Icom IC-M424 Service Manual page 12

4-2 TRANSMIT CIRCUITS
TX AF CIRCUITS (MAIN UNIT)
The AF signal from the microphone (MIC signal) is passed
through the MIC mute SW (IC502A, pins 1, 2), and then ap-
plied to the MIC AMP (IC513D).
The MIC signal from the COMMANDMICIV™ is passed
through the MIC mute SWs (IC502C, pins 9, 8 and IC502B,
pins 4, 3), and then applied to the MIC AMP (IC513D).
• MIC AMP CIRCUITS
IC513D
IC502A
MICIN MIC
AF
AMP
MUTE
HM-196
IC502B
MAIN UNIT
AF
MUTE
TX AF CIRCUITS (MAIN UNIT)
The amplifi ed AF signal is passed through the AF SW (IC901,
pins 6, 1) and applied to the noise canceller IC (IC902, pin 36).
The processed AF signal is passed through the LPF (IC905,
pins 1, 4) and AF SW (IC904, pins 1, 6), and then applied
to the D/A converter (IC514, pins 13, 14) to be adjusted in
level.
The level-adjusted AF signal is applied to the IDC AMP
(IC513B) which limits the amplitude of MIC signal to pre-
vent over deviation. The amplitude-limited signal is passed
through the LPF (IC513C), and then applied to the modula-
tion circuits.
• TX AF CIRCUITS
IC902
IC901 IC905
From the
MIC AMP circuits
AF NOISE
LPF
CANCELLER
SW
MODULATION CIRCUITS (MAIN UNIT)
The MIC signal from the TX AF circuits is applied to the VCO
(Q3, Q4 and D1–D3). The modulation signal is applied to D2,
to obtain Frequency Modulation.
The modulated VCO output signal is passed through the buf-
fers (Q5 and Q7), and then applied to the TX AMP circuits as
a TX signal, through the LO SW (D7).
• MODULATION CIRCUITS
VCO UNIT
Q2
Q8
Q3,Q4
D1,D2
D 3 Q5
From the
TX AF circuits
VCO
SHIFT BUFF
To the TX AF circuits
IC502C
AF
MUTE
OPTIONAL
COMMANDMIC
IC514
IC904
IC513B IC513C
To the modulation
D/A
AF circuits
IDC
LPF
SW
D7,D8
Q7
To the
TX AMP circuits
LO
BUFF
SW
TX AMP CIRCUITS (MAIN UNIT)
The TX signal is amplifi ed by the YGR (Q13) and power
AMP (IC1), and then passed through the LPF (L2, C111 and
C112), TX output power detector (D12 and D13), ANT SW
(D14), and two LPF (L21, L22, and C114–C116), before be-
ing sent to the antenna.
APC CIRCUITS (MAIN UNIT)
The voltage produced at the LPF (L20, C111, and C112) is
rectifi ed by D12 and D13, and is used as the TX power sens-
ing voltage.
The voltage is applied to the APC AMP (IC2, pin 3), and the
output voltage controls the gate bias voltage of power AMP
(IC1) to keep the TX output power constant.
• TX AMP CIRCUITS
Q13
From the modulation
circuits
YGR
AMP
4 - 3
ANT
IC1
D14,D16
PWR ANT
LPF
LPF
AMP SW
D12,
PWR
D13
DET
IC2
RX circuits
APC
CTRL