Service manual

Circuit Descriptions
IF section (for set with 2-antenna inputs)
Basically the IF-processing works around IC7901 TDA9801 and tuner. It demodulates
the IF signal received from the tuner at pin-1 & 2 via SAW filter 1901, processes it into
CVBS signal and outputted at pin-7. The AGC and phase alignments are possible on
this panel.
CVBS selection (2-antenna inputs only)
Due to 2-antenna inputs construction, it is possible to provide main picture/PIP picture
source either from each tuner (one for off-air and one for cable, or both off-air or cable).
It was done by the switching component of IC7916. The switching logic signals,
SEL_TUNER1 and SEL_TUNER2 are from IO expander IC7910 pin-6 & 7 respectively.
Table-1 shows the switching logic.
Table: Switching logic between Tuner-1 and Tuner-2 for Main or PIP Picture
SEL-
TUNER1 (Pin
6 of IO Expander
IC7910)
SEL-
TUNER2 (Pin
7 of IO Expander
IC7910)
Selected CVBS signals
0 0
Main Picture = Tuner 1
PIP Picture = Tuner 2
0 1
Main Picture = Tuner 1
PIP Picture = Tuner 1
1 0
Main Picture = Tuner 2
PIP Picture = Tuner 2
1 1
Main Picture = Tuner 2
PIP Picture = Tuner 1