Re: Problem with Apple IIc...for anyone who wants to help...

[Rubywand <rubywand@swbell.net>]

Labelas Enoreth writes ...
> 
> Well, when I looked at all the ICs below the keyboard and that seemed to
> connect to it, all I remember seeing offhand were the MMU, CPU, and a few
> other things which didn't seem to be directly related to the keyboard,
> except for the one chip I didn't recognize that was soldered on.
> I don't see any problems on the board itself, which was what made me think
> that the keyboard module was bad, but as I said, the other fine working
> one exhibited the same problem.

     Our IIc+ exhibits a similar problem but not so extreme-- it will sometimes
double or triple a keypress. I've been waiting to get a schematic before trying
a fix.

     The basic problem appears to be poor key debouncing by the KB input
circuit. This may relate to a defective component in a Resistor-Capacitor
timing circuit used for deciding how long to wait for contact bounce to
subside. (Unless the debounce timing is entirely digital, in which case a fix
could involve a circuit change.)

     Something I have not checked is the possibility of a bad power supply
output. Keyboard encoder and similar IC's sometimes use -5V or -12V. If one of
these were missing or off, the computer might work fairly well but screw up
when an IC counting on a negative voltage is involved. You might check your
voltages.
 


> I guess I'll investigate slightly deeper
> when I get some extra time. Heh. I got it for $5 at...somewhere...I think
> it was a thrift store. I dunno, I just have this faint notion that I
> should try to find each of the IIc rom revisions for some reason...

     Sounds like a worthy quest! 


> All
> I've ever found were the 255 and 3 for some reason. Plus, this one came
> with some good (IMO) accessories, like the thermal printer, the apple
> paddles and joystick, the RF modulator...so I guess the rest was worth
> it...

     Just the RF modulator is pretty nice. It fits like a glove and does a good
job of converting the output for color TV.




Rubywand