Icom IC-A110 Service Manual

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Contents

Icom IC-A110 Service Manual Manual pdf 7 page image

3 - 23-1-5 AM DETECTOR CIRCUIT (MAIN UNIT)The AM detector circuit converts the 2nd IF signal into AFsignals.The amplified 2nd IF signal from the 2nd IF amplifier (Q6) isapplied to the AM detector circuit (Q7). It is then detected forconversioin 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 drivea 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, pin3), and are then passed through the RMUTE swtich (IC6)and electric-volume switch (IC13, pins 14, 15) to control AFlevel. The level controlled signals are then applied to themute switch (IC30, pin 1).While in using the internal speaker or connecting outerspeaker jack, the AF signals are applied to the MOD/AFswtich (IC48, pin 7) via the AF mute swtich (IC28). Theapplied AF signals are compared at the PWM IC (IC44) withthe reference signal from the triangular wave oscillator(IC45), and are then converted to the PWM wave form. TheAF signals are amplified at the FET driver (IC47) and AFpower amplifiers (Q92, Q93). The amplified AF signals areapplied to the SP mute switch (D66, Q101, Q100) via theline 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 appliedto 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 nomodulated signal. When no voice modulation is included inthe signal, the squelch circuit cuts out the AF signal by com-paring voice audio and noise audio components in the AFdetected 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 thenapplied 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 theIF IC (MAIN unit; IC40, pin 5). The amplified signal passesthrough the RSSI section, and are then applied to thesquelch 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” signalfrom 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 signalfading and keeps the audio output level constant.AF signal from the AM detector circuit (Q7) is amplified atAGC amplifiers (Q77, Q78). The amplified signal from Q77is applied to the IF amplifier (Q4, Q5). The other amplifiedsignal from Q78 is applied to the RF amplifier (Q1), and theRX 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 strongsignal.• AGC circuitD38From the ant. RX ATTD7AMDETECTORXTALBPF IFAMPQ3"IF AGC" signal"RF AGC" signalRFAMPQ1Q73Q21st mixerBufferBPF IFAMPQ4, Q5to AF circuitIFAMPQ6Q77Q78AGCAMPIC402nd mixerAGCAMP