BACK TO DIAGNOSTICS
a. Excessive resistance
Refer to the drive and sense diagrams
for your model. Measure the resistance of the
corresponding drive and sense lines for the suspect key(s). It should be
less than 100
ohms. Flex the cable while holding the key down (an analog meter is best
suited for this
test). If the resistance varies or you observe meter deflection, you may
have a weak,
corroded or creased connector. Some keys may work with much higher resistance
(depending on temperature, humidity, machine attitude, etc). As a general
rule,
excessive resistance will get worse before it gets better. 90 ohms is the
repair manual
spec, but if it’s getting up there and there is a significant difference
from the highest
(typically code or right margin 30-70 ohms) and the lowest (typically #
5 or #6 5-20
ohms), the intermittent will soon become permanent.
b. Separating
Inspect the bottom of the keyboard for
broken welds. A few may not be a problem, but if
there are several in the same area, it is a likely cause. Try to pry a few
adjacent welds. If
they fall away easily, they were’nt doing any good anyway. Temporary
gluing, screwing,
etc., will only make matters worse.
c. Bad keyspring
If it doesn’t click, it doesn’t
work. With power off, listen closely for a sharp click on the
down-stroke and a slighter quieter up-click. Try known good keys for comparison.
This
is often corrected by simply reinstalling the keycap. Remove the keycap
by prying up
from the front of the cap. Tip the machine or hold the keyboard so that
the connectors
are pointing down and the keyspring falls to the back of the chimney. Push
the cap back
on and test. Try a different keycap. If the problem persists, replace the
spring. Twist it
anti-clockwise while pulling up gently. Using a bristol or allen wrench
that just fits
inside the new spring, place it on the mounting node of the keyswitch rocker.
Place the
forked end of a spring hook near the base of the spring and nudge it into
place. Reinstall
the keycap and test. This may take a few tries, but if you’ve given
up by now you’ve
made a good decision. Send it in.
d. Intermittent open line
Refer to the drive and sense diagram
for your model. Connect an ohmmeter to the
corresponding lines. Hold the suspect key down while you flex the cable
(an analog
meter is best suited to this task). Variations in meter deflection observed
during this test
indicate an intermittent line. Remove the circuit board to inspect for weak
solder joints
on the keyboard connector plug. Wiggle the plug from the top while you look
at the
solder joint underneath. Any movement, however microscopic, is suspect.
When
resoldering, take great care not to skid off any float-soldered components
(not that I’ve
ever done anything that dumb). Another possible source is the metal contact
in the plug.
Place a strip of tape on the non-conductive side of the keyboard cable to
insure a snug fit.
If anybody asks, you didn’t hear this hack-job idea from me, but it’s
bailed me out of a few
jams in the field.
Attempts to repair typewriter keyboards may cause further damage.
Plesase call tech support before proceeding.