Transmit Circuits; Apc Circuit; Af Circuits; Squelch Circuit - Icom IC-M73 Service Manual

• AF CIRCUITS (MAIN UNIT)
The demodulated AF signal from the demodulator circuits is
amplifi ed and fi ltered in AF amplifi er circuits.
The demodulated AF signal from the IF IC (IC170, pin 9) is
passed through the AF mute switch (IC430, pins 10, 11),
LPF (IC432) and variable register (VR UNIT: R801) which
adjusts the AF signal in level.
The level-adjusted AF signal is passed through the de-em-
phasis circuit (R286, C280 and C285) to obtain −6 dB of au-
dio characteristic, and passed through the AF switch (IC260,
pins 1, 6), and then applied to the AF power amplifi er (IC401,
pin 7) to obtain 0.6 W of output power. The power-amplifi ed
AF signal is then output from pin 1, and applied to the inter-
nal speaker, through J251.
When an external speaker-microphone or headset is at-
tached to the [SP MIC] connector (MIC UNIT: J416), the
de-emphasized AF signal is passed through the AF switch
(IC260, pins 1, 7) and applied to the AF power amplifier
(IC280, pin 4) to obtain 0.2 W of output power. The power-
amplifi ed AF signal is output from pin 10, and then applied
to the external speaker, through the [SP MIC] connector (MIC
UNIT: J416).
• SQUELCH CIRCUIT (MAIN UNIT)
The squelch mutes the AF output signal when no RF signal
is received. By detecting noise components in the demodu-
lated AF signal, the squelch circuit toggles the AF power
amplifi er ON and OFF.
A portion of the demodulated AF signal from the IF IC
(IC170, pin 9) is applied to the D/A converter (MAIN UNIT:
IC190, pin 18) for level adjustment (squelch threshold ad-
justment). The level-adjusted AF signal is output from pin 19,
and passed through the noise fi lter (R174−R176, C179 and
C180). The only fi ltered noise signal is applied to the noise
amplifi er in the IF IC (IC170, pins 7, 8).
The amplified noise components are converted into the
pulse-type signal at the noise detector section, and output
from pin 19 as the "NOISV1" signal. The signal is amplifi ed
by the Amplifi er (IC432) and then applied to the CPU (IC360,
pin 32), and the CPU outputs "AFVS" signal from pin 100,
depending on the "NOISV1" signal level, to the AF power
regulator (Q230, Q231 and Q401), which turns the AF power
amplifi ers (IC280 and IC401) ON or OFF.

• APC CIRCUIT

T5V
from the TX/RX switch (D50)
PCON
Q100
1
LOW
3
T5V
TXMS
Q351
Q50
Buffer
Pre-drive
amp.
amp.
+
APC
amp.
4
–
IC50

4-2 TRANSMIT CIRCUITS

• MICROPHONE AMPLIFIER CIRCUITS
(MAIN UNIT)
The AF signal from the microphone (MIC signal) is fi ltered
and level-adjusted at microphone amplifi er circuits.
The AF signal from the microphone is passed through the
MIC mute switch (IC430A, pins 1, 2).
While an external microphone is connected to the [SP MIC]
connector (MIC UNIT: J416), the MIC mute switch shuts out
the MIC signal from the internal microphone (MC1).
The AF signal from the MIC mute switch (IC430A, pins 1, 2)
is passed through another AF mute switch (IC430B,
pins 3, 4), and applied to the microphone amplifi er (IC200B,
pins 6, 7). The amplified MIC signal is passed through AF
mute switch (IC430, pins 8, 9), and is applied to the D/A con-
verter (IC190) and adjusted in level (=deviation adjustment).
The level-adjusted MIC (MOD) signal is passed through the
pre-emphasis circuit (R201 and C450) to obtain +3 dB of
characteristic. The pre-emphasized signal is then applied to
the limiter amplifi er (IC200D, pins 13, 14), which limits the
amplitude of the MIC signal to prevent over deviation.
The amplitude-limited MIC signal is then passed through
the splatter fi lter (IC200A, pins 1, 3), which suppresses the
3 kHz and higher audio components, and then applied to the
modulation circuit (RF UNIT: D20).
• MODULATION CIRCUIT (RF UNIT)
The modulation circuit modulates the VCO oscillating signal
using the AF signal from the microphone.
The MIC signal from the microphone amplifi er circuits is ap-
plied to the D20, and modulate the VCO oscillating signal by
changing the reactance of D20. The modulated VCO output
signal is buffer-amplifi ed by Q23 and Q24, and then applied
to transmit amplifi er circuits, through the TX/RX switch (D50).
• TRANSMIT AMPLIFIERS (RF UNIT)
The VCO output signal is amplified to the transmit output
power level by the transmit amplifi ers.
The transmit signal from the TX/RX switch (D50) is ampli-
fi ed by the buffer amplifi er (Q50), and then further amplifi ed
by the pre-driver (Q53) and power amplifier (Q54) to the
transmit output level. The power-amplifi ed transmit signal is
passed through the TX power detector of APC circuit (D91),
antenna switch (D52) and LPF (as a harmonic filter; L81,
L82, C83−C86 and C89), before being applied to the an-
tenna connector (CHASSIS: J1).
Q53
Q54
Power
amp.
4 - 2
D52
ANT
LPF
SW
To the anntena
D91
VCC