Transmitter Circuits - Icom IC-T7A Service Manual

4-1-7
AF AMPLIFIER CIRCUIT (LOGIC
UNIT)
The
AF
amplifier
circuit,
Including
an
AF
mute
switch,
amplifies
the
demodulated
signals to drive
a
speaker.
The
demodulated
AF
signals
("DETO"
signals)
from the
FM
IF IC (IC 101)
on
the
2F
unit
are applied
to
the drive
amplifier
(Q12) though
the
bandpass
filter
(C44, C45).
The
bandpass
filter
suppresses subaudible tones
and
higher
noise signal
components.
The
amplified signals
from
Q12
pass
through the
AF
mute
switch (Q10)
and
are then applied
to
the
AF
volume
control
on the
1
F
unit
via
the "AF"
signal
line.
4-1-8
AF
POWER
AMPLIFIER CIRCUIT
(2F
UNIT)
The
AF
signals
from
the
AF
volume
control
("AFV"
signals)
are amplified
at
the
AF
power
amplifier
IC (IC151).
The
amplified
AF
signals are applied
to
the loud
speaker
via the
external
speaker
jack
(1
F
unit J902).
4-1-9
NOISE
SQUELCH
UNIT
(2F
UNIT)
A
noise squelch
circuit
cuts out
AF
signals
when
no
RF
signal
is
received.
By detecting noise
components
in
the
AF
signals,
the squelch
circuit
switches the
AF
mute
switch.
Some
of
the noise
components
in
the
AF
signals
from
the
FM
IF IC
(IC101 pin 9) are applied
to
the active
filter
section
(IC101 pins
7, 8).
The
variable
register
(R504) adjusts the
active
filter
input level.
The
active
filter
section amplifies
noise
components
with
frequencies
of
20 kHz and above.
The
filtered
signals are
rectified at
the noise detector section
and
converted
into
"NOISE"
(pulse type) signals at the
noise comparator
section.
The "NOISE"
signal
is
applied
to
the
CPU
(LOGIC
unit IC1 pin 12).
The
CPU
detects the signal level
from the
number
of the
pulses,
and
outputs
an
"MM/RM"
signal
from
pin 44.
This
signal controls
the
AF
mute
switch (01
0) to
cut the
AF
signal
line.
4-2
TRANSMITTER
CIRCUITS
4-2-1
MICROPHONE
AMPLIFIER CIRCUIT
(LOGIC
AND
2F
UNIT)
The
microphone
amplifier
circuit
amplifies
audio signals with
+6 dB/octave pre-emphasis from the
microphone
to
a
level
needed
for
the
modulation
circuit.
The
AF
signals
from
the
built-in
condenser microphone
(LOGIC
unit
MCI),
or
from the [MIC] jack
via
the
"EXT MIC"
line
are applied
to
the
limiter amplifier
(LOGIC
unit
1C12
pin
3)
which has +6 dB/octave pre-emphasis
characteristics.
The
amplified
AF
signals
pass though the
splatter
filter
(IC12 pins 5-7).
The
filtered
signals
are applied
to
frequency
deviation
pots (2F
unit
R308
for
VHF,
R314
for
UHF) and
are then applied
to
the modulation
circuit
on
the
DUALVCO
board.
Q32
on
the
LOGIC
unit Is
the
PTT
control circuit
and
outputs
a
"High" signal to
the
CPU
when
transmitting.
4-2-2
MODULATION
CIRCUIT
(DUAL
VCO BOARD)
The
modulation
circuit
modulates
the
VCO
oscillating signal
(RF
signal)
using the microphone audio
signals.
The
"VMOD"
signals
change
the
reactance
of
a
diode
(D304)
to
modulate the
oscillated signal at
the
VHF-VCO
circuit
(0304,
0305, D303).
The
"UMOD"
signals are applied
to
the
UHF-VCO
circuit
via
the
"USHIFT"
line.
The
applied signals
change
the
reactance of
a diode (D302)
to
modulate the
oscillated
signal
at
the
UHF-VCO
circuit (0301
,
0302, D301).
The
VCO
output
is
buffer-amplified at
0306
and then
applied
to
the >band switch (D351, D352)
via
the
LO
amplifiers (0852, 0351).
4-2-3
POWER
AMPLIFIER CIRCUIT
(IF UNIT)
0402
is
a
drive
and
0403
is
a power
amplifier.
They
are
designed
to
use both
VHF
and
UHF
commonly. They
provide
more
than
4
W
for
VHF
and 3
W
for
UHF
with
a
1
3.5
V DC
power
source
via
one power
amplifier
system.
APC
CIRCUIT
4-2