Toshiba TW40F80 - Manual (Page 74)

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4-4. 200V Low Voltage Protection
4-4-1. Outline
When the video output power supply 200V is stopped by
some abnormality occurence, the current inside CPT in-
creases abnormally. So the CPT may be damaged. To pre-
vents this, a 200V low voltage protection circuit is provided.
4-4-2. Theory of Operation
Fig. 9-14 shows a connection diagram.
Under a normal condition Q340 is always on because of about
210V supplied from the 200V line. Accordingly Q340 col-
lector is kept at about 6.2V or the zener voltage of D341 and
Q341 is turned off.
If some abnormality occurs and 200V line voltage lowers
by less than about 160V. Q340 turns off and its collector
voltage rises. So Q341 turns on. With Q341 turned on the
voltage at pin 14 of Z801 (expander) exceeds a threshold
voltage and pin 16 of Z80 is high level and makes the power
relay turn off.
CRT-D Circuit Deflection circuit
200V 1 1
2 2
P405
R436
P301
Z801
14
16
GATE
PROTECTOR
8
17
8
17
DPC circuit
P350
R389 R390
-12V
Q340
Q341
D340
R391
D341 R392
C340
R346
R879
C894
D315
Fig. 9-14