Texas Instruments 990 Manual page 117

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Step
Symptom
7 Improper picture
displayed
8 Keyboard failure
9 Repeat test failure
Table 2-3. Power/Logic PWB Fault Isolation Procedures (Continued)
Probable Cause
Video circuitry
(figure 1-22)
UART failure
(figure 1-22)
Repeat circuit
(figure 1-22)
Troubleshooting
Procedure
Execute part 4
and scope circuit.
Exercise and trace
circuit for lowercase
g and for F7.
Exercise and trace
circuit.
Normal
Indication
Proper display
of symbols
Proper codes dis-
played on LEDs
g
= 6716
F7
=
9816
Characters
repeated at
10 ± 2 hertz
If Abnormal
Trace circuit back from
134 to 12-2. Remove and
replace faulty compo-
nent(s ).
Strike key giving incorrect
response. Trace forward
from U14 inputs.
Trace circuit forward
from J1 -10 until repeat
indication is lost. Remove
and replace faulty com-
ponent(s ).