Rgb Control - Philips 7603 series Service Manual

RGB control

The RGB control circuit enables the picture parameters contrast, brightness and saturation to be adjusted, by using a
combination of the user menus and the remote control.
Additionally automatic gain control for the RGB signals via cut-off stabilization is achieved in this functional block to obtain an
accurate biasing of the picture tube. Therefore this block inserts the cut-off point measuring pulses into the RGB signals
during the vertical retrace period.
The following additional controls are used:
• Black current calibration loop Because of the 2-point black current stabilization circuit, both the black level and the
amplitude of the RGB output signals depend on the drive characteristics of the picture tube. The system checks whether
the returning measuring currents meet the requirements, and adapt the output level and gain of the circuit when
necessary. After stabilization of the loop, the RGB drive signals are switched on. The 2-point black level system adapts
the drive voltage for each cathode in such a way that the two measuring currents have the right value. This is done with
the measurement pulses during the frame flyback. During the first frame, three pulses with a current of 8 µA are
generated to adjust the cut off voltage. During the second frame, three pulses with a current of 20 µA are generated to
adjust the 'white drive'. This has as a consequence, that a 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.
Synchronization
Inside IC 7200 part D the vertical and horizontal sync pulses are separated. These 'H' and 'V' signals are synchronised with
the incoming CVBS signal. They are then fed to the H-and V-drive circuits and to the OSD/TXT circuit for synchronization of
the On Screen Display and Teletext (CC) information.