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Thread: Issue with TRS-80 Model III

  1. #1

    Default Issue with TRS-80 Model III

    I am trying to get my dad's old TRS-80 Model III up and running. One of the issues I am having a problem solving is that it boots up into BASIC (some times), and when it does almost every command causes the PC to reboot.

    My guess is that it is an issue with one of the ROM chips. Some commands do not make it crash (like CLS), but they will end up putting junk on the screen then it will come back to a ">".

    I have removed the RAM and moved other slots around with no luck. I also noticed that long as I do not run any commands and just type in line numbers with commands it will not crash.

    Any help would be greatly appreciated.

  2. #2
    Join Date
    Mar 2013
    Chaffee, MO


    The first thing to do is start with Power "OFF". Depress and Hold DOWN the BREAK KEY. Turn on
    POWER while continuing to hold BREAK KEY DOWN. When you see cass?? release the break key and
    tap the ENTER key three times. You should be at Cassette Basic. Type in the following command to
    see how much Memory is available.


    It should be around 48082 (going from my memory) If that is correct then it is seeing all available
    memory. From there you can load software from the cassette interface. The cassette Basic has
    500 and 1500 Baud rates that load the software from the cassette tapes.

    Once in Cassette Basic, insert a floppy and just depress the RED Reset button to boot a Floppy.
    It should boot to the Operating system. TRSDOS, Multidos, NewDos, LDOS, or any of the multi
    flavors of OS's.

    When you saw the cass?? You could have typed an L (500 Baud) or H (1500 Baud) for the tape
    interface load/save rate. Then the Enter key will dump you to the Cassette Prompt.


  3. #3


    It's important to know if this is happening with a booted Disk BASIC from disk or from the built in ROM BASIC. Let us know after following Larry's steps.

  4. #4


    One of the issues I have at this time is I have no floppy disks for the system, and the power supply for the floppies does not work (I am working on fixing that).

    I was trying to find a virtual cassette that would help me trouble shoot the issue.

    I did try a PRINT MEM at the basic command prompt, but it just restarts the computer.

  5. #5
    Join Date
    Mar 2013
    Chaffee, MO


    Before you do much else get into cassette basic and just type A thru Z, and see if those characters
    are displayed. It they are OK type enter and then repeat for 0 thru 9. It just checks to see if
    the keys typed are displayed.

    If that is good, then power off the computer. lay it on it's side and remove all screws from the
    bottom. Then set it back on the base, remove the one single screw on the back. While looking down
    from over the computer watch the neck of the CRT and slowly and carefully remove the top
    of the case from the base. Raise straight up while making sure the neck of the CTR is not
    damaged. Just lay the top over towards the left and on it's side. The cables will all still be

    Then remove the three ROMS. Power up the computer and see if the display fills with the
    following repeating characters @9@9@9@9@9@9@9@9@9 for all 64 characters across
    and down the 24 lines. Hopefully, that will be fine. If it isn't post a good photo of what you see
    and tell us exactly where the characters are different.

    Also Check the Power Supply voltages with a known good Voltmeter. The +5 VDC, +12 VDC,
    and the -5 VDC Power Supply. (The -5 VDC is behind the Motherboard in the Top Right Section
    on the chassis. It goes to the RAM IC's.)

    Next step is getting a SPARE Z80 CPU to make an Address Line tester. You will need an O'scope
    or a way to see the signals on A0 through A15 making sure each will toggle from the +5 VDC rail
    to Ground. No Address line should be stuck HIGH, LOW, or at .7 VDC.


  6. #6


    So the voltages look correct, I get about 4.8v on the +5 VDC rail.

    I did remove the ROMS and booted it up and the first time I get a square block. I have attached the photo. Every time I turn it off and turn it back on I get differnt char's, but most times it is the same char on the whole screen. I never got the @9 on the screen.

    I pulled out my O'scope and checked the address lines and they go from 0 to 3.6~3.8v on the address and data lines. That seams a little low to me for 5 volt logic, but maybe that is correct.

  7. #7


    I am actually not sure how to attach a photo to this message.

  8. #8


    While it could be bitrot in the ROMs, i'd think that the RAM would be a more likely candidate. My only encounter with one of these computer was down to bad RAM - it was all getting very hot.

  9. #9


    Quote Originally Posted by Robbbert View Post
    While it could be bitrot in the ROMs, i'd think that the RAM would be a more likely candidate. My only encounter with one of these computer was down to bad RAM - it was all getting very hot.
    So after doing some work I built my own ROM dumper and compared the ROMs and the middle ROM 0x2000-0x3000 address is not working correctly.
    It returns data, but its different each time I dump it. Looking at the ROMs used for the emulators my 2 other ROMs match, but the middle one does not.

    So does anyone know where you can get that ROM? Its the 4K ROM for the Model III.

  10. #10


    So after doing a test with the ROMs I found that the middle ROM (I think the BASIC ROM?) was bad. I was able to program an EEPROM and make it work in the socket. The computer now boots up and works correctly.

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