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Thread: IBM PC AT 5170 Will Not Boot (Motherboard Problems)

  1. #1

    Question IBM PC AT 5170 Will Not Boot (Motherboard Problems)

    I recently picked up an IBM PC AT that was working perfectly (with the exception of the hard drive), but it stopped working today. It worked fine this morning, but I turned it off and turned it back on again about 10 minutes later and it no longer booted up. All of the power supply voltages are good, but nothing comes up on the screen and there are no beeps from the speaker. The only sign of life is a brief flash of the lights on the keyboard when I turn it on. I have tried removing all of the ISA cards as well as unplugging everything except the motherboard, but nothing changed. I stuck a POST card into one of the ISA slots and it displayed a code of "--", which is not listed on any of the websites describing the BIOS POST codes. It makes me think that the CPU is not even reading the ROMs at all or even beginning the POST. I have reseated all of the socketed chips (including the RAM, ROMs, and CPU), but the problem persists. I can't think of much else to try at this point, so can someone tell me what to try next?

    Thank you!
    Alex

  2. #2
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    My first port of call would be to start with the Supersoft/Landmark Diagnostic ROM:

    http://www.minuszerodegrees.net/supe...IBM%205170.htm

    Was of tremendous help when a 5170 motherboard I picked up off ebay wouldn't POST. Helped me to narrow down where the fault was which in my case was the MC146818P.
    AtariPC.net
    Atari PC | IBM Compatible Series

  3. #3

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    I just bought a set of these ROMs off of eBay and they should come in by Monday. Hopefully they will help me fix my problem!

    Thanks!
    Alex

  4. #4

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    The Supersoft/Landmark ROMs came in the mail today and I popped them into the sockets for U27 and U47. However, no beeps are played from the speaker and nothing is displayed on the monitor, just like it was before. Does this help narrow down the problem to some specific chip on the motherboard or does this just mean that my board is beyond repair?

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    It could be a power supply problem. Although it is for the 5150 line, try following the minimum diagnostic configuration procedure at http://www.minuszerodegrees.net/5150...iag_config.htm . Modify the procedure to keep the hard drive connected to power, because the 5170 PSU requires load in order for it to work (see "5170 may not start if hard drive removed" at http://www.minuszerodegrees.net/5170...ems_issues.htm ).
    Offering a bounty for:
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    It is likely a shorted tantalum capacitor on the motherboard. Check the power supply connection with a multi-meter for shorts - there shouldn't be any kind of continuity between ground and the other rails.



    http://www.minuszerodegrees.net/5150...iag_config.htm

  7. #7

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    Pretty sure a shorted cap would prevent the machine from powering on at all.

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    Unfortunately, machines that sit there like a lump on a log are the worst to troubleshoot as it could be many things. If it were a 5150 or 5160, there are troubleshooting steps for checking things like the CPU clock, although some of those involve using a scope. Not sure about 5170s.

    At any rate, if the diagnostics ROMs do nothing, then I'd suspect some piece of central circuitry like the CPU clock, CPU, buffers, things that interface directly with the CPU. And always good to sanity check video cards, power supplies, and such in other systems.

    One thought, though, the fact that it was working and then not after a power off suggests a SLIGHT possibility that the failure was caused by the system getting warm for the first time in ages and then cold again. It couldn't hurt tor try something to warm up the board or cool it down while trying to boot.

  9. #9

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    I have checked the power supply voltages and they are all good. I am using a generic AT power supply instead of the original because I wanted to eliminate as many potential problems as possible. I have also checked the power supply lines for shorts, but everything looks good. I have removed all of the expansion cards as well since the diagnostic ROMs do not require a video card and work just fine with the PC speaker.

    I will try the idea of warming up or cooling down the board during power-on to see if that helps. I have already checked the CPU clock with an oscilloscope and it is oscillating at 8MHz just like it should be. The reset line is working properly as well.

    I am running out of ideas at this point and I hope that this board will be repairable!

    Thanks for all of the help!
    Alex

  10. #10
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    Quote Originally Posted by AlexTheCat123 View Post
    I have already checked the CPU clock with an oscilloscope and it is oscillating at 8MHz just like it should be.
    Per [here] and [here], expected is 16 Mhz, not 8 Mhz.

    Quote Originally Posted by AlexTheCat123 View Post
    The only sign of life is a brief flash of the lights on the keyboard when I turn it on.
    I disconnected the keyboard from my IBM 5170, then applied +5V to the keyboard's power pins. Shortly after, the keyboard's three LED's turned on for about a second. That suggests to me that upon receipt of +5V, the keyboard does a self-test, a test that includes turning on the LED's momentarily.

    Quote Originally Posted by AlexTheCat123 View Post
    I am running out of ideas at this point and I hope that this board will be repairable!
    If bad RAM in bank 0 was the only cause, the POST card and Supersoft/Landmark diagnostic ROM would have revealed that.
    If a bad BIOS ROM was the only cause, the Supersoft/Landmark diagnostic ROM would have executed.

    The other thing that is in a socket is the CPU. Replacing it with a known-good one would be another easy-to-do step.

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