Transmitter Circuits - Icom IC-706 Service Manual

(4) WFM mode
The 2nd IF signals from the IF amplifier (05) pass through
F12, are amplified at 052 and are then applied to the FM IF
IC (IC9 pins 2, 3) where the IF signals are converted into
AF signals. The detected AF signals from pin 8 are then
applied to the AF selector switch (IC15 pin 11).
The amplified or compressed signals are applied to the VCA
section of IC22. The microphone gain setting from the O/A
converter (IC35 pin 11) is applied to the VCA control ter-
minal (IC22 pin 10). The resulting signals from pin 9 are
then applied to the buffer amplifier (063). External modu-
lation input from the [ACC] socket (pin 11) is also applied to·
063.
3-1-14 AF SELECTOR SWITCH (MAIN UNIT)
The AF amplifier amplifies the AF input signal to a suitable
driving level for the speaker.
[VOX
GAIN]
(R239)
14
AFO
-signal
MIGC from
O/A
converter (IC35)
: 10
15 [ANTI
VOX]
(R244)
VXS8
VOX OFF: 8 V
VOX ON: oV
While in FM/AM mode, the amplified signals from 063 are
applied to the limiter amplifier (IC24a) and splatter filter
(IC24b). The signals are then applied to the AF selector
switc'l (IC23) in AM mode or to the varactor diode (067) in
FM mode.
While in SSB mode, the amplified signals from 063 are
applied to the AF selector switch (IC23) and then to the
balanced modulator (IC17).
MIC
22
signal
VOXO ...
17
t+--<
to CPU
Microphone amplifier
[COMP GAIN]
COMS
(R248)
from CPU
__No/'_ •
IC22 :
2. 3
i
11
AFI
to PLL
unit
CW side
tonel
beep tone
Optional
synthesized voice
Squelch
control
The AF signal from one of the detector circuits is applied to
the AF selector switch (IC15). IC15 consists of dual
4-channel analog switches which are selected with a mode
signal and the squelch control signal.
3-1-15 AF AMPLIFIER CIRCUIT
(MAIN AND PLL UNITS)
AF selector switch
IC15
SSB/CWI
12
xo
RTTY
AM
14 Xl
X
13
FM
15
X2
•
WFM
11
X3
.......... 8
INH
9. 10
AFS1, AFS2
The AF signal from the AF selector switch (IC15) is applied
to the AF amplifier (053). The CW side tonelbeep tone and
optional synthesized voice are also applied to 053. The
amplified signal is then applied to the PLL unit.
The signals from the MAIN unit are applied to the VCA
(Voltage Controlled Amplifier, IC4) circuit. The AF gain
setting from the main CPU is converted to DC voltage at the
O/A converter (MAIN unit IC35) and applied to the VCA
control terminal (IC4 pin 8) via the "AFGC" signal line. The
AF signal from IC4 (pin 9) is power-amplified at IC5 to drive
the speaker.
3·2 TRANSMITTER CIRCUITS
3-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies microphone input
signals and outputs the amplified signals to the balanced
modulator or FM modulation circuit.
3-2-2 VOX CIRCUIT (MAIN UNIT)
The
vox
(Voice-Operated-Transmission) circuit sets trans-
mitting conditions according to voice input. The microphone
amplifier IC (IC22) includes the VOX circuit.
The microphone signals from IC22 (pin 19) pass through
the [VOX GAIN] control (R239) and are then applied to the
VOX comparator section (IC22 pin 18) to switch the keying
input of the main CPU (IC25 pin 14). When voice levels
exceed the reference level, the VOX circuit sets the trans-
ceiver to transmit.
On the other hand, a speaker drive signal from the AF
power amplifier (PLL unit IC5) is applied to the anti-VOX
comparator section (IC22 pin 14) via the [ANTI VOX] control
(R244). When the audio output level increases, this compa-
rator cuts out the VOX comparator via the MUTE terminal
(IC22 pin 16).
Audio signals from the front or rear panel [MIC] connector
enter the microphone amplifier IC (IC22 pin 22) and are
then amplified at the microphone amplifier or speech com-
pressor section. Compression level is adjusted with the
[CaMP GAIN] control (R248).
3-2-3 BALANCED MODULATOR (MAIN UNIT)
The balanced modulator converts the AF signal from the
microphone amplifier to a 9 MHz IF signal with a BFa (Beat
Frequency Oscillator) signal.
3-4