Icom IC-T7A Service Manual

An
RF
signal
from the
band switch
(D351, D352)
is
buffer-amplified
at
Q201
(for
VHF) or
Q701
(for UHF) and
then
applied to
the
drive amplifier
(PA
board
Q402)
via the
other band
switch (D201, D701).
The applied
RF signal
from the
band
switch is
amplified at a
drive
amplifier
(Q402)
and
then
amplified
again at
the
power amplifier
(Q403).
The
amplified RF
signal is
passed though
the
low-pass filter
(VHF)
or
high-pass
filter
(UHF), and
then applied
to the
antenna
connector
via the
transmit/receive
switching
circuit
(D51,
D52, D202
for VHF. D551,
D552,
D723, D724
for
UHF).
4-2-4
APC
CIRCUIT
(IF AND
2F UNITS)
The
APC
circuit
stabilizes
transmit output
power and
selects
HIGH
and LOW
output
power. The APC
circuit
consists of
APC
sensor, APC
control
(IF unit) and
APC set
(2F unit)
circuits.
The
APC
sensor
circuit (IF unit R250)
detects a
driving
current from
a
drive voltage
at the PA board.
The
detected
current is
applied to the
ope-amplifier
1C (1 F
unit IC250
pin
2)
in
the APC
control
circuit,
and compared with a
“PSET”
voltage
which is
supplied from
the
APC
set
circuit (2F unit
IC301
).
The
output voltage
from pin 1 of IC250
is
applied to
the APC
control
circuit (IF
unit
Q255
base) to
control
“VGGC”
voltage.
The
“VGGC”
APC
control signal is
separated
for VHF
(VGG1)
and
UHF (VGG2)
by resistors.
The VGG1
line
is
for
the APC
control
signal for the
drive amplifier
and the
VGG2
line is
for the
power amplifier.
Low
output
power
is obtained
by changing the
“PSET”
voltage
coming from
pin 1 of IC301
on the
2F unit. The
“PSET”
voltage is
controlled by
power set pots (R302
for
VHF, R304
for UHF)
and an
“H/L” signal
via the CPU
(LOGIC
unit
pin
56).
A
thermistor (R266)
controls APC
reference
voltage
(“PREF”
voltage)
to
reduce
the
output
power
when the
temperature is
increased.
4-2-5
ANTENNA
SWITCHING
CIRCUIT
(1FUNIT)
The
antenna
switching circuit
applies receive
signals to the
receiver
circuit and
transmit
signais to the
antenna connec-
tor.
VHF ANTENNA
SWITCHING
CIRCUIT
When
transmitting, D51, D52
and D202
are
turned
ON.
The
signal passes though
the
low-pass filter (L11,
L12,
C16-C22)
and is
then applied
to the
antenna
connector.
The
low-pass
filter suppresses
high harmonic
components.
UHF
ANTENNA
SWITCHING CIRCUIT
When
transmitting, D723, D724,
D551
and D552 are
turned
ON.
The signal
passes though the
low-pass
(L8, L9,
C10-C14)
and
high-pass (L2,
L3,
C1-C5)
filters and
is then
applied to the
antenna
connector. The
high-pass
filter
suppresses low harmonic
components.
4-3
PLL CIRCUITS
4-3-1
VHF
PLL CIRCUIT (1 F
UNIT)
The oscillated
signal at the VCO
circuit
(DUAL
VCO
board
0304,
0305,
D303)
is ampiified at a
buffer-ampilfier
(0306)
and is
again
amplified
at
another
buffer-amplifier
(0352).
The
amplified signal is applied
to the PLL 1C (IC851
pin
2),
and then
divided
by serial
data from
the
CPU
and
phase-detected
with the
divided reference
frequency. The
phase
difference is
output as
pulses.
The output signals from IC851
(pin
8)
are converted to DC
voltages (lock
voltage) by the loop
filter (R366,
C362)
and
are then
fed back
to the VHF VCO
circuit to
stabiiize
the
VCO
frequency.
The
DC
voltage
is also
applied to the
receiver
tuned
bandpass
filters
as
a “VTUNE”
signal.
PLL
CIRCUIT
VHF-VCO
Q304,
Q305. D303
4-3

Icom IC-T7A Specifications

General

Brand	Icom
Model	IC-T7A
Category	Transceiver
Language	English