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In Fig. 3, pins (1)~ (8) of 1C101, Q101 and 0102
compose
the
circuit
producing
the horizontal!
period
saw-tooth wave. The horizontal pulse turns on Q101 and
charges
C114.
By charging
and
discharging
C114
at
constant current by Q102 the saw-tooth wave shown in
Fig. 4-A is produced. R127 and R126 change the ampli-
tude of this saw-tooch wave; the range of the bar indica-
tion on the screen is changed by adjusting them.
This
saw-tooth
wave
is AC-connected
with
pins (42),
(3) of 1C101
by means of C115. DC bias of pins
and
of 1C101
is determined by the voltage divided
from
the tuning voltage
by R136
and
R135 and the
adjusting position of R133.
R133 absorbs unevenness
in the input threshold voltage of pins (12), 3) of IC101;
by adjusting R133, the position of the bar indication on
the screen can be changed. The saw-tooth wave at pins
and
of 1C101
forms the waveform
shown
in
Fig. 4-B.
To indicate the bar on the screen, pin (9) of 10101
is
set to 'L"'. In this case, pin
of 1C101
becomes the
pulse waveform
shown
in Fig. 4-C, which
is obtained
by
comparing
the
input
threshold
voltage
with
the
waveform as shown in Fig. 4-B.
The
pefiod
of this pulse waveform
is the horizontal
period and is constant, but the pulse width T is deter-
mined_by the width of the saw-tooth wave at pins
and (3) of 1C101 cut by the input threshold voltage of
pins (12) and (3) of 1C101, and when the tuning voltage
is high the average DC level of the saw-tooth wave rises,
so the pulse width becomes larger, and when the tuning
voltage
is low, the pulse width
becomes smaller. The
pulse waveform
appearing at pin
of 1C101
is dif-
ferentiated
by C116
and R137 and input to the base
of Q103, so the waveform
Q103's base is as shown in
Fig. 4-D, and Q103 is cut for a certain time after pulse
width "T",
Assuming
that UHF
is being received, the +11.4V volt-
age is applied to UHF
+B in Fig. 3, so D118 turns on
Q103
js turned
on and D112
is cut off and the opera-
tion
of the
video-out
transistor
is not affected;
but
when
Q103
is turned
OFF, D118 also turns OFF, so
D112 is turned ON, and the base potential of the video-
out
transistor
becomes
higher than usual as shown
in
Fig. 4-E, so
a BLUE
bar appears on the screen. In this
case, +B (11.4V) is not supplied to VHF
L and VHF H,
so D111 and D110 are cut off.
Pin (4)~ (6) of 1C101
is the circuit which
controls
whether or not the bar indication
is output. When pin
(4) of 10101
is "L", the pulse from pin 1 of 1C101
is
transmitted
to pin (19 of {C101 and the bar indication
appears, but when
pin
of 1C101
is ""H'', pin (11) of
1C101
is always set to ""L"' and the bar indication never
appears.
Pin
of [C101 is set to "L"' in the following 3 cases.
(1) When
the
channel-selector
button
is pressed and
the self seeking circuit is in SEARCH
mode.
In this case, D103 or D104 turns ON and pin(6 )of
1C101 is set to "H".
(2) When
the indicator
switch
is pressed
(pin 6) of
B
zene. ees
1C101 is set to ""H"').
(3) When
the synchronizing signal is not present (Pin
5 of IC101 is set to "H").
It is designed so that in cases (1) and (2), the indication
is held for a while (approx. 1.5 sec) from the time when
the search
mode
is finished or the indicator switch is
released.
1H zi
11.4V
ov
IL.4V
Input threshold
voltage
OV
IW4V
OV
OV
14y
Blue origina! signal
Bar signal
OV
Blanking
signal
t
Fig. 4
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