Icom IC-A200 Service Manual page 7

3-1-5 AM DETECTOR CIRCUIT (MAIN UNIT)
The AM detector circuit demodulates the 2nd IF signal to
AF signals.
The 2nd IF signal from Q10 is detected at the AM
detector (Q49) for conversion to AF signals. A PN junction
construction inside Q49 is used for AM detection to obtain
low output impedance. High frequency components are
removed by capacitors (C239, C241, C242) and passed AF
signals are applied to the AF circuits.
3-1-6 AF CIRCUIT (MAIN UNIT)
The AF signals are passed through the active low-pass
filter (Q13) and are amplified at the limiter amplifier (IC16,
pin 2). The amplified signals from IC16 (pin 3) pass through
the volume control (R89) on the front panel and are then
applied to the AF preamplifier (IC17, pin 3).
The limiter amplifier (IC16) has an ALC limiter circuit that
maintains the audio level even when receiving shallow or
deeply modulated signals. The AF signal level is detected at
D40 and is fed back to IC16 (pin 4) to control the amplifier
gain.
The amplified signals from the AF preamplifier (IC17) are
passed though the active low-pass filter (Q39) and the
speaker switch (Q38) and are then applied to the AF power
amplifier (IC5). IC5 amplifies the signals to a level needed
to drive a 4 Ω external speaker.
SQUELCH CIRCUIT
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3-1-7 SQUELCH CIRCUIT (MAIN UNIT)
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
comparing voice audio and noise audio components in the
AF detected signals.
The AF detected signals from Q49 are separately applied
to the active low-pass filter (IC3b) and active high-pass filter
(IC3a) to amplify voice components (lower than approx.
3 kHz) and noise components (higher than approx. 4 kHz)
respectively. Both outputs are applied to the comparator
(IC4a). When noise components are larger than audio
components, the comparator outputs "HIGH" to turn Q14
ON. Hence Q14 grounds the audio detected line and the AF
signal is cut out.
3-1-8 AGC CIRCUIT (MAIN UNIT)
The AGC (Automatic Gain Control) circuit reduces RF and
IF amplifier gain and activates the RF attenuator (D6, D7)
by decreasing the bias voltage to prevent distortion from
strong signals.
The signal from the 2nd IF amplifier (Q10) is detected at D8,
and applied to the base of Q50. When receiving a strong
signal, Q50 is turned ON to decrease the bias voltage.
Minus voltages are applied to the emitter of Q50 to operate
the AGC at low signal levels. The PLL reference oscillation
frequency (32.00 MHz) is rectified by the detector (D20,
D21, C116, C117) to obtain minus voltages (–4.5 V approx.).
The AGC release time is determined by C49 and R310. Q51
activates for strong signals.
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