Philips DPTV305, User Manual

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High Voltage Shutdown
Figure 32
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 keeps 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 negative
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, and 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 voltage 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 a -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.
Convergence
The Convergence system is split into two sections. The convergence signal processor is located on
the ACS (Auto Convergence System) module. The output amplifiers are located on the SSM.