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Thread: Superbrain QD repair

  1. #81
    Join Date
    Oct 2016
    Location
    Burnley, UK
    Posts
    163

    Default

    These updates are drifting - the endpoint elusive!

    I've recapped my one working vdu driver board. It now does not work. At least I have nothing on the screen now. I managed to work out that I'd blown an NPN power transistor - the one on a heatsink. Also didn't realize that the cap that smoked was a bipolar type, so I've swapped that cap and transistor from spare boards. I've now got 15v back at the 4th pin on the connector plug - before I replaced the transistor, it was down to 6.5v, so it must have been pulling the supply voltage down.

    The issue could also be the cheapness of the electrolytics I bought - off ebay. Too cheap to be true! I'll stick with Didgikey in future!

    It's possible my driver board is ok but maybe ive damaged something on the psu/logic board. Ran out if time to test for Hor / vert / vid signals with my scope - I spent most of the afternoon removing and socketing the RAM chips on a friends TI-99 motherboard.

    Try again tomorrow!

  2. #82

    Default

    Keep the faith Alan, and keep plugging on. You will get there in the end, I am sure.

    I'm at the stage where I have a uIDE boot but the screen is getting confused. Probably caused by something wrong in my "hidden BIOS" disassembly. But it is booting from the IDE device (it gives floppy access only - so the uIDE is merely replacing the floppy at boot). Once I have this working I can integrate the IDE driver code and we'll be away..

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