Texas Instruments 990 - Page 127

Step
2
N
3
N
Ul
4
Symptom
Screen blank, raster
unstable, previous
word select error,
keyboard self-test
failure, sync(s)
LED not lighted
Sync (S) LED not
lighted
H-Sync error only
for part 1. Video
bar on screen, but
raster unstable
Sync error may
cause keyboard self-
test to fail; will prob-
ably cause manual
keyboard to input
to fail
Table 2-9. Fault Isolation Procedures for Apparent Timer Problems
Probable Cause
Defective clock
Constant system
reset
Additional symp··
toms needed
Horizontal blanking
flip-flop or steering
circuit
Horizontal sync
flip-flop or
steering logic
Composite sync
flip-flop or
steering logic
Troubleshooting Procedure
Scope circuit.
-Scope circuit.
Execute part 1
(El).
Scope circuit.
Scope circuit.
Scope circuit.
Normal Indication
Clock period = 90 .2
nanoseconds at
U40-8.
NRL2=1 at U98-13
HSBQ = 63 .5 µ sec
period.
HSQ = 63.5 µsec
period
See figure 2-7.
If Abnormal
Trace circuit back from
U40-8. Replace faulty
component(s).
Trace circuit back from
U98-13 until source of
error is found. Replace
faulty component(s).
Trace circuit back from
V504 until fault is
found. Replace faulty
component(s).
Trace circuit back from
U50-5 until fault is found
Replace faulty component(s).
Sync on VSBQ at 050-13.
Trace circuit back from
050-12. Replace faulty
component(s).