Brightness Control; Contrast Control - Tatung OMNISCAN SERIES Service Manual

3.41 BRIGHTNESS CONTROL
(Refer to circuit diagram Fig-41)
The . brightness control is achieved by varying the
CUTOFF voltage of the CRT.
The reference voltage is derived from pin 4 of P721
via voltage divider R155-R722-R724, the reference
voltage is buffered by 0056 and fed to R, G, B
CUTOFF circuit via R057, R 119, R 179 respectively.
BLUE CUTOFF circuit for instance:
The CUTOFF voltage is controlled by 0124-0129, the
higher voltage on the emitter of 0129 the higher
voltage on the base of 0124, also the higher voltage
(CUTOFF voltage) on the emitter of 0124.
The brightness control potentiometer R722 is
BRIGHTNESS CONTROL/BLUE CUT OFF CIRCUIT
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mounted on the control panel. The control signal is
fed to the base of 0056 via pin 4 of POOs.
Thus if have any voltage change on pin 4 of P008, the
brightness of the CRT will be changed.
The transistor 0124-0129 modulates the black level
of the video signal with the clamp signal.
The level (CUTOFF) of modulation is set by the ratio
of R179-R177-R176-R175 and the user brightness
control.
R179-R175, along with the RED and GREEN channel
equivalents, enable the color point of the monitor to
optimized during set up.
Fig-41
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3.42 CONTRAST CONTROL
(Refer to circuit diagram Fig-42)
The contrast control is achieved by varying the video
amplitude of the output signal.
BLUE channel for instance:
The video amplitude of the preamplifier (0122) is
controlled by the voltage of pin 4. The higher dc
voltage on pin 4 gives the bigger amplitude on video
output (pin 8).
The contrast control circuit is implemented by 0047,
0048 and 0128. 0047 is the ABL control stage, 0048
and 0128 are the buffer stages for contrast control
signal.
The control signal is fed to pin 1 of P008 via the
emitter of 0047 and resistor divider R721-R723, then
fed to pin 4 of 0122 via 0048 and·0128.
The higher voltage on pin 1 of P008 gives the higher
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