Philips 20PT643F Service Manual page 91

change in the gain of the output stage will be compensated by a gain change of
the RGB control circuit. Pin 55 (BLKIN) of the UOC is used as the feedback input
from the CRT base panel.
Blue stretch. This function increases the color temperature of the bright scenes
(amplitudes which exceed a value of 80% of the nominal amplitude). This effect
is obtained by decreasing the small signal gain of the red and green channel
signals, which exceed this 80% level.
Beam current limiting. A beam current limiting circuit inside the UOC handles the
contrast and brightness control for the RGB signals. This prevents the CRT from
being overdriven, which could otherwise cause serious damage in the line output
stage. The reference used for this purpose is the DC voltage on pin 54 (BLCIN)
of the TV processor. Contrast and brightness reduction of the RGB output signals
is therefore proportional to the voltage present on this pin. Contrast reduction
starts when the voltage on pin 54 is lower than 2.8 V. Brightness reduction starts
when the voltage on pin 54 is less than 1.7 V. The voltage on pin 54 is normally
3.3 V (limiter not active). During set switch-off, the black current control circuit
generates a fixed beam current of 1 mA. This current ensures that the picture
tube capacitance is discharged. During the switch-off period, the vertical
deflection is placed in an over-scan position, so that the discharge is not visible
on the screen.
RGB Amplifier
From outputs 56, 57, and 58 of IC 7200 the RGB signals are applied to the integrated
output amplifier (7330) on the CRT panel. Via the outputs 7, 8, and 9, the picture tube
cathodes are driven.
The supply voltage for the amplifier is +200 V and is derived from the line output stage.