Texas Instruments 990 Manual page 113

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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