4 - 2
4-1-5 AF AMPLIFIER CIRCUIT
(MAIN AND FRONT UNITS)
The AF amplifier circuit amplifies the detected signals to
drive a speaker. The AF circuit includes an AF mute circuit
for the squelch.
AF signals from IC1 (pin 9) are applied to the de-emphasis
circuit (R101, C140). The de-emphasis circuit is an integrat-
ed circuit with frequency characteristic of –6 dB/octave.
The integrated signals pass through the high-pass filter
(Q21, R96–R100, C138, C139) to suppress unwanted lower
noise signals.
The filtered signals are applied to the AF mute circuit (IC5A),
and then passes through the [VOLUME] control (VR-A
board; R1) via the “VOL1” signal. The signals are applied to
the AF power amplifier (IC9, pin 1). The output signal from
IC9 (pin4) drives the internal (external) speaker.
4-1-6 SQUELCH CIRCUIT
(MAIN UNIT AND LOGIC BOARD)
A squelch circuit cuts out AF signals when no RF signals are
received. By detecting noise components in the AF signals,
the squelch circuit switches the AF mute switch.
A portion of the AF signals from the FM IF IC (IC1, pin 9)
pass through the squelch adjustment pot (R16), and are
then applied to the active filter section (IC1, pin 8). The
active filter section amplifies and filters noise components.
The filtered signals are applied to the noise detector section
and output from pin 14 as the “SQL” signal. The “SQL” sig-
nal is applied to the CPU (LOGIC board; IC1, pin 90). The
CPU analyzes the noise condition and outputs the “RMUM”,
“RMUTS” signals to toggle the AF mute switches (IC5A,
IC5B).
4-1-7 WEATHER ALERT DECODER CIRCUIT
(MAIN UNIT AND LOGIC BOARD)
When the weather alert function is activated and a 1050 Hz
alert tone from an NOAA weather radio broadcast is
received, the IC-M402 emits beep tones and indicates flash-
ing “ALT” on the display to inform of an emergency weather
report on the air.
AF signals from the FM IF IC (IC1, pin 9) pass through the
low-pass filter (Q28) via the “DET” signal, and are then
applied to A/D port of the CPU (LOGIC board; IC1, pin 88)
as “WXDEC” signal. The CPU has the tone decoder func-
tion. When a 1050 Hz signal is detected by software decode,
the CPU (LOGIC unit; IC1) controls beep tones and the
“ALT” indicator.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis from the microphone to a level
needed at the modulation circuit.
The AF signals from the microphone are amplified at the
microphone amplifier (IC7B) via the analog switch (IC4A, pin
1). A capacitor (C159) and resistor (R121) are connected to
the amplifier to obtain the pre-emphasis characteristics.
The amplified signals are applied to the IDC amplifier (IC7C,
pin 9) and are passed through the splatter filter (IC7D) to
suppress unwanted 3 kHz or higher signals. The filtered sig-
nals are then applied to the modulation circuit.
4-2-2 MODULATION CIRCUIT (MAIN UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
Audio signals from the splatter filter (IC7D) pass through the
frequency deviation adjustment pot (R138) and are then
applied to the modulation circuit (D2, D3, D4) to change the
reactance of D2, D3, D4 and modulate the oscillated signal
at the VCO (Q5, Q6).
4-2-3 DRIVE AMPLIFIER CIRCUIT (MAIN UNIT)
The drive amplifier circuit amplifies the VCO oscillating sig-
nal to a level needed at the power amplifier.
The VCO output is buffer-amplified by Q7 and Q9, and is
then applied to the Tx/Rx switch (D6). The transmit signal
from the Tx/Rx switch is amplified to the pre-drive (Q10) and
YGR (Q11) amplifiers to obtain an approximate 300 mW sig-
nal level. The amplified signal is then applied to the RF
power amplifier (IC3).
4-2-4 POWER AMPLIFIER CIRCUIT (MAIN UNIT)
The power amplifier circuit amplifies the driver signal to an
output power level.
IC3 is a power module which has amplification output capa-
bilities of about 35 W with 300 mW input. The output from
IC3 (pin 4) is passed through the antenna switching circuit
(D10) and is then applied to the antenna connector via the
low-pass filter.
AF
MUTE
HPF
IC9
External SP
Internal SP
(LOGIC board)
HVS
PWR
AMP
"DET" signal from the
FM IF IC (IC1, pin 9)
de-emphasis
circuit
R101
C140
(VR-A board)
• AF AMPLIFIER CIRCUIT
