Re: Apple II+ Keyboard Problem

[rubywand@aol.com (RUBYWAND)]

In article <4t5qv5$rk4@elf.wang.com>, ptaft@wang.com (Phil Taft) writes:

>
>I hope someone can help. I have a "thermally challenged" AII+ keyboard. 
>Sometimes it works fine, other times, it needs to be turned on for up to
an 
>hour before the keys work. Then it works fine. The RESET key always
works. 
>
>I've cleaned the keyboard cable sockets, and the socket for the only 
>removable chip on the keypad itself. Nothing helped. Is there anything
else
>I can do? Did I miss a chip on the motherboard?
>
>

     The KB is one of the two 'known weak' components in the II+'s
hardware design. (The other is the power supply.) 

     RESET works because it is just a switch which goes directly to the
motherboard and has nothing to do with the KB scanning/decoding circuitry.

     If your II+ boots and runs stuff okay except for having no KB input
until something warms up then the best suspects are ...

1- a bad KB connector. Sometimes connectors have internal contacts (e.g.
where a lead connects to a plug prong) which are broken and only make
contact when heat causes expansion or contraction. Use an Ohmmeter to
check the continuity of each KB connector line, especially the ones going
to pins 2, 1, 8.

2- bad or poorly socketted IC on the motherboard. Try prying up and
reseating the 74LS174 (1st IC in Row B). If this does not fix the problem,
replace the IC with a new 74LS174.

Also try prying up and reseating the four 74LS257's near the KB connector
on the motherboard.

3- a bad IC on the KB board. Try replacing the 7400's (or 74LS00's) with
new 74LS00's. One of the 7400's (IC U4) is part of an oscillator circuit
which is supposed to kick into oscillation upon power-up and act as a
clock for the main KB IC. It's a fairly good suspect since heat often
affects ability of an IC to oscillate.


Rubywand