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CIRCUIT
DESCRIPTION
1. SELF
SEEKING
CIRCUIT
The basic circuit is shown in Fig. 1.
In Fig. 1, the voltage at the tuning voltage terminal of
the electronic tuner is controlled
by charging and dis-
charging C127. That is to say, to raise the tuning voltage,
Q104
and Q105
are turned on to charge C127 with a
constant
current
(approx.
200uA
for VHF,
approx.
100KA for UHF), and to lower the tuning voltage, 0104
and
Q105
are
switched
off
to discharge
C127
by
means
of Q106
at a constant
current
(the constant
current value is the same as in charging): thus the tuning
voltage
is controlled.
First, the
mode
when
the self
seeking circuit is receiving a broadcast signal stably will
be described.
In this case, pin
of 1C181 is "H", and
pin (2) of 1C102 is ''L'', and D101 is off, so the AFC
voltage is input to pin
of 1C102 as it is. When the
AFC
voltage
becomes
slightly
higher
than
the input
threshold voltage of pin (3), pin
10 of 1C102 becomes
"H" and the tuning voltage gradually rises. The recep-
tion frequency
rises as the tuning voltage rises, so the
AFC
voltage is lower than the input threshold voltage,
pin (10) of 1C102
becomes ''L"' and the tuning voltage
gradually drops. Then, the AFC voltage rises again and
the above operation
is repeated. Since this operation is
done very quickly, the tuning voltage is stabilized and
a stable picture is obtained on the screen.
pin (1) of the two flip-flop circuits in 1C103, so pin(3)
is set to "H", the ""L" signal enters pin 8, and pin 11
is reset to "L''. Then, pin (12) of 1C102 becomes ''H"'
and
pin (8) "L''
so pin (10) of 1C102
becomes
''H"'
regardless of the AFC
voltage
and the tuning voltage
gradually rises. It is necessary
to input the reset signal
"L" to pin 6 )of 1C103 and to set pin(3)of 1C103 to
"L", to finish this mode
and to let pin
of 10102
follow
the AFC
voltage.
In the same
way,
when
the
DOWN button $102 is pressed, pin (11) of 1103 is set
to "H", pin
to "L", so pin
of 1C102 becomes
"EL
and pin (8) "H". Pin
of 1C102 is set to "L'"'
regardiess of the AFC
voltage, and the tuning voltage
drops
gradually.
This
mode
continues
until the reset
signal ''L" is input to pin (8) of 1C103 and pin @) of
1C103 is set to "L''. Accordingly, it is necessary to reset
pin © )of 1C103 in the UP mode and to reset pin(8)of
1C103
in the
DOWN
mode,
when
the next broadcast
signal-can be tuned to by the AFC voltage generated by
that broadcast signal, after the channel-selector button
is pressed and the tuning voltage changes gradually to
the next broadcast signal.
The
circuit consisting
of pins G)~ @)of 1C102 and
—~10—
1C104
generates this reset signal. It is required to dis-
criminate whether or not the signal entering through the
antenna
is the video carrier wave of a broadcast signal.
This is done by judging whether or not the correct AFC
voltage is output or whether the correct synchronizing
signal
is output
to thé synchronization
separation cir-
cuit. Pins (1) ~ @) of 1C102 are the Schmitt circuit to
convert the AFC voltage to a "L" or ''H" digital signal;
when
the AFC
voltage
is sufficiently
low, pin (4) of
1C102
is "H", pin GB), "L'" and when it is sufficiently
high, pin @)of 1102 is "L" and pin (3), "H". Pins(1)~
6) of 1C104 is the circuit which judges whether or not
the
normal
synchronizing
signal
is present; when
the
synchronizing
signal is present with the same timing as
the
horizontal
pulse,
pin
of 1C104
becomes
"'L"
periodically and C113
is discharged through R115 and
D109, so the voltage at pin (2 )of 1C104 becomes lower
than the input threshold
voltage and pin (3) of 1C104
becomes '"'H". When a normal synchronizing signal is not
present,
pin 4
of
10104
becomes
"H",
so C113
is
charged through R162 and R115 and pin 2 of !C104 is
set to "'H" and pin GB), to "L''. The reset signal (reset
during
''L'') entering
pin 6) of 1C103 during the UP
mode
is obtained from pin 40 of 1C104, and pin
of IC 104 becomes "'L'' when pin(3) of 10104 is "H" and
pin 3) of 1C102,
"H'', that is to say when a normal
synchronizing
signal is present and the AFC voltage is
high. In the same way, the reset signal (reset during 'L'')
entering
pin
of 1C103 during
DOWN
mode
is ob-
tained from pin a) of 1C104; is this case, the synchro-
nizing signal is present and the AFC voltage is low.
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