Texas Instruments 990 Manual page 116

Step
2
2a
3
4
5
6
Symptom
CRT scre:en not blank
when power first applied
Sync LED not lighted
with power applied
Unstable or no raster
No beep during part 1
of diagnostic
Failure of any portion
of parts l through 3 of
diagnostic
Terminal Ready
failure
Table 2-3. Power/Logic PWB Fault Isolation Procedures
Probable Cau:s:e
Spurious oscillations
in video circuitry
(figure 1-22)
Horizontal sync
generator
(figure 1-22)
Interface cable
connector P2
disconnected
Horizontal or ver-
tical sync circuits
(figure 1-22)
Audio logic
(figure 1-22)
Video circuitry
(figure 1-22)
Driver Ul 1
Troubleshooting
Procedure
Check circuitry from
13-4 back to source
of signal.
Check circuit back
from high side of
CR16.
Ensure that cable is
properly connected.
Check circuit back
from 13-2 and B-6.
Loop on part 1 (LI)
and scope circuit.
Execute part 4 and
scope circuit.
Execute part 4
and scope circuit.
Normal
Indication
No video detected
Logic 0@ U6-12
Cable properly
connected
0.25-second tone
Proper video and
audio responses
to diagnostic
Proper response
to diagnostic
If AbnormaJ
Replace component
producing output.
Trace circuit back until
horizonital sync pulse is
found. Remove and re-
place faulty component(s).
Secure cable.
Trace circuit back from
13-2 and J3-6 to U7-7.
Trace back from speaker
until audio signal is found . .
Remove and replace faulty
component(s).
Trace circuit and replace
faulty component(s).
Trace Circuit back from
J2-6. Re:move and replace
faulty component(s)