~~------------------
~
945424-9701
Table
2-1.
Model
911
VDT
Fault
Isolation
Procedures
Keyboard
Symptom
One
keystroke, two or more
identical characters generated,
one specific key
One keystroke, two different
characters generated, occurs
with nine keys or multiples
of
nine keys
On
power-up, keyboard in
repeat mode with some
random character
Dead data key, exclusive
of
shift, control, UCL, and
repeat keys
Tab/skip key generates two
different characters with one
keystroke
Probable
Cause
Debounce capacitor
of
encoder
either missing or wrong value
Excessive keyswitch bounce
greater than 5 msec
Solder bridge on scan lines or
multiple solder bridges on
scan lines,
of
pwb
Main 26-pin connector
not
fully
seated on mating connector
Solder bridge across repeat key
contacts
Repeat key contacts damaged
when keycap installed, and
they are shorting
Diode assigned
to
this key
either in pwb backwards,
missing
or
bad
Contacts may be spread too far,
contacts may be missing, con-
tacts damaged when keycap
installed
Solder bridge across key terminal
Flow solder missed contacts
Dummy key under double wide
keycap has contacts installed
Solder bridge on scan lines or
multiple solder bridges on
scan lines,
of
pwb
2-11
Possible
Solution
Check
C2
for presence, value,
tolerance, or solder bridge.
Replace contacts
of
keyswitch.
Strike keys with those legends
which correspond to the second
characters generated and see
which key(s) also generate dual
characters. Solder bridge(s)
along scan lines, usually on
diodes associated with these
keys.
Disassemble keyboard, check
mating
of
26-pin connector.
Check for solder bridge.
Remove keycap, check contact
functionality.
Check diode for presence, check
forward/backward resistance
with VTVM. .
Remove keycap, inspect and
check for functionality.
Inspect pwb for solder bridge.
Inspect pwb for missing solder.
Desolder contacts, pull out
with needle-nose pliers.
Strike keys with those legends
which correspond to the sec-
ond characters generated and
see
which key(s) also generate
dual characters. Solder bridge(s)
along scan lines, usually on
diodes associated with these
keys.
Digital
Systems
Division
To Next Page
Loading...
Need help?
General
