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Thread: AT&T PC6300 non-POST

  1. #1
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    Default AT&T PC6300 non-POST

    Yesterday I powered up my PC6300 to find the display scrambled, as though horizontal or vertical sync were lost. All I could see were random diagonal lines scrolling very fast. I took the CPU apart, removing the video card, which also serves as the conduit to the expansion board, and unseating and reseating every IC that was socketed. I reassembled and powered up the unit but got no display and no POST, only the continual, rapid flashing of the keyboard LEDs. I took the unit apart today to check if any pins were misplaced but could see none. I reassembled and tried power up again, with the same results.

    In the AT&T service manual (thanks to all for providing it!) it says the system ROM diagnostics will try to send any error messages to the display and to the parallel port. I tried a parallel-port diagnostic card but got no lights at all from it. I then tried an ISA / PCI diagnostic card in one of the ISA slots and powered on again.

    This time I got lights on the diagnostic card, the results are shown in the attached picture. No numbers were displayed on the seven-segment displays, just dashes. Of the six red LEDs, five came on at power on: top row is +- 5V and +- 12V; bottom row is (from inboard to outboard) RESET, FRAME, IRDY and CLK. RESET is the one that came on and then went out shortly after power was applied; IRDY never came on.

    Through all of this the display remained blank and the keyboard LEDs continued to strobe. I have no idea where to begin to diagnose this problem, so any suggestions are welcome.

    Thanks for your replies.

    -CH-

    DIAGS.jpg

  2. #2

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    Is there a battery on that board? If so, check its condition.
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  3. #3
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    The battery was removed when I acquired the machine, some years ago. It has booted successfully since then.

    BTW: I believe the display is not at fault because I hooked it up to an AT&T PC6312 WGS (286 clone) and it worked correctly there.

    -CH-

  4. #4
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    I had to try 2 or 3 parallel-port diagnostic dongles before I found one that worked with my 6300; the discussion is here: http://www.vcfed.org/forum/showthrea...agnostic-codes
    My reverse-engineering of the codes is also in that thread.

    I would try your parallel-port device on another system that is known to output codes in the same way (or, just hook it up to a system and output data to the LPT port) to ensure your dongle is working. If the 6300 is the first thing you've put it into, you don't know if the dongle is working or not.

    ISA diag cards won't pick up codes because that standard writes codes to 80h, whereas the 6300 was designed before that standard and writes its codes to 378h.

    If you know you have a good diag dongle and aren't getting anything, then you may have a power supply issue. Unfortunately, the 6300's power supply is enough of a PITA that people are replacing their PSUs.
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  5. #5
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    Thank you for the suggestion. Can I assume that a successful test on the 6312 indicates a working tester? The parallel tester that did not work on the 6300 lit up, reported several codes as POST and boot went by and stabilized after boot.

    Picture attached. By the way: Do you know what the small knurled knob on the face of the 6312 is for?

    -CH-

    6312DIAG.jpg Knob.jpg

  6. #6
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    Went back and tested 6300 with the same PP tester; no signal at all. Checked voltages at various points around the system; results reasonable as far as I can see. "5+v" means 5.x, as in 5.3 or so. Within range, especially with an unloaded system. Pic attached.

    -CH-

    6300Voltages.jpg

  7. #7
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    I use that same version of the parallel tester, albeit I externally power it from the USB port. Unless China is shipping variants in the same layout that's not a good sign.
    The blinking keyboard LED's is neither a good sign. That means POST is not going far enough to initialize the keyboard controller, let alone send diagnostic bytes to the parallel port. HAve you verified your system configuration/ram size currently matches your dip switch settings?
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  8. #8
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    I have two of those testers and two of the other format, i.e. ISA / PCI. No USB on this chassis, however.

    I have not checked DIP settings switch-by-switch and according to the reference materials, which I only located again today. (Lost in a crash earlier this year.) I was able to boot before, however, and so assumed everything must be set correctly.

    It may also be that exercising the switches clears up resistance that is preventing matters from proceeding. I guess the next step is to spend some time looking / poking at the mobo.

    -CH-

  9. #9
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    Quote Originally Posted by clh333 View Post
    By the way: Do you know what the small knurled knob on the face of the 6312 is for?
    I believe it's a volume control knob for the internal speaker.
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  10. #10
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    I also would strongly suggest making sure the parallel port dongle works first. Codes are output to LPT beginning from the very start - literally I think you should be able to see some codes even with no video card, no bus converter, and no ram installed.

    For the record I bought this one and it works (but it needs to be powered via usb - I use a battery pack):

    https://www.ebay.com/itm/PC-laptop-d...-/184003987526

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