Icom IC-M91D Service Manual Addendum page 21

4-2 TRANSMIT CIRCUITS
TX AF CIRCUIT (MAIN UNIT)
The AF signal from the internal microphone (MC250) is
passed through the MIC mute SW (IC260A, pins 1, 2), and
then applied to the MIC AMP (IC200B, pin 6).
The MIC signal from the external microphone is directory ap-
plied to the MIC AMP (IC200B, pin 6), through J290.
The amplifi ed AF signal is passed through the AF SW (IC380,
pins 6, 1) and applied to the noise canceller IC (IC382,
pin 36).
The processed AF signal is passed through the AF mute SW
(IC260C, pins 8, 9), and then applied to the D/A converter
(IC190, pin 13) to adjust the level.
The level-adjusted AF signal is passed through the pre-em-
phasis circuit (R201, C385), and then amplifi ed by the limiter
AMP (IC200A) which limits the amplitude of AF signal to pre-
vent over deviation. The amplitude-limited signal is passed
through the LPF (IC200D, pins 12, 14), and then applied to
the modulation circuits on the RF UNIT.
• TX AF CIRCUITS
Ext.MIC
MIC
J291
IC200B
IC260A
Int.MIC
INMIC
AF
AMP
MUTE
MC250
MAIN UNIT
To the modulation circuit
MODULATION CIRCUIT (RF UNIT)
The AF signal from the TX AF circuits is applied to the VCO
(Q21, Q22, and D20–D22). The modulation signal is applied
to D20, for Frequency Modulation.
The modulated VCO output signal is passed through the buf-
fers (Q23 and Q24), and then applied to the TX AMP circuits
as a TX signal, through the LO SW (D50).
• MODULATION CIRCUITS
Q21,Q22 Q23
Q24
D2-D22
BUFF
BUFF
MOD
RF UNIT
From the TX AF circuit
IC380
From the demodulator circuit
AF
SW
IC382
NOISE CANCEL IC
IC260C
IC190
AF MICO
D/A
MUTE
MOD
PRE
LIMIT
LPF
AMP ENPHA
IC200D IC200A
D50
To the TX AMP circuit
LO
SW
TX AMP CIRCUIT (MAIN UNIT)
The TX signal is sequentially amplifi ed by the buffer (Q50),
YGR (Q53) and power (Q54) AMPs, and then passed
through two LPFs (L57, C70, C71 and L80, C81, C82), TX
output power detector (D91), ANT SW (D52), and a two stage
LPF (L81, L82, C83–C86 and C89), before being sent to the
antenna.
APC CIRCUIT (MAIN UNIT)
The voltage produced at two LPFs (L57, C70, C71 and L80,
C81, C82) is rectifi ed by D91, and is used as the TX power
sensing voltage.
The voltage is applied to the APC AMP (IC50, pin 3), and the
output voltage controls the gain of the pre-driver (Q53) and
power (Q54) AMPs, to keep the TX output power constant.
• TX AMP AND APC CIRCUITS
Q50
From the VCO
BUFF
RF UNIT
4 - 2
Q53 Q54 D52
PRE
PWR ANT
DRIVE
SW
AMP
IC50
D91
APC
PWR
TDETV
CTRL
DET
To the RX circuit
J1
LPF