ViewSonic P90F Service Manual page 22

converter to drive horizontal deflection output Q433 via L401. The width, pincushion, trapezium
information are modulated on the DC converter by changing the duty of the B+ drive.
Moreover, the B+ duty is controlled with a charging circuit at same time. When the Q302 is
OFF, the voltage at pin16 of I301 will linearly increase with a charge time of determining by
R326 and C308 to modulate B+ duty
Horizontal DC centering circuit is build by Q423, Q424. By controlling the small current flow in
or out of L405, it will provide DC center compensation for CDT.
8-3-4 TIMEBASE CONTROL (I301), VERTICAL
The vertical free running frequency is determined by C312 (pin 22 of I301).The amplitude of
the outputat pin 23 can be adjusted via .
Icon provides a DC shift at the saw-tooth output at pin 23 and the EW drive output
EWDRV (pin24) in such a way, that the whole picture moves vertically while maintaining the
correct geometry.
The adjustments for vertical size and vertical position also affect the wave-forms of horizontal
pincushion, vertical linearity, and vertical linearity balance, focus parabola, pin unbalance and
parallelogram correction. The result of this interaction is that no readjustment of these
parameters is necessary after an adjustment of vertical picture size or position.
8-3-5
VERTICAL OUTPUT STAGE
The power amplifier driving the vertical yoke assembly is a DC design-based on power
amplifier I310 TDA8172.
The vertical deflection coil is connected to pin5 of I330 via P401A. The saw-tooth signal is
derived from Pin 23 of I301 and fed to pin1 of I310 via R306 / C322. The DC bias voltage is
derived from pin13 of I301 and fed to pin7 of I310 via derived R325 / R327 to optimize the
vertical position.
There are two supply voltage for I310, -15V is applied at pin4 via R384, +15V DC voltage is
applied at pin2 and pin6 via D310 for the output stage during the retrace time. The supply
voltage for the output stage during the retrace time is derived from the fly-back generator
output on pin3 of I310 and applied through C388 to pin6.
8-4 HIGH VOLTAGE GENERATOR AND BLANKING CIRCUIT
I101 is a current mode PWM controller which provides the drive signal for high voltage
generator. It is synchronized by horizontal blanking signal coming from deflection circuit via
Q111. Q121, Q124 are high voltage output switches. T101 provide the high voltage (26.2kV) to
CDT anode. Pin 13 provide feedback to pin2 of I101 to control the duty when high voltage is
varied according to the beam current. Thus it gives a very good high voltage and size
regulation. A very precise geometry distortion is achieved. R176 provides high voltage
adjustment.
Pin5 of T101 senses high voltage for OVP protection. When high voltage is over 33kV, it raises
the emitter voltage of Q114. When emitter voltage of Q114 is 6.8V higher than Q115 collector,
Q116 start to conduct and shutdown the PWM (I101) by reducing the voltage at pin1. This is
also called X-Ray protection. R137 set the protection point.
Pin 8 of FBT sense the beam current. When beam current rise, base voltage of Q126 drop and
so the emitter does. The voltage across C108 drops to pull down the voltage at pin17 of I501.
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