6-2. PA Board
1. High Voltage Generator and Regulator Circuit
The high voltage regulator circuit of this model uses the switching regulator system in order to decrease
the power consumption.
The high voltage generator and regulator circuit consists of the converter block that supplied the high
voltage pulse to FBT, the regulator block that supplies to the converter block and the control block that
supplies the control signal which stabilizes the high voltage output, to the regulator block.
The PWM control IC that is IC501 receives, the HV RTN voltage, which is the anode voltage detection
output voltage via buffer amplifier IC801 (2/2). The anode voltage is already divided by the resistance-
type-voltage-divider that is built inside the RBT at the specified ratio so that the divided voltage is sent to
IC501. The anode voltage detected signal is inputted to the positive polarity (+) input terminal (pin-1) of
the error amplifier while the reference voltage (the voltage that is divided by resistors R505 and R506) is
input to the negative polarity (-) input terminal (pin-19). The PWM output signal is supplied from pin-12
of the error amplifier so that the pin-1 input signal and the pin-12 input signal have the unity level. Q102
is driven by the PWM signal.
On the other hand, the Q109 gate input that is the converter block receives the drive pulse all the time.
Amplitude of the drain pulse voltage increases in proportional to the duration of the time during when the
drive pulse and the ON-time of Q102 (when the power supply voltage is applied to the Q109 drain)
overlap. The anode voltage is generated by multiplying the drain pulse voltage by FBT.
IC501 controls the anode voltage in such a way that it widens the Q101 ON-time for a higher anode
voltage when the anode voltage happens to decrease as a CRT current flows through the CRT. The anode
voltage is maintained to a constant voltage in such a manner that the Q102 ON-time is controlled.
The anode voltage is switched ON/OFF by turning ON/OFF the IC501 (pin-19) reference voltage with
D524 by the HV CTRL signal that is supplied from the E board.
2. Excessive Anode Voltage Protector Circuit
The excessive anode voltage protector circuit is installed in this model for the purpose of protecting CRT
when the high (anode) voltage increases up to an abnormal voltage due to failure of the feedback system
of the high voltage regulator circuit. The excessive anode voltage protector circuit consists of IC502 (2/
2), D902, D502, D517, D518, D519 and their peripheral circuit components.
The anode voltage is monitored from the pulse voltage that is developed across the tertiary winding of the
FBT. This voltage is rectified by D801 of the PC board and by C801 of the PA board to monitor the
anode voltage.
The monitoring voltage is inputted to the negative polarity (-) input terminal of the comparator IC502 (2/
2) while the protector operating voltage that is obtained by voltage-dividing the reference voltage is input
to the positive polarity (+) input terminal. Because the negative input terminal has a lower voltage than
the positive input terminal, the comparator output goes high normally. If anode voltage increases due to
failure of the parts of the feedback loop system of the high voltage regulator circuit, the voltage across the
tertiary winding also increases. As the result, negative polarity input terminal of the comparator IC502
(2/2) also increases. When it exceeds the protector operating voltage, the comparator output goes low
that turns off IC501 trigger input that is the control IC, through D502, D501 and Q502. As the result the
HD DRIVE signal is turned off so that oscillation is stopped. Because the PWM output signal is supplied
from IC501, the switching FET Q102 is also turned off that stops supplying the power to the converter.
As the result, the high voltage converter stops its operation and the high voltage output is held down.
IC501 has the latch function with D517 and D518 so that the low state is maintained once the comparator
output goes low.
6-4BVM-D20F1A/D20F1E/D20F1U/D24E1WA/D24E1WE/D24E1WU |
ACEC
