High Voltage Circuit; High Voltage Control Circuit; High Voltage Output Circuit; High Voltage Protector Circuit - NEC MultiSync FP1350 Service Manual

3. HIGH VOLTAGE CIRCUIT

The high voltage circuit consists of the high voltage control circuit, and Q5R1 and its peripheral circuits made
up for the high voltage output circuit and the high voltage protection circuit for instance of high voltages circuit
irregular operation.

1) High Voltage Control Circuit

ICD5P1 is high voltage control integrated circuit.
The high voltage detection voltage is output to TP-HV from the FBT's internal bleeder resistor.
The AFC pulse output from the horizontal output transformer (T561) is input to 4pin of ICD5P1, and the
sawtooth wave is generated in ICD5P1. This sawtooth wave is compared with the high voltage detection
voltage and the drive pulse applied to the gate of Q5R1 is generated. This pulse width becomes narrower
when the high voltage rises, and becomes wider when high voltage drops.

2) High Voltage Output Circuit

The output circuit operates depending on the drive pulse applied to the gate of Q5R1, and the fly-back pulse
is produced at pin 1 of T5R1 (FBT). The drive pulse width changes with respect to the high voltage
fluctuation resulting from changes in the CRT beam current and operation frequency, the peak value of the
corrector pulse changes, and high voltage stabilization is performed.

3) High Voltage Protector Circuit

This circuit consists of the X-ray protector which stops the operation of the control circuit when the high
voltage rises abnormally, and the overcurrent protector which operates when the current flowing into FBT
(T5R1) has increased. Once the X-ray protector and the over current protector have started operation, their
operation is not canceled unless the power supply is switched off, and then on again.
* X-ray protector
When the high voltage rises, the voltage of T5R1 (FBT) pin 10 rises. This voltage is compared with the
reference voltage in IC2001, and when the reference voltage is exceeded, the comparator operates and
output voltage of IC2001 (5pin) is high level, then the operation of the high voltage control circuit is stopped.
* Overcurrent Protector
When the current flowing into T5R1 (FBT) pin 2 increases, Q5F1 operates and Q5F3 is switched on.
Also when the current flowing into T5R1 (FBT) pin 8 increases, Q5F2 operates and Q5F3 is switched on.
When Q5F3 is switched on, Q5F4 is switched on and the voltage of ICD5P1 10pin rises.
Then latch circuit inside ICD5P1 operates and the high voltage control circuit is stopped.
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