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Thread: Model 4P Boot Woes

  1. #1

    Default Model 4P Boot Woes

    Hello! I acquired a 4P in reasonably good physical shape a few months ago, and I've been trying to get it working ever since.

    When I received it, I had no disks, so I requested some from Ira Goldklang. The 4P will display the customary boot messages, asking for a floppy. If I hold F3 and P, it will ask for a Model 3 ROM; if I hold Period (as I recall), it runs a RAM test that displays no errors. So far so good.

    I received a bundle of floppies from Ira and set off to test them. But when I attempt to boot from any of them, the disk drive activity light comes on and stays on, and the CRT is filled with a single, repeating character, that appears to be randomly chosen. Often, it's a capital A with an umlaut over it, but I've seen other characters at times as well.

    First thing I tried was to replace the RAM chips with some extra 4164s I had around the house. While it passed the RAM test with the original ICs, I figured I might as well fill the second bank while I was at it. No change in booting, and the RAM test still runs without error.

    Second attempt: I acquired a new-old-stock (according to the seller) Z80A and swapped the CPU. (Thanks for socketing every major IC, Tandy!) No change. Still displays the boot messages without a floppy, still passes a RAM test, still displays a screen full of a single, seemingly random character when attempting to boot a disk.

    The last thing I tried, this morning, was to use a Gotek floppy drive emulator that I flashed FlashFloppy onto, according to the directions I found in the March 2019 issue of TRS8Bit. (Wonderful newsletter, I can't recommend it enough.) No change whatsoever.

    I'm somewhat disheartened at this point, and somewhat stumped as well. The next thing I can think of to try is to replace the disk controller IC, but I have yet to research prices on those, and I figured I'd come ask for advice before I spend any more money and time on this system. Anyone have any ideas for what I should try next? For what it's worth, I don't have access to an oscilloscope, and I wouldn't know what to do with one if I did, but I've got a multimeter around I can test things with if needed.

    Thanks, forum! Looking forward to hearing any advice.

  2. #2
    Join Date
    Jun 2015
    Location
    Sioux Falls SD
    Posts
    360

    Default

    Do you have the service manual? It has troubleshooting information. If you don't have it, search on archive.org

  3. #3
    Join Date
    Mar 2016
    Location
    Pikesville, Maryland
    Posts
    216

    Default

    I could sell you a replacement motherboard (very cheap) and you could try it out ...... it no change then send it back and get your money back

    Just make sure you have a model 26-1080A unit as this is the model # of the motherboards I stock (4P Gate Array)

    Visit my website at plaidvest.com/newsoft ............

  4. #4

    Default

    Are you sure the drive 0 floppy drive is healthy? Clean, aligned, running at the right speed. Maybe try switching it for drive 1. You just need to switch the jumper block found near the drive interface connector on the drives between the two so drive select is correct.

    Even just try unplugging/re-plugging the 34 wire drive cable into the drives and motherboard in case the connector are dirty - even though it sounds like you have had it apart to the point where this may have happened already.

  5. #5
    Join Date
    Jun 2015
    Location
    Sioux Falls SD
    Posts
    360

    Default

    Quote Originally Posted by spt87a View Post
    Are you sure the drive 0 floppy drive is healthy? Clean, aligned, running at the right speed. Maybe try switching it for drive 1. You just need to switch the jumper block found near the drive interface connector on the drives between the two so drive select is correct.

    Even just try unplugging/re-plugging the 34 wire drive cable into the drives and motherboard in case the connector are dirty - even though it sounds like you have had it apart to the point where this may have happened already.
    The 4P used a coded cable with pins removed for drive select. The drives themselves are selected as all drives. The TRS-80 line was consistent in this. It might have changed in the 4D due to double-sided drives.

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