Icom IC-725 Service Manual page 12

4-2-5
RF
FILTER CIRCUIT (PA UNIT)
The
PA
UNIT
has
6 Chebyshev
low-pass
filters.
The
signal
from
the
power
amplifier (Q5, Q6), applied to
one
of
the low-pass
filters
depending on
the transmit
frequency
range,
suppresses high
harmonic components.
The
filter
switching
voltage,
obtained
at
the
PLL
UNIT,
is
applied to
the
PA UNIT
via P7.
FREQUENCIES
AND
APPROPRIATE
FILTERS
FILTER
FREQUENCY RANGE
(MHz)
LI
Below 2
L2
2~4
L3
1
CO
L4
8~15
L5
15~22
L6
22-30
The
filtered
signal
passes through the
SWR
detector
circuit (L51)
and
is
then
applied
to
the antenna connector.
The
forward signal
from
L51
is
detected
at
D7 and
applied
to
the
MAIN UNIT
as
the
"FOR"
voltage.
The
reflection
signal
from
L51
is
detected
at
D8
and
applied
to
the
MAIN
UNIT
as the
"REF"
voltage.
4-2-6
ALC
CIRCUIT (MAIN
UNIT)
The
ALC
(Auto Level Control)
circuit
stably controls the
RF
output
power
using the
[RF
POWER]
control.
The "FOR"
voltage
from the
PA
UNIT
is
applied to IC11
pin
2
and IC10
pin
3.
The
"POC"
voltage controlled
by the
[RF
PWR]
control
is
also applied to IC1
1
pin
3 as
the reference voltage.
When
the
"FOR"
voltage
exceeds
the
"POC"
voltage,
ALC
bias voltage
from
IC11 pin
1
controls the IF
amplifiers
to
reduce
the output
power
until
the
"FOR" and "POC"
voltages are equalized.
In
AM
mode,
IC11
operates
as
an averaging
ALC
amplifier,
because a
capacitor
on
the optional
AM
•
FM
UNIT
(C51)
is
connected
to
the
cathode
of D76.
Q54
turns
ON
and
the
"POC"
voltage
is
shifted for
40
W
AM
output
power (maximum).
The
ALC
bias voltage from IC11 pin
1
is
also applied
to
the inversion-amplifier
(IC1
1
pin 6) to control
an
intensity
of
the
[TX] indicator,
showing
the
ALC
level.
An
external
ALC
input
from
the [ALC] jack
is
applied
to
the
buffer amplifier (Q53).
ALC
operation
is
identical to
that of
the
internal
ALC.
4-2-7
APC
CIRCUITS (MAIN
UNIT)
The
APC
circuits
protect the
final
transistors
from
high
SWR
and
excessive
current.
The "REF"
voltage
from
the
PA
UNIT
is
applied
to
Q56.
When
the
"REF"
voltage
exceeds
the
reference
voltage,
determined by
R203
and
R204,
Q56
turns
ON
and
the
"POC"
voltage
is
shifted
for
12
W
output power.
The "ICH" and
"ICL" voltages are applied to the
Ic
APC
amplifier
(IC10,
pins 5
and
6)
and
then
to
the
ALC
bias
voltage
line
to
prevent excessive current
flow.
4-2-8
CW
KEYING CIRCUIT (MAIN
UNIT)
When
the
CW
key
is
closed,
the
"KEY"
signal
line
becomes "LOW." Q38
outputs
8
V
to
control break-in
operation, sidetone signal
and
transmit
signal.
When
the
[BK
IN]
switch
is
pushed
IN,
8
V
from
Q38
charge C252 and
026
is
turned
ON,
turning
ON
Q52.
Q52
grounds the
SEND
line
for transmitting.
The [DELAY]
control (R244) adjusts the transmit release
delay
time.
The 8
V
from
038
charges
C249
and D91
is
turned OFF,
disconnecting
C249
from 040.
040
then oscillates
a
sidetone
signal.
R268
prevents sidetone click noise.
TTie
8
V
from
038
is
applied
to
a time constant
and
then
to
the
balanced modulator
(IC6) to
create
a
carrier
signal.
R241
in
the time constant adjusts
a
transmit delay timing
for
12 msec.
ALC
CIRCUIT
4
—
5