Texas Instruments 990 Manual page 138

Step
5
6
7
Symptom
Display compressed
to strip about 0.38 mm
(0.5 in.) in height
Display is horizontal
or vertical line
No raster (blank
screen)
Table 2-15. Fault Isolation Procedures for CRT Monitor (Continued)
Probable Cause
Sweep board/
vertical choke
Yoke, sweepboard,
or flyback inter-
connection
Sweep board
Sweepboard
Flyback trans-
former
Trouble~shooting Procedure
Check continuity through
vertical choke circuit.
Visually inspect, then check
wavefom1s per schemat:lc.
Listen for flyback output.
Check horizontal output
transistor.
Check horizontal output
transistor.
Check flyback resistance.
Normal Indication
Normal display height
Raster is visible.
Barely audible hum
15 V
±
0.3 V horizontal
transistor collector
15V±0.3V
Resistance from any low
voltage winding to another
is approximately 0.5 ohm.
From the high voltage
winding to any low voltage
winding, the resistance is
approximately 6 ohm.
If Abnormal
Replace vertica:l choke
or check vertical
circuit.
Repair open cir
0
cuit.
Check horizontal
and vertical trans-
istor.
If approximately
0.5 V, disconnect
flyback connector
and measure vertical
output transistor
collector. If the vertical
collector measures
approximately 13.5 V,
replace: horizontal out-
put transistor.
If zero,, check flyback
resistance and 1 5 V
Picofus:e (if applicable).
R1eplac~e
defective
flybac:k.