Icom IC-A24 Service Manual page 8

(2) FM DETECTOR CIRCUIT
The amplified signal is applied to the limiter amplifier section
of the IF IC (IC1, pin 5) and is then applied to the quadra-
ture detector (IC1, pins 10, 11) to demodulate the 2nd IF
signal into AF signals.
The demodulated AF signals are output from pin 9 of the
IC1 and are applied to the AM/FM switch (IC2, pin 7).
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 (IC18, pins 1, 2), and
then pass through the low-pass filter (IC18, pins 5, 7, 8, 10).
The filtered signals are amplified at the OP-amplifier (IC18,
pins 13, 14), and are then applied to the AF power amplifier
(IC6, pin 4) to obtain the specified audio level after being
passed through the electric volume (IC17, pins 21, 22). The
amplified AF signals are applied to the internal speaker (SP1)
via the [SP] jack (J5) 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 sig-
nals, the squelch switches the AF mute switch.
The AGC signal from the AGC amplifier (Q10) is amplified
again at the RSSI amplifier (IC13) and is then applied to the
CPU (LOGIC unit; IC1,pin 29) as the "RSSI" signal.
• 2ND IF AND DEMODULATOR CIRCUIT
R95
Q18
AM AF signal
R97
to the AM/FM
R96
switch (IC2, pin 6)
FM AF signal to the
AM/FM switch (IC2, pin7)
R5
Q16
C111 R92
C108
R89
Q17
R91
C109
C112
R94
FM
detector
R72
R73 R274
C95 C93 C92
The CPU analyzes the noise condition and outputs the con-
trol signal to the expander IC (IC5). The expander IC (IC5,
pin 4) outputs the squelch control signal as the "AFC" sig-
nal. The signal is applied to the AF out control circuit (Q36,
Q35) to control the power amplifier (IC6) and cut the AF
signal line.
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 applied
to the AGC amplifier circuits (Q14; for 1st/2nd IF amplifiers,
Q10; for RF amplifier, Q9; for RF attenuator). The signal
from the AGC amplifiers is applied to the 1st/2nd IF ampli-
fiers (Q13, Q15, Q16) and RF amplifier (Q11) to reduce the
amplifier gain and RF attenuator (D29) to attenuate the RF
signals when strong signals are received.
When strong RF signals disappear, then the AGC signal is
released to keep the constant audio output level.
• AGC CIRCUIT
Q18 Q17
AM 2nd lF
DET AMP
AGC
AMP
AGC
AMP
AF signal to the AM/FM switch (IC2)
AM
R5
2nd IF filter
R87 C104
450 kHz
Q15
FM
C101
C103
D40
R86
D41
5
IF amp.
9 10 11
C201
C91
R71 X1
R5
4 - 2
Q16 Q15
2nd lF 2nd lF
AMP AMP
2nd IF signal from
the 2nd mixer (IC1)
Q14
to the 1st IF amplifier (Q13)
Q10
to the RF amplifier (Q11)
FI2
(29.6 MHz)
3 2
Mixer
IC2 TA31136F
16
1st IF (30.05 MHz) from
the IF amplifier (Q13)
14
PLL IC
IC3
X2
29.6 MHz