Icom IC-725 Service Manual page 10

BFO
FREQUENCY
IN
EACH MODE
MODE FREQUENCY
(MHz)
USB
9.0130
CW(Tx) 9.0106
LSB
9.0100
CW
(Rx)
9.0098
AM NO OUTPUT
4-1-10
AF
AMP
CIRCUIT (MAIN
UNIT)
The
AF
signal
from
the
AF
input
mode
selector switch
is
applied
to
the
AF
preamplifier (Q35, Q36).
The
CW
sidetone
signal
is
applied
to
Q36.
The
output
from
the
AF
preamplifier
is
applied
to
the [AF]
control
(FRONT
UNIT, R1
b)
and
the 2.8
kHz
cut-off active
low-pass
filter
(Q37).
The AF
signal
is
power-amplified
at
IC9
to
drive the speaker.
4-1-8
DEMODULATOR
CIRCUITS
(MAIN
UNIT)
The
IC-725
has 2
detector
circuits,
a product detector and
a diode detector
to
demodulate
the
SSB,
CW
signal
and
AM
signal respectively.
In
SSB
or
CW
mode,
the
2nd
IF signal
from
the IF amplifier
(Q29)
is
mixed
with the
BFO
signal at the
product detector
(ICS) to
demodulate
the
2nd
IF signal
into
an
AF
signal.
The
detected
signal
passes
through
the
AF
input
mode
selector switch
(ICS).
In
AM
mode,
the
2nd
IF signal
from
029
passed through
Cl
21
is
detected
at
D62 and
passes through
the
AF
input
mode
selector switch
(ICS).
4-1-11
AGC AND
S-METER CIRCUIT
(MAIN
UNIT)
The
receiver gain
is
determined by
the voltage
on
the
AGC
line (030, collector).
When
strong signals
are
received,
the
AGC
circuit
decreases
the voltage
on
this
line.
The
IF signal
from
the
IF amplifier (029)
passes
through
Cl
1
7, is
detected
at
D59 and
D60, and applied
to the
base
of
030.
A
time constant
(Cl
13,
R120)
is
connected
to
the
AGC
line
that
determines the
AGC
release
time.
The
time constant
is
controlled
by
the
[AGC]
switch.
When
the
[AGC]
switch
is
pushed OUT, Cl 12 and R119
are
connected
in
parallel
with the
AGC
line to
obtain a slow
AGC
release
time.
4-1-9
AF
INPUT
MODE
SELECTOR SWITCH
(MAIN
UNIT)
The
AF
signal
from a
detector
circuit
or the optional
AM
•
FM
UNIT
is
applied
to
the
AF
input
mode
selector switch
(ICS).
ICS consists
of
4 analog switches and
they are
selected with
a
mode
signal
from IC15 and
the squelch
control signal.
The
AF
signal
is
applied
to
the
AF amp
circuit.
ICS
AF
INPUT
MODE
SELECTOR SWITCH
MODE
ACTIVATING
PIN
NUMBERS
CONTROL
PIN
NUMBER
USB,
CW
2-^1
13
AM
3^4
5
FM 10-^11
12
ANY MODES
(for
S-meter)
9->8
6
The
AGC
bias voltage
is
applied to the
differential
amplifier
(IC4,
pin
6)
where
the difference
between
the bias
and
reference voltages
is
detected.
The
resulting
S-meter
signal
passes
through the
meter
switching
circuit (ICS)
and
is
then applied
to
the
meter on
the
front panel.
The
reference voltage
is
adjusted with R116. ICS pins
8 and
9
are
connected
inside
the 1C
in
receiving.
The
FM
S-meter
signal
from
the optional
AM
•
FM
UNIT
is
applied
to
the
meter
switching
circuit (ICS) via
D57.
The
signal
is
also applied
to the
squelch
circuit
(IC4 pin
2).