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Timing Trigger Circuit
This circuit is designed to delay the lighting up
of the
indicator
LEDs
(TUNE,
LOCKED,
STE-
REO) when the power is switched on, and to turn
them off immediately when the power is switched
off again. (See Fig. 4-10).
When
the power is switched
on, Q2. is turned
off immediately
by a negative voltage applied via
D,.- The base voltage of Q,, is thereby increased
gradually,
the
increase
being
controlled
by the
Rii3/C1;3
time constant. Once the voltage reaches
+1.2V (approx.), Q,7 and Q., are both turned on,
thereby tuming on all relevant indicator LEDs.
When
the
power
is switched
off again,
the
negative voltage applied via D,, is cut immediately,
resulting in Q., being turned
on. C,,3; therefore
discharges directly via Q.. to turn Q,7 and Q2, off.
All
indicator
LEDs
will
consequently
turn
off
immediately.
This
same
result
is also
achieved
when
the FUNCTION
selector is in the AM posi-
tion. This is due to the fact that the base of Q,, is
connected
to ground by the FUNCTION
selector,
thereby
turning Q,7 and Q», off.
Re-lock Trigger Circuit
When
the power
is switched on, this re-lock
trigger circuit activates a sweep of the local oscil-
lator frequency, covering a range of up to 100kHz
both
sides of the frequency
indicated by the dial
pointer
at the time. If a signal of antenna input
level in excess of 20dBf is detected in this range,
the
frequency
is automatically
re-locked
by the
APC circuit. (See Fig. 4-11).
In
this case, when
the power
is switched
on
Q.3 is turned off immediately by the negative volt-
age applied via D,,, resulting in the voltage changes
at different
places describing
different curves
as
shown
in Fig. 4-12.
The
voltage
applied
to the
variable capacitance diode in the local oscillator is
represented by curve (e), thereby causing the local
oscillator frequency to change, or '"'sweep" within
a limited range. If an input signal whose antenna
input level exceeds
20dBf is detected during this
sweep,
the APC circuit FET
switch is turned on,
Fig. 4-11
Re-lock trigger circuit
10
Muting Control Circuits
This tuner features 3 major muting actions.
(1) Inter-station muting in the FM band and mut-
ing of weak FM stations.
(2) Muting of switching noises when
FUNCTION
selector and IF BAND switch are operated.
(3) Muting when POWER
switch is turned on and
off.
All muting
action is controlled by the muting
gate included
in the AF MUTE
IC (PA1002-A).
(See Fig. 4-13).
(1) Inter-station Muting in FM Band
(3) Power Switch Muting
The muting trigger employed when the POWER
7
res ON
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oe
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Cee
ee ah
1
5
es
ee
ON
I
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canter
oo
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Fig. 4-12
Voltage waveforms
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