Icom IC-706 Service Manual page 10

Microphone signals from the AF selector switch (IC23) are
applied to the balanced modulator (IC17 pin 6). The BFO
signal from the PLL unit is applied to IC17 (pin 8) as a
carrier signal.
IC17 is a double balanced mixer IC and outputs a double
side band (OSB) signal with -40 dB of carrier suppression.
R191 and R193 adjust the balanced level of IC17 for
maximum carrier suppression. The resulting signal passes
through a 9 MHz IF filter (F14 in SSB/CW/RTTY modes) to
suppress unwanted sideband signals.
In AM mode, R195 is connected to upset the balance of
IC17 via 023 for leaking the BFO signal as a carrier signal.
The CW keying/RTTY TX signal is applied to IC17 pin 6.
3-2-4 FM MODULATION CIRCUIT (MAIN UNIT)
The microphone signals from 063 are applied to the limiter
amplifier (IC24a) and the splatter filter (IC24b). The 1750
Hz European tone signal from the main CPU (IC25 pin 40)
is also applied to IC24a pin 2 for European repeaters. The
subaudible tone signal (67.0-254.1 Hz) from the main CPU
(IC25 pin 37) is also applied to IC24b pin 2 for repeater use.
The resulting signals are applied to the VCO circuit (040,
067) via R274 to change the reactance of the varactor
diode (067) for FM modulation. The modulated signal is
bUffer-amplified at 041 and bypasses the 9 MHz IF filter.
3-2-5 IF AMPLIFIER (MAIN UNIT)
The 9 MHz IF signal from a modulation circuit passes
through the 9 MHz IF filter (F14 in SSB/CW/RTTY modes;
FI5 in AM mode; through in FM mode). The signal is
amplified at IC6, passes through R511 and L55, and the
low-pass filter (L52, C82-C84) in non-FM modes or FI3 in
FM mode. The signal is then applied to the mixer (IC5).
The signal is mixed with the 2nd LO signal (60 MHz) and
converted to a 69 MHz IF signal at IC5. The 69 MHz IF
signal passes through a bandpass filter, the IF amplifier
(IC4) and the 69 MHz IF filter (Fll) and is then converted to
the displayed frequency at the balanced mixer (IC3) with
the 1st LO signal. The mixers (IC3, IC5) and IF amplifiers
(IC4, IC6) are used in both receiving and transmitting
commonly.
The ALC voltage is supplied to the transmiVreceive switch-
ing circuit (013/014 and 022/023) of IF amplifiers. 013/014
and 022/023 function as PIN attenuators for ALC operation.
3-2-6 RF CIRCUIT
(MAIN AND PA UNITS, HPF BOARD)
The RF circuit amplifies the displayed frequency signal to
obtain 100 W of RF output power for HF/50 MHz bands and
10 W for 144 MHz band.
The HF/50 MHz RF signal from the balanced mixer (IC3)
enters the HPF board and then passes through one of 6
high-pass filters. (Refer to p. 3-1 for used RF high-pass
filter.) The 50 MHz RF signal passes through a low-pass
filter additionally. The filtered signal returns to the MAIN
unit, is amplified at IC1, and is then applied to the PA unit.
The 144 MHz RF signal from the balanced mixer (IC3)
bypasses the filters in the HPF board via "TOTR" line and
passes through the bandpass filter (L99-L101 , C329-C333)
in the MAIN unit. The signal is amplified at IC38 and ICl
and is then applied to the PA unit.
The signal from ICl enters the PA unit and is amplified at
the predrive amplifiers (016, 01) and drive amplifier (02) in
sequence. For the 144 MHz RF signal, 10 W of RF output
power is obtained at 02 and the signal is applied to the
low-pass filter circuit via RL1.
The HF/50 MHz RF signal from 02 is amplified at power
amplifier (018. 019) to obtain a stable 100 W of RF output
power. The signal is then applied to one of the low-pass
filters in the FILTER unit.
3-2-7 LOW-PASS FILTER CIRCUIT
(FILTER AND PA UNITS)
The low-pass filter circuit consists of 8 Chebyschev low-
pass filters to suppress the higher harmonic components.
The HF/50 MHz RF signal from the power amplifier passes
through the transmiVreceive switching circuit (FILTER unit
RL1) is applied to one of the 7 low-pass filters. According to
the operating frequency, the band signals (L1-L7) from the
MAIN unit select a low-pass filter. The filtered signal is then
applied to the [ANT1] connector.
• TRANSMITTER CONSTRUCTION
9.0115 MHz IC5
IC3
3-5
144 MHz
MAIN unit I PA unit
I
I
018. 019
I
I
I
I
01. 02. 016
[ANT1]
FILTER
unit
[ANT2]
144 MHz