Transmitter Circuits - Icom ic-775 Service Manual

4-1-21
AF AMPLIFIER CIRCUIT
(MAIN
UNIT)
The
AF
amplifier amplifies
the
AF
input signal to
a
suitable
driving level for
the speaker.
The
AF
signal
from
the
AF
low-pass
filter
(IC1 b)
is
applied
to
the
AF
amplifiers (IC24a, IC7a).
The
CW
side tone/
transmit
monitor
signal
and beep tone are
amplified at
tC7b
and
IC24b, respectively.
The
amplified
signals
from
IC7a,
IC7b and lC24b
are
applied to the
[TONE]
control
(VR-A
unit
R32)
and
then
to
the
VGA
(Voltage Controlled Amplifier,
IC16)
circuit.
The
AF
gain
setting
from the
main
CPU
(pins
42-51)
is
con-
verted to
DC
voltage
at
R58-R77
and
applied to
the
VGA
control
terminal
(IC1
6
pin
8)
via
IC20a.
The AF
signal
from
1C16
(pin 9) is
power-amplified
at
IC6 to drive
the speaker.
4-2-2
VOX
CIRCUIT (MAIN
UNIT)
The
VOX
(Voice-Operated-Transmission)
circuit
sets trans-
mitting conditions
according
to
voice
input.
The
microphone
signal
from ICS
pin
4 passes through the
[VOX-GAIN]
control
(VR-A
unit
R45) and
is
amplified at
IC23b.
The
signal
is
then applied
to
the
VOX
comparator
(IC23d
pin
13)
to
switch Q23, IC17, IC22,
Q5.
When
voice
levels
exceed
the
comparator
level,
the
VOX
circuit
sets the
transceiver
to
transmit.
On
the other hand, a speaker
drive signal
from the
AF
power
amplifier (IC6)
is
applied to
the
antl-VOX comparator
(IC23a
pin 3)
via the
[ANTI-VOX]
control
(VR-A
unit
R44).
When
audio output
level
increases,
this
comparator
cuts out
the
VOX
comparator
input using
06.
4-2
TRANSMITTER
CIRCUITS
4-2-1
MICROPHONE AMPLIFIER CIRCUIT
(MAIN
AND
IF
UNITS)
The microphone
amplifier circuit
amplifies
microphone
input
signals
and outputs the amplified signals
to
the
balanced
modulator,
DSP
(PSN)
modulation
circuit
or
FM/AM
modu-
lation
circuit.
Audio signals
from the [MIC] connector are amplified
at
VCA
1C (IC8).
The
amplified signals
from
pin
4 are applied
to
the
[MIC
TONE]
control
(VR-A
unit
R43) and then
to the
VCA
section of
IC8
(pin
7).
The
microphone
gain setting
from
the
main
CPU
(pins
32-41)
is
converted
to
DC
voltage
at
R38-R57 and
applied
to
the
VCA
control terminal (ICS
pin
8)
via
IC20b.
The
resulting signals
from
pin
9 are then
applied to
IClOa
(IF unit) via
the
"MIG2"
signal
line.
Exter-
nal
modulation input
from
the
[ACC(1
)]
socket
(pin
4)
is
also
applied
to
IClOa
(IF unit).
The
amplified signals
from IClOa
are then applied
to
the
AF
selector switch (IC9)
for
SSB
modulation or the
amplifier
(IClOb) for
FM/AM
modulation.
When
the
DSP
modulation
Is
turned
ON,
pins 3/5
and
13/14
of
the
AF
selector switch (IC9)
are connected. Therefore,
the amplified signal
is
applied
to
the
DSP
unit
and
the
resulting signal (15
kHz
IF)
is
applied
to
the mixer (ICS) via
IC9.
4-2-3
BALANCED MODULATOR
(IF
UNIT)
The balanced modulator converts the
AF
signal
from
the
microphone
amplifier to
a
455 kHz
IF signal
with
a
BFO
(Beat
Frequency
Oscillator) signal.
Microphone
signals
from
the
AF
selector switch (IC9 pins
1
,
3)
are applied
to
the
balanced modulator
(ICS pin
6).
The
BFO
signal
from the
PLL
unit is
applied
to
ICS
(pin
8)
as a
carrier signal.
ICS
is
a double balanced mixer
1C
and outputs a
double
side
band (DSB)
signal with
-40
dB
of carrier
suppression.
R273 and
R274
adjust the
balanced
level
of
ICS
for
maxi-
mum
carrier
suppression.
The
resulting signal
passes
through the
455 kHz
IF
filter
(FI4;
FI6
when
PSN
modu-
lation) to
suppress unwanted sideband
signals.
When DSP
modulation
is
turned
ON,
ICS
functions
as a
mixer.
ICS mixes the
1
5
kHz
IF signal with
a
440 kHz LO
to
obtain
a
455 kHz
IF signal.
In
CW
and
RTTY
modes,
IC9 pins
2 and 3
are
connected
to
upset the balance of ICS via
R268
for
leaking the
BFO
signal
as a
carrier signal.
CW
keying
is
controlled at
01
0
via
the
"WFM4"
signal.
.
TRANSMITTER CONSTRUCTION
4-5