Icom IC-725 Service Manual page 11

4-1-12
SQUELCH CIRCUIT (MAIN
UNIT)
The
squelch
circuit
mutes
the
audio output
when
the
S-
meter
signal
is
lower than the [SQL] control setting
level.
The
S-meter
signal
from
IC4 pin
7
is
applied
to
the
comparator
(IC4 pin
2) to
be
compared
to
a
threshold
level
controlled
by the [SQL] control.
The
squelch control
signal
is
applied
to control
terminals of the
AF
input
mode
selector switch
(ICS).
When
the S-meter signal
is
lower than the threshold
level,
the
comparator
turns
"HIGH" and
then
Q32
turns
OFF
to
deactivate the
AF
input
mode
selector switch. This
signal
is
applied to
Q34, turning
OFF
the [RX]
indicator
and
is
also applied
to
the [MIC]
connector
pin
4.
4-2
TRANSMITTER
CIRCUITS
4-2-1
MIC AMPLIFIER (MAIN
UNIT)
Audio signals
from the [MIC]
connector are
applied to the
[MIC]
control
and
amplified
at
the
mic
amplifier (Q45).
External
modulation
input
from
the [ACC(1)]
socket
pin
4
is
also applied to
Q45
via
R255.
The
AF
signals
from
Q45
or
CW
keying
signal
is
applied
to
the
balanced modulator
(IC6).
Q44
cuts the signals
from
Q45
in
CW
or receiving.
4-2-2
BALANCED MODULATOR
(MAIN
UNIT)
Output
signals
from
the
mic amplifier or
CW
keying signal
are applied to
the
balanced modulator
circuit (IC6) to
be
converted
to
a 9
MHz
IF
signal
using a
BFO
signal.
The
BFO
signal,
buffer-amplified at Q42,
is
applied
to
ICS
pin
7
as a
carrier signal.
ICS outputs
a double sideband
signal
and passes
through a 9
MHz
filter
to
create
an
SSB
signal.
R177 and R179
adjust
the
balance
level
of
ICS
for
maximum
carrier
suppression.
In
CW
mode,
the
CW
keying
signal
upsets the balance
to
create a
carrier signal.
4-2-3 IF
CIRCUITS (MAIN
UNIT)
The
9
MHz
IF
signal
passes through
one
of
the three
9
MHz
filters
where unwanted sideband
or out-of-band
signals
are removed.
The
filters
are selected with
mode
selecting signals
and
the
"T8" voltage
line.
The
optional
CW
narrow
filter is
not
used
in
transmitting.
The
resulting signal
is
amplified at Q22,
and
is
then
mixed
with the
2nd
LO
signal to
be converted
to
a 70.45
MHz
IF
signal at IC1. IC1
is
used
in
receiving
and
transmitting.
The
FM
signal
from the
optional
AM
•
FM
UNIT
is
amplified
at
Q22
and
is
then applied to IC1
The
70.45
MHz
IF signal
is
amplified
at
the IF
amplifier
(Q7)
and
is
then converted
to
the displayed
frequency
at
the
balanced mixer
(Q2, Q3).
The
gates
of
the
IF
amplifiers (Q7, Q22)
are controlled
by
ALC
bias voltage
from the
ALC
circuit.
R89, con-
nected to
the gate
of
Q22,
improves
the
temperature
characteristics of
the transmitter gain.
R85
adjusts the
transmitter gain.
4-2-4
RF CIRCUtTS (MAIN
AND
PA
UNITS)
The converted
signal
from
Q2
and
Q3
is
applied to
the
bandpass
filter
where
the
unwanted
LO
signal
emission
is
reduced.
The converted
signal
is
amplified
at
Q1, and
is
then applied
to
the
PA UNIT
via J11.
Incoming
signals
from the
MAIN
UNIT
are amplified
at
the
predrive amplifier
(Q1),
drive amplifier (Q2, Q3)
and power
amplifier (Q5, Q6) to
obtain stable
100
W
RF
output
power.
The
predrive amplifier
is
a
class
A
amplifier with a
Vcc
of
13.8 V.
The
drive
and power
amplifiers
are
class
AB
push-pull amplifiers with
a
Vcc
of
1
3.8 V.
A
stable bias
voltage
is
applied
to
these amplifiers.
D1 controls
a
bias
voltage
to
the drive amplifier. Q4,
D2
and
D3
supply a
bias voltage to
the
power
amplifier.
A
0.012
Q
resistor (R26),
inserted
in
the 13.8
V Vcc
line, is
provided
for
the
Ic
APC
circuit.
A
voltage
generated
at
both terminals of
R26
is
applied
to
the
MAIN UNIT
via
the
"ICH" and "ICL"
signal
line.
Thermal
switch
SI
and
thermistor
R32
detect
the
temperature
of
Q6
and
Q5
respectively,
and
control the
cooling fan speed.
TEMPERATURE
°C
("F)
Below
50
(122)
50~90
(122~194)
Above
90
(194)
THERMAL SWITCH
(SI)
OFF OFF
ON
RESISTANCE
OF
R32
HIGH
LOW
LOW
COOLING
RECEIVE
STOP
LOW
HIGH
FAN SPEED
transmit
LOW
HIGH
COOLING FAN
CONTROL
CIRCUIT