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Thread: Apple 3.5 inch Unidisk Drive Analog Card Schematic/Repair Advice?

  1. Default Apple 3.5 inch Unidisk Drive Analog Card Schematic/Repair Advice?

    So I have two Apple 3.5 inch Unidisk drives. One works fine, but the eject motor doesn't push the disc out all the way, but I can live with that. The other won't work at all. When I connect it to my //e, and insert a boot disc, the light flashes, then nothing. I've tracked the issue to the analog card in the drive, as when I swapped the drive itself between the two, the failure remained with the defective board. I've been poking at it with multimeters for months now with no idea what I'm looking for or luck in tracing the problem beyond the board.
    I really don't want to throw this thing out. Aside from the high cost of replacement, this was part of my grandfather's //e, so there's a lot of sentimental value as well. So if anyone has a schematic, or any advice on how to trace the fault, I would be very grateful.

  2. #2
    Join Date
    Oct 2016
    Cincinnati, Ohio USA


    First thing would be to replace the capacitors. They often short out when they get old and draw the voltages down too low for the drive to work. Next would be to swap chips between the working board and the defective board, if they are socketed.
    PCjr, DTK PC-XT Turbo, 386DX 33, 486 laptop, Pentium 120, Pentium III 500, various old laptops, Commodore Colt, all working. I also have a 286 that I need to see if I can repair.

  3. Default

    I replaced the large capacitors about a year ago, but it made no difference. Also, there are no socketed or through mount chips, only surface mount, so chip swapping is out of the question, as I do not want to damage my working board.

  4. #4


    I'm not a fan of replacing capacitors, unless you know the capacitor has failed. An ohm meter will show a short on a power rail, if that is an issue. I'm not that familiar with the 3.5 in drives but I've worked on the 5.25 in drives before. Can you show us some pictures of the drive without a cover on it?

  5. #5
    Join Date
    Jan 2012
    Connecticut, USA


    I have reworked a bunch of these 3.5 inch drives. Haven't had to replace any caps yet. Sometimes the eject failure is caused by the gear deteriorating. If that's not the case cleaning and regressing wakes up most of these drives just fine.

  6. Default How do I post a picture?

    I have no idea how to post a picture in this thread, however, I may have found the fault. There's a small surface mount I want to say diode that seems to have shorted out.

  7. Default

    /Users/fromadistantstar/Desktop/back board.jpg
    Lets see if this picture posts.

  8. #8
    Join Date
    Oct 2008
    Kamloops, BC, Canada
    Blog Entries


    You're gonna have to at the least attach it to the post or use an image hosting service like imgur. You can't just link directly to files on your own hard drive!
    [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
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    = Excellent space heater

  9. Default Boar pics

    back board.jpgtop board.jpg
    So my entire user interface just changed for this site last night, gotta say that I like it. Also his is the board I need a schematic for.


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