[psu.micro.ibm] Strange "keyboard" problem- help!

<PNU@psuvm.psu.edu> (04/11/91)

We have an honest to god IBM PC-AT, 8 Mhz, 512 Kb motherboard.

It exhibits unusual behaviour when data is typed at the keyboard.
For example, pressing the CTRL key will cause the number keys (top
of keyboard, not numeric keypad) to generate odd sequences. For example,
pushing CTRL and then the "2" key generates a CTRL-C. This stays in
effect until the left CTRL and Alt keys are held down simultaneously
(lots of trial and error went into finding that fix!).

The right Alt key causes the keyboard to lock up, again until the
left CTRL-Alt combination is used. For no apparent reason, the keyboard
occasionally goes into shift-lock mode, which can only be cleared
by hitting the shift key twice in rapid succession.

Here's what we did to try to fix the problem:

    Remove all option boards except the disk controller and
    display adapter (EGA).

    Replace the EGA board with a CGA monitor and controller card.

    Swap keyboards with another system.

    Replace the entire motherboard.

    Booted from a clean copy of DOS (i.e. no config.sys or autoexec.bat).

    Tried DOS 3.2 as well as 4.0.

So far, nothing has helped.

As far as I can see, the only options remaining are:

     1. The disk controller (floppy/hard).

     2. The power supply.

Which is it most likely to be? Since the disk drives all seem to function
perfectly, our instrument repair people claim it must be the power supply.
Is this possible? How could a power supply cause this kind of problem?
More importantly, how could a power supply cause this kind of problem
without affecting other parts of the system?

This is really starting to annoy me. I seem to be the guy who gets
stuck fixing other peoples computers, so a while back I started the
policy of replacing the motherboard on any AT which causes problems
(at $175 for an exchange motherboard, this is usually money well spent).
Now I look like an idiot because we apparently trashed a perfectly good
motherboard and the problem still persists. Someone help me come up with
a rational explaination for this problem before my reputation as a computer
guru suffers irreparable damage?!

                                            Jon Peters
                                            (Not a computer technician,
                                             but I like to pretend)