Panasonic BT-M1950Y Service Manual page 110

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4. Screen control
Ordinarily, screen voltage is supplied from the FBT, but in this series, a separate circuit is provided for improving screen
regulation.
The H-OUT collector pulse is rectified by D301 and C303, and is used as the DC screen voltage for controlling 0301.
This control is performed in the following manner.
DC reference voltage determined by zener diode D305 is applied via R307 to pin 6 of opamp IC301. This DC voltage
comes from D404.
The screen voltage is divided and applied as DC voltage to pin 5. Since IC301 functions as a differential amplifier, if the
pin 5 voltage exceeds pin
6
(screen voltage increases), a High output is produced from pin 7. This increases the 0301
base current and the screen output voltage can be reduced. The screen control (R313) determines the 0301 operating
point.
D3O1
0502
H.OUT
escreen reset
,--------------------------<1>-----~scREEN
OUT
FROM D4O4
+12V
0304
Fig. 4-1 Screen control and reset
The screen voltage is reset at the time of power supply startup. This is in order to prevent an unstable CRT output in
the period before the power supply reaches stable operation.
IC302 prevents screen glow. Until D305 switches on, IC302 holds IC301 pin 6 at Low potential. High potential from
IC301 pin 7 prevents the screen voltage output.
0302 prevents picture disturbance. During power supply startup. skew 1 and skew 2 signals from the MPU are both
High. Normally, since at least one of these is Low, either D308 or D309 is on and consequently D310 and 0302 are
off. Since both are High during power supply startup, 0~08 and D309 are off, while D310 and 0302 are on. The
power supply is diverted to prevent screen voltage output.
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