Adjustable Selectivity Employing Ceramic Filters - Sony CRF-230 Service Manual

The noise
is
amplified, rectified into
negative
d-c
voltage
by
diodes
D2l0
and D2 I
l,
and applied to
the
base
of
transistor
Q2
l0
to turn
it
on.
Since the
base
voltage
of
Q208
is controlled by
the collector
voltage
of
Q210,
the amplification
of
transistor
e208 in-
creases
due
to
increased
base bias, and
transistor
Q210
is held
conducting
quite
reliably. As
before,
transistor
Q404 cuts
off
the detector output.
Positive feedback through resistor R252
and
capacitor
C235
from
transistor e2
l0
aids
in
turning
off
Q209.
Adjustable Selectivity
E
mploying
Ceram
ic
Filters:
The bandwidth
in
a-m reception can
be altered
by
changing the
coupling
between
the
sections
of
ceramic filters
in
the
a-m
i-f circuit.
When
bands SW2
through
SWl9
are
selected, ceramic
filter CF50l
is
automatically set
to
narrow bandwidth by
switch
5501. Similarly, when
bands
LW, MW
or SWI
are
selected, a wide bandwidth
is automatically
selected.
Ceramic
filter
CF502 can be manually set
to
narrow or
wide bandwidth
by
switch 5502.
When set
to
SHARP, switch
5502
also connects
in
the
high+ut
filter
consisting
of
coil L507
and
capacitors
C537,
C538 and C541.
The net result of
the switch manipulations on
the
ceramic
filter
circuits
are summarized
in
Table. The
overall selectivity
curves
of
the
a-m
i.f strip
at each
of
the
four
possible
switch
combinations are shown
in
Fig.
l-5.
Band
setting
cF501
(3501-1-4)
cF502 (5502-1-41
High-cut
filter
(s502-5,
6)
Overall
response
(Fie.
l-5)
Band
width
C.oupling
Band
width
Coupling
sw2
through
swl9
Sharp
c523
R528
Sharp
f*
ON A
Broard
OFF
LW
MW
swl
Broard
c522
R527
r
f-]
our
Sharp
ON C
Broard
OFF D
8-