Philips DPTV400 Series, Руководство по обучению

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Sweep Failure detection and Blanking (Figure 14)
The Shutdown circuit will shut the High voltage Off if the Horizontal or Vertical Sweep should fail. It will
also shut the High voltage Off if the Power Fail line goes High or the +200-volt source should fail.
The Vertical pulse is fed to zener diodes 6824 and 6814, which keep capacitor 2837 charged. This
pulse is rectified by 6815 to keep the base of 7808 at a negative voltage. This keeps the transistor
turned Off. In the same manner, the Negative Horizontal pulses keep the base of 7810 at a nega-
tive voltage to keep it turned Off. The Protect line is normally Low, keeping 7809 turned Off. The
+200-volt source is fed through zener diodes 6812 and 6816, resistor 3850, the base-emitter of
7906, the base emitter of 7812 to keep transistor 7813 turned On. This turns transistor 7814 On,
which switches the On voltage to the High Voltage module. The conduction of 7812 keeps the volt-
age on the G1 line at approximately -18 volts, which turns the CRTs On. If the Vertical Pulse should
fail, transistor 7808 will turn On, which will turn 7906, 7812, 7813, and 7814 Off. This will turn the
HVG module Off. In addition, when 7812 turns Off, the G1 voltage will go to -200 volts, blanking the
CRTs. The same sequence will occur if Horizontal should fail. The Protect line should go High, or
the +200-volt source should fail.
FIGURE 14 - SWEEP FAILURE DETECTION AND BLANKING Page 20