Re: Keyboard Problem

[dempson@actrix.gen.nz (David Empson)]

D. Cousins <dcousins@spots.ab.ca> wrote:

>     Yeah, I did find several keys that don't work, and I'll tear it down to
> see if their the culprits.

That sounds promising.  It might be a jammed keyswitch, dry joint (less
likely), circuit board fault on the keyboard, etc.

> I guess I can try to find new microswitches from an electronics store, and
> resolder them, if their defective. If it's the motherboard, what chip(s)
> would be a fault?

The AY-3600-PRO is the keyboard encoder.  It is probably OK.

I'm not entirely clear on how this chip operates, but I imagine it
derives the "any key down" flag by detecting at least one row/column
pair.  Once the initial delay has elapsed, it will continue to repeat as
long as "any key down" is true.  This means that a track, cable or
keyboard fault is most likely.

The delay and repeat rate are controlled by discrete components
connected to the AY-3600-PRO.  I don't think a problem with these would
affect this, but it wouldn't hurt to check.

On the original IIc motherboard, the components are C46 (47 pF), R6
(51Kohms) and C45 (0.22, not sure what units - presumably microfarads).

C46 goes between pins 1 and 2 of the AY-3600-PRO, R6 is between pins 2
and 3, and C45 is between pin 31 and ground.

I'm not sure which components deal with the repeat rate and which are
the initial delay, but the first two go together.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand