Icom IC-M422, Service Manual

Page: 9 / 35

4 - 2
4-1-6 SQUELCH CIRCUITS
Noise squelch circuit mutes AF output signals when no RF
signals are received. By detecting noise components in the
demodulated AF signals, the squelch circuit switches the AF
mute switch ON and OFF.
A portion of the demodulated AF signals from the FM IF IC
(IC2, pin 9) are passed through the squelch adjustment pot
(R214) to be adjusted its level. The level adjusted AF signals
are passed through the active fi lter (IC2, pins 7, 8; R211–R213,
C211, C212, C217). The fi ltered signals are then applied to
the noise amplifier section in the IC2 to amplify the noise
components only.
The amplified noise components are converted into the
pulse-type signal at the noise detector section, and output
from pin 14 as the “SQL” signal. The “SQL” signal is applied
to the squelch amplifi er (IC11, pin 1) to be amplifi ed its level,
then output from pin 4. The amplifi ed “SQL” signal is applied
to the CPU (LOGIC BOARD; IC1, pin 5). Then the CPU out-
puts “RMUTEM” signal from pin 64 according to the “SQL”
signal level to toggle the AF mute circuit (IC4) ON/OFF.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
The microphone amplifi er circuit amplifi es the audio signals
from microphone (MIC signals) 6 dB/oct pre-emphasis
characteristic.
The MIC signals from the microphone (HM-150B/SW; MC1)
are passed through the microphone mute switch (IC5, pins
10, 11) and amplifi ed at the microphone amplifi er (IC7, pin
6) to 6 dB/oct pre-emphasis characteristics. The am-
plifi ed MIC signals are limited its level at the IDC amplifi er
(IC7, pin 5), and fi ltered out 3 kHz and higher audio signals
at the LPF (IC8, pins 5, 7).
The filtered MIC signals are passed through the deviation
adjustment pot (R327) to be adjusted its level. Then the level
adjusted MIC signals are applied to the modulation circuit.
4-2-2 MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
with the audio signals from the microphone.
The level adjusted MIC signals are applied to the modulation
circuit (D2) to modulate the VCO oscillating signal by chang-
ing the reactance of D2 at the VCO (Q4, Q5, D3, D4).
The modulated VCO output signals are amplifi ed at the buf-
fer amplifiers (Q6, Q7), then applied to the transmit ampli-
fi ers via the TX/RX switch (D7).
4-2-3 TRANSMIT AMPLIFIERS
The VCO output signals are amplified to transmit output
power level by the transmit amplifi ers .
The buffer-amplified VCO output signals from the TX/RX
switch (D7) are applied to the pre-drive (Q10), YGR (Q12),
and power (IC3) amplifi ers to be amplifi ed to the transmit out-
put power level. The power amplifi ed transmit signal is passed
through the power detector (D12, D13), antenna switch
(D14) and a two-stage LPF (L21, L22, C127–C130, C132),
and then applied to the antenna connector (CHASSIS UNIT; J1).
4-2-4 APC CIRCUIT
The APC (Automatic Power Control) circuit stabilizes trans-
mit output power and controls transmit output power High (25 W)
or Low (1 W).
The power detector circuits (D12, D13) detect the transmit
output signal level and converts it into DC voltage. The de-
tected voltage is applied to the APC amplifi er (IC12, pin 3). The
voltage of the “T5V” line is applied to another input (pin 1)
via the transmit output power adjustment pot (R112) as the
reference voltage.
The output voltage from the APC amplifi er controls the bias
of the power amplifier (IC3) to control the output power by
comparing the detected voltage and the reference voltage.
Thus the APC circuit maintains a constant transmit output
power.
Power
amp.
APC
amp.
YGR
amp.
+
–
HV
TXDET
• APC CIRCUIT
to the anntena
T5V
R112
T5V
TMUTE
H/L
from TX/RX switch (D7)
Q12
Pre-drive
amp.
Q10
IC12
Q17
IC3
LPF
ANT
SW
D12
Power detector
D13
D14