Texas Instruments 990 Manual page 122

Table 2-6. VDT Controlle.r Preliminary Fault Isolation Procedures
~
Step Sympt<Jim
Probable Cause Trouble.shooting Procedure
Normal Indication
U
Ab111ormal
\0
.i;:..
Ul
CRT screen not blank
Video output logic
Scope circuit.
Trace back from J4-2 (JS-2
.a::i.
N
(figure 1 -44)
for 2nd controller) until
..i;:..
~
fault is found. Replace
-..J
faulty component(s).
0
-
2
Test LED on
Built-in test and Scope circuit.
Logic 1 at U83-12
Trace back from U83-12
master clear logic
until fault located. Replace
(figure 1 .3 5)
faulty component(s).
3
Sync LED not Video output logic Scope circuit.
Trace back from J4-2 {JS-2
lighted (figure 1-44)
for 2nd controller) until
fault is found. Replace
faulty component(s).
N
4 Unstable or missing
Timing logic
Scope circuit.
Trace back from
14-2
{JS-2
I raster
(figure 149, 1-50)
for 2nd controller) until
N
0 fault is found. Replace
faulty component(s).
5
Basic clock signal Oscillator Scope circuit.
CLK
==
90 .9
nse~ Tracie back from U4Q . .S.
(CLK) faulty
Replace faulty compo-
nent(s).
0
<9:
Ci)"
-