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From: Rubywand <rubywand@swbell.net>
Newsgroups: comp.sys.apple2
Subject: Re: ][ + control key locked
Date: Sat, 12 Jul 1997 07:26:10 +0000
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wshek@ibm.net writes ...
>
> Anyone have any experience with keyboard encoder chips and such?
>
> I have a ][+ with a keyboard symptomatic of control key always pressed when
> it isn't. ie. press the single key 'g' get beep....press 'm' get cr etc.
> Keyboard appears to be functioning properly by process of elimination but I
> don't have another encoder chip to try.
> Has anyone purchased parts for a ][+ recently? Would like as near original as
> possible.
Are you sure that your CTRL key is not stuck? (Try diddling the key.)
Another possible source of the problem is a KB Controller IC pin making
poor contact with the line coming from the CTRL key circuit. (Try removing and
re-socketing the KB IC.)
U1, a 7400 quad nand gate IC, could have gone bad or be making poor
contact at some pins. This could result in an 'always-pressed CTRL key' signal
at pin 11.
When the CTRL key is not pressed, pins 12 and 13 of U1 (connected to the
CTRL key switch) should be at nearly 5V and pin 11 (which goes to pin 19 of the
KB Controller IC) should be at logic "0" (roughly 0V - 0.25V).
If U1 pins 12 and 13 are at 0V (or very close) when the CTRL key is not
pressed, your CTRL key switch is, probably, stuck.
If U1 pins 12 and 13 are at/near 5V and pin 11 is not near 0V, U1 is
probably bad or making poor contact at some pins.
If U1 pin 11 is near 0V and pin 19 of the KB Controller IC is not, then,
there is either a break in the line connecting the pins or one or both IC's are
making poor socket contact.
If pin 19 of the KB Controller IC is near 0V when the CTRL key is not
pressed and near 4-5V when the CTRL key is pressed, then, if you still have
'stuck CTRL key' symptoms, there is a good chance that the KB Controller IC is
messed up.
Note: all of this assumes that your KB is like the one diagrammed in the Apple
][ Reference Manual on page 101.
The keyboard is known to be a weak point in ][+ design. Probably, Apple
did not expect the machines to be in service at the turn of the century!
After a plague of KB problems, we ended up yanking the entire thing,
enlarging the KB opening, and installing a home-brew KB. It uses the key matrix
from a surplus reed-switch KB and a decoder circuit designed around the SMC
KR2376-ST KB Controller IC.
Rubywand