Icom IC-A23, Руководство по обслуживанию

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4 - 2
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.
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.
R87 C104
Mixer
16
IF amp.
2nd IF filter
450 kHz
PLL IC
IC3
X2
29.6 MHz
X1
(29.6 MHz)
RSSI
IC2 TA31136F
12
1st IF (30.05 MHz) from
the IF amplifier (Q13)
S-meter signal to the RSSI
amplifier (IC13, pin 1)
11 10 9
8 5 3
FM AF signal to the
AM/FM switch (IC2, pin 7)
AM AF signal to the
AM/FM switch (IC2, pin 6)
R5
2
12
14
Active
filter
FI2
Noise
detector
FM
detector
C93 C95 C92
R72
R71
C91
Q15
C103 R86 C101
Q16
C109
R91
R92 C108
R89
Q17
C112
R94
C111
Q18
R97 R96
R5
R5
R73 C94
C201
R74
R5
AGC
AMP
AGC
AMP
AF signal to the AM/FM Switch (IC2)
AM
DET
Q15
to the RF amplifier (Q11)
to the 1st IF amplifier (Q13)
2nd IF signal from
the 2nd mixer (IC1)
Q16
Q17
Q18
Q14
Q10
2nd IF
AMP
2nd IF
AMP
2nd IF
AMP
• 2ND IF AND DEMODULATOR CIRCUITS
• AGC CIRCUIT