Akai VS-2EGN

11-3-6 AUDIO CIRCUIT
1)
In
Fig.
7-113
(D)
a
beat
component
at
5.5
MHz
cor-
responding
to
the
frequency
difference
between
the
video
and
audio
carriers,
is
picked
up
by
C25,
C26,
L11,
and
CF3
(ceramic
filter),
and
is
fed
to
IC2
terminals
1
and
2,
and
is
further
detected.
R38 (1 kohm) is a terminating resistor.
2) The audio signal fed to IC2 has its amplitude made
constant,
and
fed
to
the
FM
detector,
and
fed
to
the
FM
detector.
After
detection,
the
low
frequency
signal
passes
through
the
electronic
volume
control
buffer,
is
level
adjusted
at
VR3,
and
becomes
an
audio
output.
3) FM detector
The
detector
used
here
is
called
the
peak
differential
detecting
circuit.
Its
features
are
that
a
single
tuned
cir-
cuit
suffices
and
that
a
large
output
is
available.
Fig.
7-168
shows
the
FM
detector,
and
the
part
within
IC2
is
its
equivalent
circuit.
The
tuning
circuit
(LT,
CT,
C30)
resonates
at
f2
(Fig.
7-169)
between
terminal
9
and
earth,
and
to
fl
between
terminal
10
and
earth.
Signals
at
terminals
9
and
10
are
fed
to
the
bases
of
TR1
and
TR6
respectively,
and
fed
to
TR2
and
TRS
via
the
emit-
ter
follower
for
peak
value
detection.
This
detection
output
is
fed
to
the
differential
amplifier
(TR3,
TR4),
and
the
differential
output
is
picked
up
from
the
collec-
tor
of
TR4.
Thus,
the
output
of
excellent
linearity
may
be
obtained.
4) Electronic volume
The
level
is
varied
by
controlling
the
base
bias
current
of
the
transistor.
Normally
IC2
pin
©
is
locked
to
con-
stant
bias
by
D8
and
R40.
In
Fig.
7-113(D)
during
the
v
TO CONSTANT
VOLTAGE CIRCUIT
Fig. 7-168 FM detector
OUTPUT (Vv)
DIFFERENTIAL OUTPUT-
Fig.
7-169
FM
detector
output
characteristic
Mute
mode,
Mute
terminal
becomes
HIGH
level,
TR6
conducts,
and
the
audio
signal
is
cut
off.
Mute
is
effect-
ed
during
channel
switching
and
search.
Since
the
AFT
operation
cannot
be
made
during
the
Mute
mode,
TR9
is
turned
OFF,
AFT
line
is
isolated,
and
the
voltage
is
locked
to
the
center
AFT
voltage
6.5V,
determined
by
R14
and
R15.
11-3-7 BAND SWITCHING CIRCUIT
In
Fig.
7-113(D),
the
band
switching
circuit
is
composed
of
D9,
D10,
D11,
TR10,
TR11,
TR12,
etc.
Any
of
the
channels
(VU,
VH,
and
VL)
specified
by
the
operation
PCB
becomes
LOW,
the
trarisistor
(any
of
TRIO—12)
of
that
specified
channel
conducts,
power
is
fed
to
the
tuner
and
the
demodulator
AGC
circuit,
and
the
band
is
switched
selected.
nn
Service
Manual
VS-2EGN
———_—___$________
97