Icom IC-M422 Service Manual

Also see for IC-M422: Instruction manual Service manual

Contents

4 - 24-1-6 SQUELCH CIRCUITSNoise squelch circuit mutes AF output signals when no RFsignals are received. By detecting noise components in thedemodulated AF signals, the squelch circuit switches the AFmute switch ON and OFF.A portion of the demodulated AF signals from the FM IF IC(IC2, pin 9) are passed through the squelch adjustment pot(R214) to be adjusted its level. The level adjusted AF signalsare passed through the active filter (IC2, pins 7, 8; R211–R213,C211, C212, C217). The filtered signals are then applied tothe noise amplifier section in the IC2 to amplify the noisecomponents only.The amplified noise components are converted into thepulse-type signal at the noise detector section, and outputfrom pin 14 as the “SQL” signal. The “SQL” signal is appliedto the squelch amplifier (IC11, pin 1) to be amplified its level,then output from pin 4. The amplifi ed “SQL” signal is appliedto the CPU (LOGIC BOARD; IC1, pin 5). Then the CPU out-puts “RMUTEM” signal from pin 64 according to the “SQL”signal level to toggle the AF mute circuit (IC4) ON/OFF.4-2 TRANSMITTER CIRCUITS4-2-1 MICROPHONE AMPLIFIER CIRCUITThe microphone amplifi er circuit amplifi es the audio signalsfrom microphone (MIC signals) within +6 dB/oct pre-emphasischaracteristic.The MIC signals from the microphone (HM-150B/SW; MC1)are passed through the microphone mute switch (IC5, pins10, 11) and amplified at the microphone amplifier (IC7, pin6) to obtain +6 dB/oct pre-emphasis characteristics. The am-plified MIC signals are limited its level at the IDC amplifier(IC7, pin 5), and filtered out 3 kHz and higher audio signalsat the LPF (IC8, pins 5, 7).The filtered MIC signals are passed through the deviationadjustment pot (R327) to be adjusted its level. Then the leveladjusted MIC signals are applied to the modulation circuit.4-2-2 MODULATION CIRCUITThe modulation circuit modulates the VCO oscillating signalwith the audio signals from the microphone.The level adjusted MIC signals are applied to the modulationcircuit (D2) to modulate the VCO oscillating signal by chang-ing the reactance of D2 at the VCO (Q4, Q5, D3, D4).The modulated VCO output signals are amplified at the buf-fer amplifiers (Q6, Q7), then applied to the transmit ampli-fi ers via the TX/RX switch (D7).4-2-3 TRANSMIT AMPLIFIERSThe VCO output signals are amplified to transmit outputpower level by the transmit amplifi ers .The buffer-amplified VCO output signals from the TX/RXswitch (D7) are applied to the pre-drive (Q10), YGR (Q12),and power (IC3) amplifiers to be amplified to the transmit out-put power level. The power amplifi ed transmit signal is passedthrough the power detector (D12, D13), antenna switch(D14) and a two-stage LPF (L21, L22, C127–C130, C132),and then applied to the antenna connector (CHASSIS UNIT; J1).4-2-4 APC CIRCUITThe APC (Automatic Power Control) circuit stabilizes trans-mit output power and controls transmit output power High (25 W)or Low (1 W).The power detector circuits (D12, D13) detect the transmitoutput signal level and converts it into DC voltage. The de-tected voltage is applied to the APC amplifier (IC12, pin 3). Thevoltage of the “T5V” line is applied to another input (pin 1)via the transmit output power adjustment pot (R112) as thereference voltage.The output voltage from the APC amplifier controls the biasof the power amplifier (IC3) to control the output power bycomparing the detected voltage and the reference voltage.Thus the APC circuit maintains a constant transmit outputpower.Poweramp.APCamp.YGRamp.+–HVTXDET• APC CIRCUITto the anntenaT5VR112T5VTMUTEH/Lfrom TX/RX switch (D7)Q12Pre-driveamp.Q10IC12Q17IC3LPF ANTSWD12Power detectorD13D14