Philips 7629, Service Manual

Page: 70 / 374

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 +200V 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 synchronized 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.