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Thread: Osborne 1 Repair

  1. #1
    Join Date
    Apr 2018
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    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/
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  3. #3
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    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?

  4. #4

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    It's always difficult to pinpoint failed 4116s unless they're burning hot. Before blaming RAM chips though make sure the PSU is ok. The Osborne 1 uses an Astec AC8151 variant putting out +12/-12/+5. The -12 gets regulated down to -5 for the RAM chips so if you don't have -12 you'll be in trouble. Check the 4116s for -5V on pin 1, +12 on pin 8 and +5 on pin 16.

    I know this won't help you in the short term but I got fed up tracking down dead 4116s in Commodore PETs so I built a tester using a ZX Spectrum. I put a ZIF socket on the left-most lower RAM location and use Benny Alford's diagnostic ROM. I can get through 16 4116s in a few minutes.
    www.binarydinosaurs.co.uk - UK home computer history
    Where RIFA capacitors come to die
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  5. #5
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    Default

    Quote Originally Posted by Witchy View Post
    It's always difficult to pinpoint failed 4116s unless they're burning hot. Before blaming RAM chips though make sure the PSU is ok. The Osborne 1 uses an Astec AC8151 variant putting out +12/-12/+5. The -12 gets regulated down to -5 for the RAM chips so if you don't have -12 you'll be in trouble. Check the 4116s for -5V on pin 1, +12 on pin 8 and +5 on pin 16.

    I know this won't help you in the short term but I got fed up tracking down dead 4116s in Commodore PETs so I built a tester using a ZX Spectrum. I put a ZIF socket on the left-most lower RAM location and use Benny Alford's diagnostic ROM. I can get through 16 4116s in a few minutes.
    That's a really good suggestion. I have another osborne, so I might stick a ZIF on that one.

    Thanks!

  6. #6

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    @Witchy: I did the same as you, testing DRAM on a ZX Spectrum. One of the DRAMs I tested blew the Spectrum up! I think it damaged the power supply but haven't got round to repairing it yet.

    I'd suggest the OP proceeds with caution on this one - be ready to disconnect the Spectrum's power the moment you see it has failed to boot. In my case I left it running for a few seconds before it blacked out.

    As the OP is in the US, he may find it difficult to procure a Spectrum. The US version was marketed under the name Timex Sinclair 2000 series.
    Last edited by JonB; June 19th, 2018 at 10:44 PM.

  7. #7
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    Default

    Building this will be well worth the effort in the time it saves you.
    https://fjkraan.home.xs4all.nl/digaud/arduino/Z80exer/

    I built one and used it to repair a trso80 model 1 with multiple faults that was proving very difficult to diagnose using normal methods. I have modified the code and laid out a shield pcb and schematic so it will using via the CPU socket or the model 1 expansion connector.

    Pair it up with a $10 logic analyzer off ebay and locating faults becomes easy. Instead of the confusing oscilloscope signals you see clean signals of just the operation you specify with no extraneous signals to confuse the issue.

    It takes about 90 mins to wire up and get working

  8. #8

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    That looks pretty nifty, particularly if it can pinpoint stuck RAM chips. Pity I don't have a spare Arduino Mega.
    www.binarydinosaurs.co.uk - UK home computer history
    Where RIFA capacitors come to die
    facebook.com/binarydinosaurs

  9. #9
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    Default

    Quote Originally Posted by David_M View Post
    Building this will be well worth the effort in the time it saves you.
    https://fjkraan.home.xs4all.nl/digaud/arduino/Z80exer/

    I built one and used it to repair a trso80 model 1 with multiple faults that was proving very difficult to diagnose using normal methods. I have modified the code and laid out a shield pcb and schematic so it will using via the CPU socket or the model 1 expansion connector.

    Pair it up with a $10 logic analyzer off ebay and locating faults becomes easy. Instead of the confusing oscilloscope signals you see clean signals of just the operation you specify with no extraneous signals to confuse the issue.

    It takes about 90 mins to wire up and get working
    that is fantastic. I didnt know about this. One more project I need to build.

  10. #10
    Join Date
    Sep 2016
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    Melbourne, Australia
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    Default

    I just laid out and ordered a shield that will plug directly into the Mega2560. I should have 20 bare boards in a little over a week.
    The shield I designed supports both the standard z80 cpu socket and the TRS-80 Model 1 expansion edge connector, I've modified to original code to support both.

    The board will look something like this.
    Untitled-1.jpg
    My component library didn't have a 3d model for the push button and the pins that plug into the arduino are missing, but you get the idea.
    You just need an Arduino Mega2560 ($10 on ebay) and to make the appropriate ribbon cables I'll be making the code available once I add a few enhancements.
    At this stage I expect I'll sell a bare board for about $10.

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