Icom IC-04A Service Manual page 9

4-1-4
S-METER
CIRCUIT (PLL UNIT)
A
portion of signals
passed from FI202
is
amplified at S-
meter
amplifier
Q205
and Q206, and
is
detected
at
voltage doubler
rectifiers
D201 and D202.
These
signals are
then
applied to
meter comparator
IC105A
on the
MAIN
UNIT.
4-1-5
AF CIRCUIT (MAIN
UNIT)
AF
signals
output
from
pin
9 on IC201 are applied to the
MAIN
UNIT, and pass through
a
de-emphasis
circuit
consisting of
R127 and Cl
17.
These
signals are
then
amplified
at
AF
controller
Q105.
This
de-emphasis
cir-
cuit
is
an integrator circuit
with
frequency
characteristics of
6dB/oct.
The
AF
amplifier circuit
consists of
Q1
16~Q122.
The
input section
(Q1
1
6 and
Q1
1 7)
functions as a differen-
tial
amplifier to
ensure
stable
operations
and
a suitable
frequency response by the negative
feedback network
of
R152 and R149.
The
AF
power
amplifier
circuit is
a
complementary
SEPP
circuit
with a Darlington
connec-
tion of
Q1
1
9~Q1
22. This
circuit
drives
the speaker.
When
the
power
source voltage
is
more
than
10V,
D106
and
voltage regulator
Q106
limit
output voltage
and output power,
stabilizing
the
bias.
A
standby current
suppressor (Q1 32)
is
installed
to sup-
press the current
and
residual
noise while the
squelch
is
closed.
4-1-6
SQUELCH CIRCUIT (MAIN
UNIT)
Noise
components from
pin
9
of
IC201
are fed to active
filter
IC101B through
SQUELCH CONTROL
R126.
This active
filter is
a
high-pass
filter,
and
amplifies ap-
proximately
20kHz
noise
components. The
noise
com-
ponents
are
then
rectified
by
D103
and
converted to
DC
voltage
at
R1
1 9,
R1 20,
Cl
1 1
,
and Cl
1
2.
The
DC
voltage
passes through
inverters
IC103B and
IC103A
to obtain a
TTL
level
from
a
linear level.
AF
controller
0105
is
turned
OFF
by the
"HIGH"
voltage
level
from
pin
2
of
IC103A. Output
signals
from
pin
2
of
IC103A
are also
fed
to
MPU
IC401
in
the
LOGIC UNIT
through
D104
as a
squelch
signal.
When
no RF
signal
is
received, noise rectified
output
voltages
from
D103
are
"HIGH".
Pin
2 of
IC103A
thus
becomes "HIGH" and
0105
turns OFF.
AF
output
is
then cut OFF.
In
transmit
mode,
T
-l-
5
signals are applied to pin
9 of
IC103B
via
D 105
to turn
0105
OFF.
4-1-7
1
St
LO
CIRCUIT
(VCO
AND
PLL UNITS)
450MHz
band LO
signals
from the
VCO
UNIT
are buffer
amplified
at
0208
and fed to transmit/receive swit-
ching
circuit
D208
in
the
PLL UNIT.
The
signals are
then
applied to
the base of
1
st
mixer
Q203
as
1
st
LO
signals.
4-2
TRANSMITTER CIRCUITS
4-2-1
MICROPHONE
AMPLIFIER CIRCUIT
(MAIN UNIT)
AF
signals
from the
INTERNAL
MICROPHONE
or
from
EXTERNAL MIC
JACK
J202
are amplified
at a limiter
amplifier
consisting of
Q101~Q104.
This limiter
amplifier
is
formed by
a
negative
feedback
circuit
with frequency
characteristics set at
6dB/oct.
from
300Hz
to
3kHz.
This
causes the
limiter
amplifier
to function as a
pre-emphasis
circuit.
Output from the
limiter
amplifier
is
similar
to a rectangular
waveform
and
includes
harmonic components.
Harmonic components
higher
than
3kHz
are
attenuated
by splatter
filter
IC101A.
AF
signals
from
IC101A
pass through modulation
ad-
justing
trimmer pot
R226
in
the
PLL
UNIT
and
are
then
applied to the
anode
of
D703
in
the
VCO
UNIT
for per-
forming frequency modulation.
4-2-2
BUFFER AMPLIFIER CIRCUIT
(PLL UNIT)
430
or
440MHz
band
signals
output from the
VCO
UNIT are buffer amplified
by
0208
and pass through
transmit/receive
switching
circuit
D209. They
are
then
amplified at predriver
021
1
,
and
driver
021
2,
thus ob-
taining a
wideband
of
1
50mW.
4-2