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Muting
Circuit:
When
the
receiver
is
detuned
from
a
signal,
the
signal
decreases.
Since less
i-f
signal
is then rectified
by
diodes D2O7
and
D208,
the
negative
output
voltage
of
these
rectifiers can
no
longer back
the
positive
voltage applied
to the
base
of
transistor
Q207
through adjustable resistor
R265 (FiC.
l-3).
This
allows Q207
to
conduct lowering its collector
voltage.
Since
the collector
of
Q207 is connected
to
the
base
of
PNP
transistor
Q2
10, Q2
l0
conducts when
its
base
voltage
decteases
(with
respect
to
ground),
thereby
causing
transistor
Q209
to
conduct.
The
collector
voltage
of
Q209 then
drops
to
near
ground potential.
The voltage
at the collector
of
Q209
is
fed
to
the
base
of
transistor Q405
through MUTING switch
5904.
Since
this voltage
is
so
low
Q405
cannot
conduct
and
complete the
emitter circuit
of
transistor
Q404. This
prevents Q404
from amplifying the
detector output.
When
tuned
to a
signal,
the
opposite
actions
occur. I-f
signal
through
capacitor C228
is
rectified
into
negative
d-c
voltage
by
diodes
D207
and
D208.
This
voltage
cuts
off
transistor Q207 and
eventually
turns
on
transistor Q405, thereby enabling transistor
Q404
to
amplify
the
detector
output. The
muting
level can
be
adjusted
by
potentiometer R265.
The
muting
level
is usually
set
approximately 20
dB
lower
than
the
signal
level.
If
the
receiver
tuning
is shifted
within
range
B
of Fig.
1-4,
the
difference
between the
collector voltages
of
transistors
Q212
and Q2l3
becomes
large enough drops
across
resistors
R254
and
R255,
thus lowering the
base
voltage
of
PNP
tran-
sistor
Q2
l0
and results
in its conduction.
As before,
the
conduction
of
Q2
l0
begins
a
chain
of
events
which
prevents Q404
from amplifying.
If
the
tuning
is shifted
within
range
C
of Fig. l-4,
the difference
between
the collector
voltages
of
transistors
Q2
l2
and
Q2
13
is
so
small
that
transistor
Q2
l1
is turned
off
within this
range as
well
as
in
range
A.
Noise
components caused
by
detuning, however,
are
coupled
to
transistor Q208
through
capacitor
C234
and
resistor R249
from
the detector
output.
C-f-a-f--
A-_-+-B-)--c
Fig.
l-4
Discriminatorcharacteristic
!
v
trom
d
r
sc.rmr
notor
O2lA
.27a
_
wlt--"
a209
R252
R249
9
7
Fig.
l-3
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