Under Voltage Protect; X-Ray (High Voltage) Protect - Toshiba 1997 G Series Troubleshooting Manual

10. 200V Under Voltage Protect.
The flyback transformer T 461 produces about 200V at
the cathode of D406. This is dropped to about 6.8 V on
the base of Q340. Under normal operation, Q340 is
turned on. This keeps Q341 turned off. The collector
of Q341 is about 0 V.

11. X-Ray (High Voltage) Protect.

The cathode of D885 is about +40V. This developes a
+25V reference to pin 11 of Z801. The cathode of D471
developes a nominal voltage of +22V to pin 13 of Z801.
As long as the X-Ray monitor point voltage is LESS
than the reference voltage at pin 11 of Z801, operation
is normal.
When the 200V drops to about 160V, Q340 turns off,
and Q341 will turn on. When Q 341 turns on, its collec-
tor voltage will go to about 6 V, sending the high to pin
14 of Z801, resulting in shut down.
Figure 22. 200V Under Voltage Protect.
If the cathode voltage of D471 increases, excessive High
Voltage may also be occuring.
When D471 cathode voltage goes GREATER than the
reference voltage, shut down occurs.
Figure 23. X-Ray (High Voltage) Protect.
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JRS 12-97