Philips 20PT643F Service Manual page 85

RF Signal Processing
The incoming RF signal goes to the tuner (pos. 1000), where the 45.75 MHz IF signal is
developed and amplified. The IF signals then exits the tuner from pin 11 to pass through
the SAW filter (pos. 1002/1003). The shaped signal is then applied to the IF processor
part of the UOC (pos. 7200).
Tuner AGC (Automatic Gain Control) will reduce the tuner gain and thus the tuner
output voltage when receiving strong RF signals. Adjust the AGC takeover point via the
Service Alignment Mode (SAM). The tuner AGC starts working when the video-IF input
reaches a certain input level and will adjust this level via the I
signal goes to the tuner (pin 1) via the open collector output (pin 22) of the UOC.
The IC also generates an Automatic Frequency Control (AFC) signal that goes to the
tuning system via the I
The demodulated composite video signal is available at pin 38 and then buffered by
transistor 7201.
Video Source Selection
The Composite Video Blanking Signal (CVBS) from buffer 7201 goes to the audio
carrier trap filters 1200, 1201, or 1202 (depending on the system used), to remove the
audio signal. The signal then goes to pin 40 of IC 7200. The internal input switch selects
the following input signals:
Pin 40: terrestrial CVBS input
Pin 42: external AV1 CVBS input
Pin 44: external Side I/O CVBS or AV2 (or comb filter) luminance (Y) input
Pin 45: external AV2 (or comb filter) chrominance (C) input
C bus, to provide frequency correction when needed.
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C bus. The tuner AGC
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