Icom IC-706MKIIG Service Manual page 10

3-2-4 FM MODULATION CIRCUIT (MAIN BOARD)
The microphone signals from Q961 are applied to the limiter
amplifier (IC981a) and the splatter filter (IC981b). The 1750
Hz European tone signal from the main CPU (IC2001 pin
40) is also applied to IC981a pin 2 for European repeaters.
The sub-audible tone signal (67.0–254.1 Hz) from the main
CPU (IC2001 pin 37) is also applied to IC981b pin 5 for
repeater use.
The resulting signals are applied to the VCO circuit (Q1011,
D1012) via R1002 to change the reactance of the varactor
diode (D1012) for FM modulation. The modulated signal is
amplified at the buffer amplifier (Q1013) and bypasses the 9
MHz IF filter.
3-2-5 TRANSMITTER IF CIRCUIT (MAIN BOARD)
The 9 MHz IF signal from the modulation circuit passes
through the 9 MHz IF filter (FI671 in SSB/CW/RTTY modes;
FI681 in AM/FM-N modes; through in FM mode). The signal
is amplified at IC571, and then passes through the total gain
adjustment volume (R579), and the crystal filter (FI561). The
signal is then applied to the 2nd mixer (D551).
The signal is mixed with the 2nd LO signal (60 MHz) and
converted to a 69 MHz IF signal at the 2nd mixer (D551).
The 69 MHz IF signal passes through a bandpass filter, IF
amplifier (IC521) and 69 MHz IF filter (FI511), and is then
converted to the displayed frequency at the 1st mixer (D271)
with the 1st LO signal. The mixers (D271, D551) and IF
amplifiers (IC521, IC571) are used commonly for both
receiving and transmitting.
The ALC voltage is supplied to the transmit/receive switch-
ing circuit (D521/D522 and D572/D573). D521/D522 and
D572/D573 function as PIN attenuators for ALC operation.
3-2-6 RF CIRCUIT
(PA UNIT, MAIN AND HPF BOARD)
The RF circuit amplifies the displayed frequency signal to
obtain 100 W of RF output power for HF/50 MHz bands and
50 W for the 144 MHz band, 20 W for the 440 MHz band.
The HF/50 MHz RF signals from the 1st mixer (D271) via the
low-pass filter enter the HPF board and then pass through
one of 8 high-pass filters (Refer to 3-1 for used RF high-pass
filter). The 50 MHz RF signals pass through a low-pass filter
additionally. The filtered signals return to the MAIN board,
are amplified at the YGR amplifier (IC231), and are then
applied to the PA unit.
• TRANSMITTER CONSTRUCTION
SSB
IC931
MIC Amp.
AM
FM, AM
IDC
FM
FM tone
BFO
60.0 MHz
9.0115 MHz
IC1041
Crystal
filter
Q1011
D551
FI671, FI681,
or optional filter
MAIN BOARD
The 144 MHz RF signals from the 1st mixer (D271) via the
low-pass filter (L251–253, C253–259) bypass the filters and
pass through the bandpass filter (L182–L184, L195, L196,
C181–C186, C195–C197) in the MAIN board. The signals
are amplified at the RF amplifier (IC221) and YGR amplifer
(IC231) and are then applied to the PA unit.
The 440 MHz RF signals from the 1st mixer (D271) via the
low-pass filter (L251–253, C253–259) are amplified at RF
amplifier (IC222) and passed through the bandpass filter
(FI371–FI373) in the MAIN board. The filtered signal is
amplified at the YGR amplifer (IC231) and is applied to the
PA unit.
The signals from IC231 enter the PA unit and is amplified at
the drive amplifiers (Q101, Q121) in sequence. The ampli-
fied signals are applied to the band switch (RL1).
The HF/50 MHz RF signals from the band switch (RL1) are
amplified at the drive (DRIVER board; Q161) and power
(Q171, Q172) amplifiers to obtain a stable 100 W of RF out-
put power. The power-amplified signals are then applied to
the [ANT1] connector via one of the 7 low-pass filters in the
FILTER board.
For the 144/440 MHz RF signals from the band switch
(RL1), 50 W for 144 MHz band or 20 W for 440 MHz band
of RF output power is obtained at the drive (DRIVER board;
Q211) and power amplifier (Q231). The power-amplified sig-
nals are applied to the [ANT2] connector via the antenna
switching circuit and low-pass (144 MHz band) or high pass
(440 MHz band) filters.
3-2-7 ALC CIRCUIT (MAIN BOARD)
The ALC (Automatic Level Control) circuit reduces the gain
of IF amplifiers in order for the transceiver to output a con-
stant RF power set by the RF power setting even when the
supplied voltage shifts, etc.
The HF/50 MHz RF power signal level is detected at the
power detector (FILTER board; D9), buffer-amplified at IC1b
and applied to the MAIN board as the "HFOR" voltage.
The 144 MHz and 440 MHz RF power signals are detected
at the power detectors (PA unit; D262, D263) and (PA unit;
D312, D313) respectively. The detected signals are applied
to the MAIN board as the "VFOR" or "UFOR" voltages.
HPF
BOARD
1st LO
HPF
FI1
•
Crystal
•
•
filter
except FM
HPF
BPF D231
BPF
FM only
50 MHz
BPF
144 MHz
BPF
440 MHz
3 - 5
HF+50 MHz
FILTER
Q171
Q161
BOARD
Q172
Amp.
Amp.
IC231
Amp. Amp. 144/440 MHz
Q101
Q121
Q211 Q231
144 MHz
Amp. Amp.
PA UNIT
440 MHz
[ANT1]
LPF
[ANT2]
LPF
HPF