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Thread: Keybaord troubleshooting

  1. #1
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    Default Keybaord troubleshooting

    (first port here, be gentle please)

    Just got an AST Bravo 4/25s to run some old software stored on 5 1/4" and 3 1/2" disks. When it powers up, BIOS complains that setup information was lost (CMOS battery was replaced) and asks to press Ctrl-Alt-Esc to enter Setup. I cannot get past this screen. The original keyboard that came with the unit blinks lights on boot, but keystrokes don't register. I've tried using a known good PS/2 keyboard via a DIN-to-PS/2 adapter - no dice. If I boot without any keyboard attached, I do get a "no keyboard" error on the screen, so kb presence is being detected.

    Any ideas as to the next steps? Is there something about the adapter that could make my PS/2 keyboard not compatible with the old hardware? Should I look for a blown fuse or a faulty component on the motherboard (though I thought a fuse cuts 5VDC, and it seems 5VDC is present)?

    TIA!

  2. #2
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    What happens when you press the "caps lock" or "num lock" key? Do you get the corresponding LED to illuminate? I guess I'd check the pads around the keyboard connector on the motherboard. Not infrequently, these will be lifted off or otherwise the circuit will be broken. Simple enough to repair.

  3. #3

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    Check for possible corrosion from a nearby battery.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  4. #4
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    The supplied keyboard is flashing its LED's at start-up. The power for the LED's will be coming from 5V supplied to the keyboard.

    If that flashing is happening within just a few seconds of the computer being powered on, then that flashing is often the keyboard turning the LED's on and off as part of the keyboard's self test (a test initiated by the keyboard itself when it gets +5V).

    Quote Originally Posted by legalalien View Post
    If I boot without any keyboard attached, I do get a "no keyboard" error on the screen, so kb presence is being detected.
    I agree. Normally, the computer's POST sends a command to the keyboard and the keyboard responds.

    Quote Originally Posted by legalalien View Post
    I've tried using a known good PS/2 keyboard via a DIN-to-PS/2 adapter - no dice.
    Quote Originally Posted by legalalien View Post
    Is there something about the adapter that could make my PS/2 keyboard not compatible with the old hardware?
    For AT class computers, standard PS/2-to-DIN adapters have always worked for me.

    I am unfamiliar with the AST Bravo 4/25s. A photo of the AST Bravo 4/25s that I see on the Internet shows that the computer has a keylock switch on its front panel. On some computers, a locked keylock stops a user (but not the POST) from using the keyboard. Of those, if the keylock switch is in the locked postion, some computers will display a 'keyboard locked' type error, and others not. So that is a possible line of investigation (e.g. is keylock cable still attached to motherboard, attached to correct connector).

    If the previous owner indicated something like, "It was working 100% before it went into storage. After storage, I replaced a leaking battery.", then per Stone's post, damage (visible or not) from a leaking battery is a possibility.

  5. #5
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    Quote Originally Posted by Chuck(G) View Post
    What happens when you press the "caps lock" or "num lock" key? Do you get the corresponding LED to illuminate? I guess I'd check the pads around the keyboard connector on the motherboard. Not infrequently, these will be lifted off or otherwise the circuit will be broken. Simple enough to repair.
    No response to num or caps lock. I'll check the keybaord socket for any broken connections.

    Quote Originally Posted by Stone View Post
    Check for possible corrosion from a nearby battery.
    Quote Originally Posted by modem7 View Post
    I am unfamiliar with the AST Bravo 4/25s. A photo of the AST Bravo 4/25s that I see on the Internet shows that the computer has a keylock switch on its front panel.
    ...
    If the previous owner indicated something like, "It was working 100% before it went into storage. After storage, I replaced a leaking battery.", then per Stone's post, damage (visible or not) from a leaking battery is a possibility.
    Will check for possible corrosion. The previous owner did, in fact, say almost verbatim that the unit worked before it went into storage and the battery was replaced. No mention of it leaking though.


    Thanks for the tips! I'll post an update on my progress.

  6. #6
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    Update: turns out keyboard fuse was blown (5v one end, 1.7v on the other, same 1.7v on pin 5 of the socket), Replaced the fuse, so now I can see 5v getting to the KB socket. Keyboard input, however, is still not working. I don't have an oscilloscope to confirm the CLK and DATA signals.

    The area around the battery and the motherboard, in general, are pretty clean. I now wonder whether the KB controller is bad .. which would mean that I need to find a replacement motherboard.

    Any other ideas?

  7. #7
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    Quote Originally Posted by legalalien View Post
    Update: turns out keyboard fuse was blown (5v one end, 1.7v on the other, same 1.7v on pin 5 of the socket), Replaced the fuse, so now I can see 5v getting to the KB socket.
    And the keyboard LED's were flashing at computer power-on when the 5V fuse was blown ?

    Quote Originally Posted by legalalien View Post
    Keyboard input, however, is still not working. I don't have an oscilloscope to confirm the CLK and DATA signals.
    Offline, I have assisted members fix the keyboard circuitry on their faulty motherboard. They used a logic probe, with CLK and DATA activity showing on the probe as pulses. Assistance was possible because a motherboard circuit diagram was available and so I was able to direct the member as to which pins of which components to measure and report on.

    A multimeter can be misleading. For example, a multimeter measurement by one member of a particular output pin of the 8255 chip in the keyboard interface circuitry of an IBM XT showed neither a high nor a low. On a good IBM XT motherboard, the pin normally sits at high. At face value, it suggested a partially faulty 8255. It was later discovered (via logic probe) that the pin was oscillating between high and low (behaving the way it was due to other things).

    And so if you want to confirm CLK and DATA activity, perhaps use a logic probe ([example]) to get good confidence.

    I expect that in many cases, AT clone motherboard makers used keyboard interface circuitry that was very close to what IBM used. What IBM used in the IBM AT is shown at [here].

    Quote Originally Posted by legalalien View Post
    I now wonder whether the KB controller is bad .. which would mean that I need to find a replacement motherboard.
    Not necessarily. Understandably, for compatibility reasons, the motherboards of many AT clones have a keyboard controller that is functionally compatible with the IBM AT (IBM 5170) motherboard's keyboard controller, and so in many cases, the keyboard controller from an IBM AT can be used as a replacement. At [here] are the results (in the 'Self reminder' field) of me trying an IBM AT keyboard controller on some of my AT clone motherboards. It does not always work, as member roadrash discovered with his Dell System 200.

    The IBM AT's keyboard controller chip is an 8042 (or 8742) chip that has been programmed with custom code. If you want to experiment, and assuming that the keyboard controller on your motherboard is also a programmed 8042/8742 chip, then there are people/companies at [here] who can create an IBM AT keyboard controller for you. The code for the IBM AT's (IBM 5170's) 8042/8742 keyboard controller chip is at [here].

    If you acquire such a replacement, and it does not change things, additional possibilities would include:
    * Supplied IBM AT keyboard controller fully functional, but incompatible with your motherboard.
    * Supplied IBM AT keyboard controller faulty in some way. (E.g. Not fully tested by supplier / failed in transit )

    Or perhaps you have another AT class motherboard, and can try the 8042/8742 based keyboard controller from it, knowing however that there is the possibility of incompatibility.

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