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Thread: Turbo XT clone - dead motherboard

  1. #1
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    Unhappy Turbo XT clone - dead motherboard

    Hi,

    I have one of those Taiwanese XT clone boards like the one discussed in this Russian forum

    http://www.phantom.sannata.ru/forum//index.php?t=26329

    But mine looks much better

    IMG_152654530174F.jpg

    despite of its gorgeous appearance it does not boot Appareance it's not everything

    I have followed minuszerodegrees Minimum Diagnostic Configuration (is this trademaked or patented? ) with the following results:
    - There are no beeps
    - PSU fan works (I know for sure that the PSU it's OK since I got it from a working system)
    - I do not have a multimeter to check voltages but some chips on the board get warm included the 8088-1 so I think there are not shorted capacitors
    - I have tested several RAM chips on bank 0 (I left populated only this bank) unsuccessfuly
    - I have noticed that the the DM74S74N flip flop in U39 gets extremely hot It burns when touched and I was afraid that it would catch fire

    Se the photo:

    IMG_152654535503F.jpg

    Any ideas ?? Could this be the root cause of the MB failure?

    Thanks

    BTW the MB has a stick over the BIOS chip that read TD3.91, there is this BIOS dump on minuszerodegrees:

    BIOS source Motherboard: Unbranded XT clone (CPU: 8088 @ 4.77 MHz)
    Supplier giobbi at the Vintage Computer Forums
    BIOS chip type 2764
    BIOS contains the strings "86 TD3.86 ID: 75102637"
    "COPAM 1985"
    "Author: Thomas Lao"
    Comment BIOS chip is labelled: TD 03.86
    Download Clone XT BIOS - TD3.86 ID 75102637.bin
    and in the Russian forum they have posted version TD3.93 I repost it here in case there is someone interested (I have tested it on PCem and boots fine)

    td393.zip
    Last edited by dieymir; May 17th, 2018 at 11:12 AM.

  2. #2
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    Default

    I can't see your second photo--I get the message:

    Invalid Attachment specified. If you followed a valid link, please notify the administrator

  3. #3
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    Quote Originally Posted by Chuck(G) View Post
    I can't see your second photo--I get the message:
    Corrected, thanks

  4. #4
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    Unfortunately, replacing ICs randomly will not be a very fruitful experience. You'll need some tools; even a logic probe and cheap multimeter would be great improvements at this point.

    Otherwise, you're in the position of one trying to repair a vintage auto and having only a rusty screwdriver.

  5. #5
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    If you have access to an EPROM burner you could try the 5160 Supersoft Diagnostics ROM.
    I think they may work on a Non-IBM board.

    http://minuszerodegrees.net/supersof...mark%20ROM.htm

    I've had several boards that appeared totally dead, and SuperSoft ROMs at least identified
    the area of the error. The chip getting that hot doesn't sound good, I would check the PS
    voltages and see if they are correct.

  6. #6

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    Quote Originally Posted by dieymir View Post
    - There are no beeps
    - PSU fan works (I know for sure that the PSU it's OK since I got it from a working system)
    - I do not have a multimeter to check voltages but some chips on the board get warm included the 8088-1 so I think there are not shorted capacitors
    - I have tested several RAM chips on bank 0 (I left populated only this bank) unsuccessfuly
    - I have noticed that the the DM74S74N flip flop in U39 gets extremely hot It burns when touched and I was afraid that it would catch fire

    First, as mentioned, trying to troubleshoot without some basic tools is pretty much futile. A half-decent set of basic tools shouldn't set you back more than $10 (US). A cheap DVM (sometimes free with Harbor Freight coupons) would be another $5-$10. It could still tell you voltages and check for shorted components.

    Having said that, (In reverse order of your diagnosis) I'd say that given your description, the 74S74 is likely bad. Problem being is that you don't know if it's just the IC or something driving it too hard (adjacent filter cap shorted, shorted trace/pins, etc). How did you check the RAM chips? I'd remove the P/S from the unit if you're sure it's OK...at least until you've troubleshot the board first. And obviously you won't get any beeps until you get some semblance of a boot going (i.e.---repairing the MoBo)

  7. #7
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    Quote Originally Posted by mikey99 View Post
    I think they may work on a Non-IBM board.
    They do, I've used them on a variety of boards.
    WTB: IBM RT Parts and Accessories, and AOS.
    Twitter / Discord / YouTube / Twitch

  8. #8

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    One other thing you can do is resetting the system while pin IOCHRDY has been tied to GND. Have a look for the pinout of the ISA bus on my site. Then measure the voltages at the data bus and address bus. Generally everything above 2.5 Volt is considered (H) or 1, below 0.7 is (L) or 0. If in between, then you have a problem. The address bus should be outputting the address FFFF0 and the data bus should read EA. FFFF0 is the first address the 8088 CPU outputs after a reset and EA stands for "jump far" (unless you have a weird BIOS). If neither of the buses outputs these values, you have a serious problem. If the address bus outputs FFFF0 but the data bus outputs something else, check what the EPROM outputs. If it outputs EA, then one of the buffers between the EPROM and ISA bus is broken. If it outputs FF most probably it is not selected and thus start checking the logic between the CPU and the EPROM.

    If the output is OK, then something else is going on. I have built a debugger for the PC based on this design and it enables me to step through the BIOS. Best is to use a BIOS that comes with the source code as well. If at a certain point the BIOs does THIS and you expected THAT, you probably have found a possible reason why the board won't start. Now you have to find the reason why the PC does THIS instead of THAT.

    Good luck!
    With kind regards / met vriendelijke groet, Ruud Baltissen

    www.baltissen.org

  9. #9
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    Yes you are right, I'll get first a basic set of diagnosing tools. Now another question, What is that basic set of tools?:
    - A multimeter
    - ...

    Thanks

  10. #10
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    A logic probe

    If you're going to burn diagnostic ROMs, an EPROM programmer

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