Icom IC-A20MK II Service Manual page 7

3-1-6 SQUELCH CIRCUIT (MAIN UNIT)
The squetch circuit cuts out AF signals when no RF signal is
received.
Detected signals are applied to the limiter amplifier and
noise amplifier sections (IC1004, pin 5) The noise
components from 1C1004 (pin 9) pass through the [SQL]
control (R1035) and are then rectified by the noise detector
(D1006) and are converted to DC voltage. The DC voltage
controls the squelch control circuit (Q1008-Q1010).
3-1-7 AGC CIRCUIT (MAIN AND RF UNITS)
The AGC (Automatic Gain Control) circuit reduces signal
fading and keeps the audio output level constant.
When receiving a strong signal, the AM detector voltage
increases, and the collector voltages of Q1003 and Q2005
decrease. When the collector voltage of Q1003 decreases,
base bias voltages of the 2nd IF amplifiers (Q1001, Q1002)
decrease.
When the collector voltage of Q2005 decreases, base bias
voltages of the 1st IF amplifier (Q2004) and RF amplifier
(Q2002) decrease. Thus, total gain is decreased and
protected from distortion.
When receiving a VOR signal, a 30 Hz sine wave is
detected. To prevent the AGC function from operating with
the 30 Hz sine wave, Q1005 turns ON and C1018 is
connected in parallel with C1017.
3-1-8 AF CIRCUIT (MAIN UNIT)
The AF circuit drives the speaker (SP3001).
The AF signal selected at IC1005 is amplified at the AF
preamplifier (Q1011) and is applied to a low-pass filter
(Q1012). Through the [VOL] control (R1035), the AF signal
is amplified at the AF power amplifier (IC1006). 1C1006
drives the speaker (SP3001).
SQUELCH CIRCUIT
+5
SQL
signal
to IC3001
pin 36 Q1009
to Q1012 <
асі (а M З
j
AF signal
from IC1005
pin 1
3-1-9 ANL CIRCUIT (MAIN UNIT)
The ANL (Automatic Noise Limiter) circuit (D1004, C1015,
R1019~1022) reduces pulse noises.
The AM detector output signal (DETO) from D1003 is
applied to the cathode of D1004 passing through R1019,
where it is divided by R1019 and R1020. The DETO signal
is also applied to the anode of D1004, passing through
R1022 and R1021.
When the [ANL] switch (S1201) is OFF, the anode voltage
of D1004 is higher than the cathode voltage. Thus, the
received signal passes through D1004 and is applied to the
AF selector (1C1005).
When the [ANL] switch is ON, C1015 is grounded. The
detector output, including pulse noises, is applied to the
cathode of D1004 only. If pulse noises are received, the
cathode voltage of D1004 becomes higher than the anode
voltage and D1004 turns OFF. Thus, while pulse noises are
received, the detected signal is not applied to IC1005.
ANL CIRCUIT
to 1C1005 D1004 R1019
R1020
switch
$1201
from
Q1007
В | > from DETO
Fig. 3-3
nS from Q1001
2nd limiter amp
IF signal
Q1008
D1006 noise amp
<
Orr
R1035 IC1004
Fig. 3-2