Icom IC-A110 Service Manual Download

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3-1-5 AM DETECTOR CIRCUIT (MAIN UNIT)

The AM detector circuit converts the 2nd IF signal into AF
signals.

The amplified 2nd IF signal from the 2nd IF amplifier (Q6) is
applied to the AM detector circuit (Q7). It is then detected for
conversioin to AF signals.

The AF signals are applied to the AF circuit.

3-1-6 AF CIRCUIT (MAIN UNIT)

The AF circuit amplifies the demodulated AF signals to drive
a speaker.

The AF signals are passed through the low pass filter (IC31,
pin 1), and then applied to the MOD/AF select swtich (IC22,
pin 7).

The AF signals are amplified at the AF amplifier (IC42, pin
3), and are then passed through the RMUTE swtich (IC6)
and electric-volume switch (IC13, pins 14, 15) to control AF
level. The level controlled signals are then applied to the
mute switch (IC30, pin 1).

While in using the internal speaker or connecting outer
speaker jack, the AF signals are applied to the MOD/AF
swtich (IC48, pin 7) via the AF mute swtich (IC28). The
applied AF signals are compared at the PWM IC (IC44) with
the reference signal from the triangular wave oscillator
(IC45), and are then converted to the PWM wave form. The
AF signals are amplified at the FET driver (IC47) and AF
power amplifiers (Q92, Q93). The amplified AF signals are
applied to the SP mute switch (D66, Q101, Q100) via the
line filter (L62–L64, L81, C456, C457, C462, C463). The fil-
tered AF signals are applied to the internal speaker via the
[EXT SP] jack (J7).

While in connecting the head set, the AF signals are ampli-
fied at the AF power amplifier (IC37, pin 8), and then applied
to the ACC connector (J6, pin 6) via the impedance conver-
tor (T1).

3-1-7 SQUELCH CIRCUIT

(MAIN AND FRONT UNITS)

The squelch circuit cuts out AF signals when receiving no
modulated signal. When no voice modulation is included in
the signal, the squelch circuit cuts out the AF signal by com-
paring voice audio and noise audio components in the AF
detected signals.

The 2nd IF signal from the IF IC (MAIN unit; IC40, pin 3)
passes through the 450 kHz ceramic filter (Wide mode: FI3,
Narrow mode: FI4) to remove out-of-band signals, and then
applied to the IF amplifier.

A portion of the amplified 2nd IF signal from the IF amplifier
(MAIN unit; Q4) is applied to the IF amplifier section on the
IF IC (MAIN unit; IC40, pin 5). The amplified signal passes
through the RSSI section, and are then applied to the
squelch amplifier (MAIN unit; IC11, pin 1) as “RSSI” signal.

The amplified “RSSI” signal at IC11 is output from pin 4 as
“SQLI” signal, and is then applied to the CPU (FRONT unit;
IC1, pin 74). The CPU analyzes the noise condition and out-
puts the “AFMUT” signal via the output expander IC
(FRONT unit; IC5) to switch the SP mute switch (Q100,
Q101), etc. When connecting the head set, “RMUTE” signal
from the CPU via the output expander IC (FRONT unit; IC5)
is applied to the RMUTE circuit (MAIN unit; IC6, Q80, Q81).

Even when the squelch is closed, the mute switch (IC30)
opens at the moment of emitting beep tones.

3-1-8 AGC CIRCUIT (MAIN UNIT)

The AGC (Automatic Gain Control) circuit reduces signal
fading and keeps the audio output level constant.

AF signal from the AM detector circuit (Q7) is amplified at
AGC amplifiers (Q77, Q78). The amplified signal from Q77
is applied to the IF amplifier (Q4, Q5). The other amplified
signal from Q78 is applied to the RF amplifier (Q1), and the
RX attenuator (D38) via the buffer amplifier (Q73).

These amplifiers reduce the amplifier gain of IF amplifiers
(Q4, Q5) and the RF amplifier (Q1) while receiving a strong
signal.

• AGC circuit

D38

From the ant.

RX ATT

DETECTOR

XTAL

BPF

IF
AMP

"IF AGC" signal

"RF AGC" signal

RF
AMP