Icom IC-725

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