Icom IC-2SA Service Manual page 8

4-1-6 AF AMP CIRCUIT (АҒ AND VR UNITS)
The AF signal is applied to Q1 and Q2 on the AF UNIT.
Q1 is an active filter that functions as a high-pass filter
to suppress tone signals for the tone squelch operation.
Q2 is also an active filter that functions as a low-pass
filter to suppress higher noise signals.
The filtered signal is applied to the [VOL] control (R1)
on the VR UNIT via the AF mute circuit (Q3). When the
squelch is closed, Q3 cuts the AF signal as the AF mute
switch. The AF signal is power-amplified at the AF
power amplifier (IC1) to drive the speaker.
The AF voltage regulator (Q4—Q6) supplies power to
the AF power amplifier. The AFS signal from the MIN
UNIT controls Q6 and mutes AF output while receiving
no signal or no specified tone/DTMF signal.
4-1-7 SQUELCH CIRCUIT (DET UNIT)
Some of the noise components in the AF signal from IC1
pin 11 are applied to IC1 pin 13 via C11, R8, C13 and
C14. The [SQL] control (R2) on the VR UNIT adjusts the
pin 13 input level.
The active filter section in IC1 amplifies noise components
of frequencies of 20 kHz and above, and outputs the
resulting signals from pin 14. Output signals are гес-
tified by D1 and are converted to DC voltage.
The rectified voltage triggers the squelch switch (Q1).
The collector of Q1 outputs the squelch signal. The
signal is applied to the CPU (IC1, pin 27) on the LGC
UNIT through the BUSY signal line. Тһе CPU outputs the
RMUTE and BUSY LED signals.
The RMUTE signal, decoded at the output expander
(IC1) on the IO UNIT, activates the AF mute circuit (Q3)
on the AF UNIT to cut the AF signal. The BUSY LED
signal is applied to Q1 on the LGC UNIT, turning OFF
the receive indicator.
TRANSMITTER CIRCUIT BLOCK DIAGRAM
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER (MIC UNIT)
AF signals from the built-in condenser microphone or
from the [MIC] jack are applied to IC1 pin 3, and are
pre-emphasized to +6 dB/octave through Сб and R4
connected to pin 2. ІСІ functions as the microphone
amplifier and the limiter.
The output signals from ІСІ pin 1 pass through the
splatter filter circuit (IC1 pins 5 and 6) where signals
of 3 kHz and above are attenuated. ІСІ pin 7 then
outputs the signals. The signals are applied to the
modulation circuit (PLL UNIT, D2) in the VCO to produce
an FM signal.
The VCO circuit (Q2, L2, D2) on the PLL UNIT oscillates
the transmit frequency with AF signal modulation.
4-2-2 DRIVE AMPLIFIER (MIN UNIT)
The VCO output, buffer-amplified at Q5 on the PLL
UNIT, is applied to the the transmit/receive switching
circuit (014) on the MIN UNIT. Тһе VCO output is then
amplified at the predrive amplifier (Q7) and the drive
amplifier (Q6).
The voltage controlled by the APC circuit is applied to the
collector of Q6 and Q7 to protect the RF power module
from damage by an antenna mismatch.
4-2-3 RF POWER AMPLIFIER (MIN UNIT)
IC1 is a power module which provides stable 5 W output
power.
An RF signal from the drive amplifier (Q6) is applied to
IC1 pin 1. The amplified signal is output from pin 4,
and applied to the antenna connector through the diode
Switching and low-pass filter circuits.
PLL UNIT
Fig. 2
4—2