Icom IC-A20MK II Service Manual page 9

3-2 TRANSMITTER CIRCUITS
3-2-1 MICROPHONE AMPLIFIER AND ALC
CIRCUITS (MAIN UNIT)
The microphone amplifier circuit provides an AF signal for AM
modulation.
AF signals from the microphone (MC3001) or from the [MIC]
jack (02002) are applied to the AF amplifier (IC1001, pin 2)
through R1067. R1067 adjusts the microphone input level.
The output signal from IC1001 (pin 3) passes through R1071,
a buffer amplifier (IC1003, pin 6), and a low-pass filter
(IC1003, pin 3). The signal (MOD) is applied to the AM
modulator (Q2022) on the RF UNIT.
The ALC (Automatic Level Controller) prevents signal distor-
tion caused by strong input signals. A portion of the output
signals from the AF amplifier (IC1001, pin 3) is detected by
the ALC detector (D1013). The detected voltage is applied to
the amplifier (IC1001, ріп 4) and controls the gain of IC1001.
3-2-2 MODULATOR CIRCUIT (RF UNIT)
Using an AF signal from the microphone amplifier circuit, the
modulator circuit modulates an LO signal from the VCO.
During transmission, the LO signal from OUT1 of the VCO
UNIT is output through D2011 апа an attenuator
(R2034-R2036). This signal is then applied to the AM
modulator (Q2022).
Q2022 amplifies the LO signal with a gain controlled by an AF
signal (MOD) to make low level modulation.
MICROPHONE AMPLIFIER AND MODULATOR CIRCUITS
Fi
ALC AMP
MC3001
3-2-3 PRE-DRIVE AND DRIVE AMPLIFIERS
(RF UNIT)
The modulated RF signal from 02022 is amplified at the
APC (Automatic Power Control) amplifier (Q2021), pre-drive
amplifier (Q2020) and drive amplifier (Q2019). The drive
amplifier outputs approx. 1 W (PEP).
By using troidal coils (L2018, L2020, L2022) as matching
transformers between these stages, signals over a wide
frequency range can be amplified without adjustment.
3-2-4 RF POWER AMPLIFIER AND APC
CIRCUITS (RF UNIT)
From 118 MHz to 136.975 MHz, the RF power amplifier
gives 5 W (PEP) of output power.
The amplified signal from the drive amplifier (Q2019) is
power-amplified at the RF power amplifier (Q2018). While
transmitting, the antenna switching circuit (D2012, D2021) is
turned ON and the signals are applied to the antenna
connector through a low-pass filter (C2001~C2005, 12001,
L2002).
The APC (Automatic Power Control) circuit (D2013, D2014,
Q2021) protects the RF power amplifier (Q2018) from a
mismatched output load.
MAIN UNIT RF UNIT
from VCO UNIT
OUTI
MIC AMP
AND AF
to APC amp
О2021
FiL
IC1003