Icom ic-775 Service Manual page 18

An
external
ALC
input
from
the [ALC] jack
or
the [ACC]
The
selected
AF
signal
is
applied
to
the
AF
amplifier
circuit
sockets
is
applied
to
the
buffer amplifier (Q4). External
ALC
in
the
MAIN
unit,
operation
is
identical to that of
the
internal
ALC.
4-2-10
APC
CIRCUfT
(IF
UNIT)
4-2-13
RF,
SWR,
ALC,
COMP,
Id,
Vd
METER
CIRCUITS
(IF
UNIT)
The
APC
(Automatic
Power
Control)
circuit
protects the
power
amplifiers
on
the
PA
unit
from high
SWR
and
exces-
sive current.
The
reflected
wave
signal
appears and
increases
on
the
antenna connector
when
the antenna
is
mismatched. D1
of
the
SWR
detector
circuit (L2,
D1,
D2)
in
the
CTRL
unit
detects the
signal
and
applies
it
to
IC2a
in
the
IF unit
as
the
"REF"
signal.
When
the
"REF"
signal level
increases,
lC2a decreases
the
"ALC"
voltage
via IC1
b
to
activate the
ALC.
For the current
APC,
the
power
transistor current
is
ob-
tained
by
detecting the voltages
("ID-i-"
and
"ID-")
which
appear
at
both terminals of
a
0.012
Q
resistor
(R35)
on
the
PA
unit.
The
detected voltage
is
applied
to
the
differential
amplifier
(ICIa
pins
2, 3).
When
the current
of
the
final
transistors
is
more
than
1
8 A, IC1 a controls the
ALC
line
via
IC1 b
to
prevent excessive
current
flow.
When
activating
the antenna tuner
with
manual
tuning, the
"PODN"
signal turns
Q7
ON. As
a
result,
the
"POC"
voltage
is
shifted to
reduce
the output
power
to
50 W.
4-2-11
TEMPERATURE
PROTECTION
CIRCUIT
(PA UNIT)
The
cooling fan
(MF1)
is
activated while transmitting or
if
the
temperature
of the
final
transistors
exceeds
the preset
value.
While
transmitting,
Q5
and
08
are turned
ON, and
provide
a voltage
to
MF1.
MF1
rotates
at
medium
speed
in
this
case.
Thermistor
R29
detects the
temperature
of
Q4b.
When
the
Q4b
temperature
exceeds 60°C
(140T),
R29
activates
07
and
09
to
accelerate the cooling
fan.
MF1
rotates
at
high
speed
at
80
"C
(176T) or
more.
R29 keeps
the cooling fan
rotating
even
when
receiving
until
the
Q4b
temperature drops
to
40
°C (104°F) or below.
While
transmitting,
RF,
SWR,
ALC,
COMP,
Id
or
Vd
meter
readings are
available
and can be
selected with the
[METER]
switch.
(1
)
Power
meter
circuit
The "FOR"
voltage from the
CTRL
unit
is
applied
to
the
power meter
amplifier
(IC2b pin
5).
The "POM"
signal
from
IC2b
(pin
7) is
applied
to
the meter
switch (IC6 pin 15).
R456
and C21 0
are
used
for
peak
hold.
(2)
SWR
meter
circuit
The "FOR"
voltage, amplified at IC2b,
is
applied
to
the
SWR
meter
circuit (IC4, ICS, 01, 02).
The "REF"
voltage, ampli-
fied at
lC2a,
is
also applied
to
the
SWR
meter
circuit.
IC4b
outputs a
signal
according
to
the
ratio
of
"FOR"
to
"REF"
voltage.
The
signal
is
applied
to
the
meter
switch (IC6
pin
14).
(3)
ALC
meter
circuit
The
ALC
bias voltage
is
applied to
the
inversion-amplifier
(IC3b
pin 6)
and
is
then
applied
to
the
meter
switch (IC6
pin
12
).
(4)
COMP
meter
circuit
A
portion of the
RF
speech compressor
(035)
output
is
detected
at
D33,
applied
to
the buffer-amplifiers
(037,
IC7a
pin
3),
and
is
then applied
to
the
meter
switch (IC6
pin
1).
(5)
ID
meter
circuit
A
portion of
the current
APC
circuit (IC1 a)
output
is
applied
to
the meter
switch (IC6 pin 5) via
R31
9.
(6)
Vd
meter
circuit
The
"ID-"
voltage
is
divided
at
R326 and R327 and
is
applied
to
the
meter
switch (IC6 pin
2).
(7)
Meter switch
According
to
the transmit meter
selection
and
operating
condition,
IC6
selects the
meter
signals
and
outputs from
pin
3 as
the
"MET
signal.
The "MET"
signal
is
applied
to
the
S/RF
meter
(DISPLAY
unit
MEl)
via
the
main and sub
CPUs.
4-2-12
MONITOR
CIRCUIT
(IF
UNIT)
Transmitted
IF signals
can be
monitored
to
check
voice
characteristics
when
[MIC
TONE]
is
adjusted
or
the transmit
low-pass/high-pass
filters
(DSP
functions)
are
in
use.
A
portion of
the
9
MHz
transmit IF signals from
045
are
amplified
at
062
and
are then demodulated
at
IC20
with the
MBFO
signal
from the
PLL
unit.
The
demodulated
AF
signal
is
applied
to
the
AF
selector (IC21).
The
CW
side
tone signal
(300-900
Hz) from the
PLL
unit
is
also applied
to
the
AF
selector via the
AF
gate
(063).
063
switches the
side
tone signal with a keying
signal.
Meter switch
IC6
(IF)
4-7