Demodulator Circuit (Man Unit); H3E Detector Circuit (Main Unit); Agc Circuit (Main Unit); S-Meter Circuit (Main Unit) - Icom IC-M700PRO Servise Manual

The detected voltage is also applied to the noise blanker
AGC circuit (Q12, Q10) and is then fed back to the noise
amplifier (IC8) as a bias voltage. The noise AGC circuit
prevents closure of the noise blanker gate for long periods
by non-pulse-type noise. The time constant of the noise
blanker AGC circuit is determined by R58 and C114.

3-1-5 DEMODULATOR CIRCUIT (MAN UNIT)

This circuit mixes the 2nd IF and BFO signals to pick up
the AF components (except H3E mode). The 2nd IF signal
from the 2nd IF amplifier (Q33) is applied to the balanced
mixer (IC10, pin 1). The 9.0116–9.0130 MHz BFO signal
from the PLL unit is also applied to IC10 (pin 10). AF sig-
nals are output from pin 12 and are then applied to the AF
circuits.

3-1-6 H3E DETECTOR CIRCUIT (MAIN UNIT)

The 2nd IF signal from the 2nd IF amplifier (Q33) is applied
to the AM detector circuit (D39) to be demodulated into AF
signals. The detected signals are amplified at the buffer
amplifier (Q45), and are then applied to the AF circuits.

3-1-7 AGC CIRCUIT (MAIN UNIT)

The AGC (Automatic Gain Control) circuit reduces IF ampli-
fier gain to prevent the receiver circuit from distorting and
to keep the audio output at a constant level.
A portion of the IF signals from the 2nd IF amplifier (Q33)
is detected at the AGC detector circuit (D31) and is then
applied to the AGC amplifier (Q41) to control the AGC time
constant line. The reference voltage of the AGC line is con-
trolled by the "RFG" line which comes from the CPU for the
RF gain setting.
When receiving a strong signal, the detected voltage
increases and the voltage of the AGC line is decreased by
the AGC amplifier (Q41) via the –5 V voltage line. The AGC
line is used for the bias voltage of the IF amplifiers (Q8,
Q16, Q32, Q33), so that these amplifiers reduce gain.
When the strong signal disappears, the AGC line voltage is
released by C245/R268 and C670/R813.
The AGC switch (Q42, D38) turns the AGC circuit OFF
when the AGC OFF function activates. The AGC-fast switch
(Q131) sets the AGC line as fast-release during scanning,
A1A mode selection and DSC operation.
• AGC CIRCUIT
1st IF signal
8 V
AGC time
constant line
RFG (RF gain control)
– 5 V
Q8 D52 Q16
AGC-fast control
Q131 C670
CW, DSC,
SCAN
0 to 5 V

3-1-8 S-METER CIRCUIT (MAIN UNIT)

The S-meter indicates the AGC level on the display, since
the AGC level varies with the received signal strength.
The AGC bias voltage (AGC time constant line) from the
AGC amplifier (Q41) is inverted and amplified at the meter
amplifier (IC19b). The amplified signal is applied to the
CPU via the "RSM" line.
3-1-9 AF AMPLIFIER CIRCUITS
(MAIN UNIT AND LOGIC BOARD)
AF signals from the demodulator or H3E detector circuits
pass through the active low-pass filter (IC20b) and squelch
gate (IC12a), and are then applied to the electronic volume
control (IC36). The CPU (IC132, pin 37) outputs the volume
control signal (1 to 5 V) according to the [VOLUME] control
setting.
The AF output signal from IC36 (9 pin) are supplied to
the LOGIC unit via J23. The signals are amplified at the
AF power amplifier (IC2007) and are then applied to the
internal speaker via microphone connector (pins 3, 4) and
external [SP] jack via the MAIN unit.
The speaker switch relay (RL2001) is connected to the (–)
terminal of internal speaker for the [SPEAKER] switch func-
tion.
3-1-10 SQUELCH CIRCUIT (MAIN UNIT)
The transceiver has two squelch circuits, voice activated
squelch for J3E/H3E and S-meter squelch for A1A/F1B/
J2B.
(1) AF ACTIVATED SQUELCH
A portion of the AF signal from the active low-pass filter
(IC20b) is amplified at the limiter amplifier (IC20a) and is
then applied to the one-shot multi-vibrator (IC22c, IC22d).
The one shot multi-vibrator functions as an F-V converter
which generates a signal only when audio signals are
received.
The output signals pass through the NOR gate (IC22b) and
then the 3 Hz low-pass filter (IC21a) to remove the remain-
ing noise components. The filtered signal is applied to the
window comparator (IC21b). The NOR gate (IC22b) deac-
tivates the audio activated squelch during A1A/F1B/J2B
mode operation.
2nd IF filters
Q32
C245
R813 C268
AGC amplifier
Q41
3 - 2
H3E derector
Q33
Demodulator
D31 AGC detector
AGC OFF control
Q42