new ACIA chip installed all of the Status Register bits were 0s, except for bit-0.

    We toggled in the 2SIO echo program and saw that the Altair was constantly streaming the last character entered on the terminal emulator. We also found that entering a 0 would echo a 1, 1 would echo a 2, and 2 would echo a 3. On the 'scope we could see that data bit-0 from the ACIA was always a 1.

    We toggled in a little program to write alternating 1s and 0s to the Control Register and looked at the ACIA data bus. We could see all of the lines wiggling except for bit-0. We are assuming that one of the SN74367 bus receiver/driver chips are bad and ordered replacements. Hopefully replacing the SN74367 will finally fix the 2SIO.

    The next project will be modifying the 2SIO for faster baud rates, and making MDOS diskettes. Then we will look in the warehouse for a period correct terminal to use with this system. Probably an ADM-3A.
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  9. #9
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    We replaced two SN74367 bus receiver/driver chips on the 2SIO board and both serial ports are now working OK. We jumpered the first serial port for 2400 baud. We will try 9600 baud next week.

    The next project is making an MDOS or CP/M floppy for this system. We toggled in a bootloader, downloaded the PC2FLOP program for the Micropolis floppy, and ran it. It asked which floppy drive we wanted to use, we entered 0, It lit the LED on the floppy drive and then it said that the floppy drive was not ready.

    Floppy_Not_Ready.jpg

    We disassembled the floppy to see if the motor was running. It runs all the time. We also found that we didn't have the drive door latched correctly so the floppy wasn't spinning. We ran out of time to make an MDOS floppy so we will try again next week.
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  10. #10
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    We made some good progress on the Altair today. The double-density Micropolis diskette drive would not write an MDOS diskette using the PC2FLOP program from Mike Douglas and his MDOS image. We tried a single-density diskette drive we borrowed from a TEI system. That drive worked much better and we were able to write the first half of the double-density diskette image onto the single-density drive. It booted MDOS and ran BASIC. We will try to fix the double-density drive next week.

    MICROPOLIS MDOS VS 4.0 - COPYRIGHT 1978
    >FILES
    DIR 03 0000
    RES 03 0014
    MDOS 0C 001A
    LINEEDIT 14 000E
    ASSM 14 0012
    SYMSAVE 14 0003
    FILECOPY 14 0003
    COPYFILE 14 0004
    DISKCOPY 0C 000B
    DEBUG-GEN 14 0020
    SYSQ1 04 0005
    SYSQ2 04 0007
    BASIC 0C 0050
    FEATURES 0C 0003
    UTILITY 10 0006
    >BASIC
    MICROPOLIS BASIC VS 4.0 - COPYRIGHT 1978
    READY
    PRINT "HELLO WORLD"
    HELLO WORLD
    READY
    PRINT 1 + 2
    3
    READY
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