Icom IC-725 Service Manual page 9

4-1-5
IF
CIRCUITS (MAIN
UNIT)
The
1st IF signal
from
Q15
is
converted
to
a 9
MHz
2nd
IF signal at
the
2nd
mixer
(iCI).
IC1
is
a
DBM
(Double
Balanced
Mixer).
EXACTNESS 2ND
IF
FREQUENCY
MODE FREQUENCY
(MHz)
SSB
9.0115
CW
9.0106
AM,
FM
9.0100
The 2nd
LO
signal (61.44
MHz)
from the
PLL UNIT
via
J4
is
applied
to
the
2nd
mixer.
The
converted
2nd
iF signal
passes through
D4
(D35
for
transmitting)
and
is
applied
to
the
MCF
(FI2) to
suppress unwanted
signals.
The
signal
output from
FI2
passes through the noise
blanker gate
(D5~D8) and
is
amplified
at
the
2nd
IF
am-
plifier (Q21).
The
signal
enters
one
of
the three 9
MHz
filters
(FIS, FI4,
optional
CW
narrow
filter)
or optional
AM
•
FM
UNIT
via
D52.
The
filters
are selected with
mode
selecting signals
(SSB
•
CW, AM, CW-N) and
the
"T8"
voltage
line.
The
signal
from a 9
MHz
filter is
amplified
at
the
2nd
IF
amplifiers
(Q27~Q29) and
applied
to
the
demodulator
circuit.
Dual-gate
FETs
are
used on
the 1st
and 2nd
IF
amplifiers
(Q1
5,
Q21
,
Q27).
The 2nd
gates of
Q1
5,
Q21 and
Q27
are
controlled
by
AGC
bias
voltage.
A
rapid
time constant
is
used
for
Q27
to
prevent raising
the
edge
distortion of
receive
signals.
R140, connected to the gate
of
Q28, improves the
temperature characteristics of
the receiver
gain.
R138
adjusts the receiver gain.
4-1-6
NOISE
BLANKER
CIRCUITS
(MAIN
UNIT)
The
IC-725
uses a noise
trigger
noise blanker
circuit
that
cuts out
pulse-type noise signals
at
the noise blanker gate
(D5-D8).
A
portion
of
the signals
from FI2
is
amplified
at
the noise
amplifiers (Q16, IC2)
and detected
at
the noise detector
(D1
2,
D1
3).
The detected voltage from the noise detector
is
applied
to
the noise blanker switch (Q1
9).
The
threshold level of
the noise blanker switch
is
set
at
0.9
V.
When
the
detected voltage
exceeds
the threshold
level,
Q20
outputs a blanking signal to activate
the noise
blanker gate
(D5~D8).
A
portion
of
the
detected voltage
is
applied
to
the noise
AGC
circuit (Q18)
and
fed
back
to
the noise
amplifier (IC2)
as
noise
AGC
voltages.
The
time constant of
the noise
AGC
circuit
is
determined by R43,
R47 and
C60. This
AGC
circuit
does not
operate
to
detect pulse-type noise.
When
the operating
frequency
or
mode
is
changed,
the
"DNB"
signal
line
becomes "LOW,"
turning
Q20 ON. The
noise blanker gate prevents
PLL
click
noise.
4-1-7
BFO
CIRCUIT (MAIN
UNIT)
A
9
MHz
signal oscillated
at
the
BFO
circuit
(031, XI)
is
buffer-amplified at
Q42 and used
at
the
balanced
modulator
(IC6)
and
a product detector
(ICS).
The
BFO
frequency
is
shifted
with a
mode
signal
using
D67~D69.
In
USB
mode,
the
"USB"
signal line
becomes "HIGH,"
turning
ON
D69.
The frequency
is
then adjusted with
C294
to set
the
USB
carrier point.
At
CW
mode
transmitting,
the
"CW"
signal
line
becomes
"HIGH"
and
033 becomes OFF,
turning
ON
D68.
The
frequency
is
then adjusted with
L83
to set the
CW
transmit carrier point.
In
LSB mode,
the
"LSB"
signal line
becomes
"HIGH,"
turning
ON
D67.
The
frequency
is
then adjusted with
L82
to
set the
LSB
carrier point.
IF
CIRCUIT
062
4
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