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Thread: Osborne 1 Repair

  1. #1
    Join Date
    Apr 2018
    Location
    Fort Worth, TX
    Posts
    2

    Default Osborne 1 Repair

    Hello,

    I bought an non-working Osborne off EBay in hopes of repairing it in my spare time, but I seem to be way in over my head in terms of debugging prowess. I'm curious if I could get a little bit of help.

    The symptoms are as follows:
    • Won't boot
    • Beeps continuously
    • CRT shows boxes and random characters


    I've been poking around with a scope and found a few oddities. I'm no electrical engineer, so I could be barking up the wrong tree...

    So far I've replaced the Z80A, the '393s in A13 and B14, and the '175 in D14 to no avail. I chose to replace those on a hunch after looking around with my scope. I've included a few photos of the waveforms I think seem problematic.

    Any way, any suggestions or tips would be appreciated!

    Thanks!

    IMG_0072.jpg
    74LS393 in B14 : pin 12 (CLK)

    IMG_0073.jpg
    74LS393 in B14 : pin 11 (2QA)
    Notice the "blip" that only lasts a few ns.

    IMG_0074.jpg
    MCM4116 : pin 2

    IMG_0075.jpg
    Z80 : pin 24 (WAIT)

    IMG_0076.jpg
    74LS175 in D14 : pin 7 (Q1)

    IMG_0079.jpg
    CRT

  2. #2

    Default

    Could be almost anything, but my hunch would be RAM. Those 4116 chips are not the most reliable of things. In memory mapped devices like these, bad RAM can make the machine exhibit all kinds of crazy behaviour...lights flashing...drives on...screen with garbage on it.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  3. #3
    Join Date
    Apr 2018
    Location
    Fort Worth, TX
    Posts
    2

    Default

    I figured that much. I have 12 4116s on-hand, but I can't find a faulty IC by piggy-backing the chips. Do you know the typical failure mode of the component?

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