Icom IC-A23 Service Manual page 8

4-1-5 AF CIRCUIT (RF UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.
AF signals from the AM detector (Q18; While in AM mode)
or IF IC (IC1, pin 9; While in FM mode) are applied to the
AM/FM switch (IC2, pin 6 or 7). The output signals from pin
1 are applied to the AF amplifier (IC510, pin 6), and then
pass through the low-pass filter (IC510, pins 2, 1). The fil-
tered signals are amplified at the OP-amplifier (IC14), and
are then applied to the power amplifier (IC6, pin 4) to obtain
the specified audio level after being passed through the
electric volume (IC505, pins 1, 2). The amplified AF signals
are applied to the internal speaker (SP1) via the [EXT SP]
jack (J2) when no plug is connected to the jack.
4-1-6 SQUELCH CIRCUIT (RF AND LOGIC UNITS)
A squelch circuit cuts out AF signals when no RF signals are
received. By detecting noise components in the AF signals,
the squelch switches the AF mute switch.
A portion of the 2nd IF signal from the 2nd IF amplifier (Q15)
is fed back to the IF IC (IC1, pin 5). The IF signal is ampli-
fied at the IF amplifier section in the IC, which then detects
the receiver signal strength at the RSSI section for conver-
sion into DC voltage.
The DC voltage is applied to the CPU (LOGIC unit; IC1, pin
3) via the "RSSI" signal after being amplified at the RSSI
amplifier (IC13).
The CPU analyzes the noise condition and outputs the con-
trol signal to the shift resistor (IC5). The shift resistor (IC5,
pin 14) outputs the squelch control signal via "AFC" line. The
signal is applied to the AF out control circuit (Q36, Q35) to
control the power amplifier (IC6) and cut the AF signal line.
• 2ND IF AND DEMODULATOR CIRCUITS
AM AF signal to the
AM/FM switch (IC2, pin 6)
FM AF signal to the
AM/FM switch (IC2, pin 7)
R5
C111 R92 C108
Q18
Q17
R91
R97 R96 R94
C112
R74
R5
Active
filter
FM
detector
R73 C94 R72
C95 C93 C92
4-1-7 AGC CIRCUIT (RF UNIT)
The AGC (Automatic Gain Control) circuit reduce signal fad-
ing and keeps the audio output level constant.
AF signals from the AM detector circuit (Q18) are converted
into DC voltage at the AGC amplifier circuits (Q14; for
1st/2nd IF amplifiers, Q10; for RF amplifier) by detecting the
driving current at the AM detector. The DC voltage from the
AGC amplifiers is applied to the 1st/2nd IF amplifiers (Q13,
Q15, Q16) and RF amplifier (Q11) to reduce the amplifier
gain when strong signals are received.
• AGC CIRCUIT
Q18
Q17
AM
DET
AF signal to the AM/FM Switch (IC2)
R5
Q16
R87 C104
Q15
R89
C101 R86 C103
C109
8 5
Noise
detector
IF amp.
RSSI
9 10 11
C91 C201
R71 X1
R5
4 - 2
Q15
Q16
2nd IF
2nd IF 2nd IF
AMP AMP AMP
Q14
AGC
AMP
to the 1st IF amplifier (Q13)
Q10
AGC
AMP
to the RF amplifier (Q11)
2nd IF filter
450 kHz
FI2
(29.6 MHz)
3 2
Mixer
IC2 TA31136F
12 16
1st IF (30.05 MHz) from
the IF amplifier (Q13)
S-meter signal to the RSSI
amplifier (IC13, pin 1)
2nd IF signal from
the 2nd mixer (IC1)
12 14
PLL IC
IC3
X2
29.6 MHz