Icom IC-M23 Service Manual

Also see for IC-M23: Instruction manual

Contents

Icom IC-M23 Service Manual Manual pdf 9 page image

4 - 24-2 TRANSMITTER CIRCUITSTX AF CIRCUITSAF signal from the microphone (MIC signals) is passedthrough the MIC mute SW (IC260A), amplified by MIC AMP(IC200B), and then applied to the D/A converter (IC190),through another MIC mute SW (IC260C).The level-adjusted MIC signal is pre-emphasized by R201and C385 to obtain +6 dB/oct of frequency response, andthen passed through the limiter (IC200A) to prevent overdeviation. The amplitude-limited MIC signal is filtered by thesplatter filter (IC200D) to remove 3 kHz and higher signals,and then applied to the VCO (Q21, Q22, D22) for frequencymodulation.TX AMPLIFIERSThe frequency-modulated signal from the VCO (Q21, Q22,D22) is passed through the buffers (Q23 and Q24) and LOSW (D50), and then sequentially amplified by the bufferAMP (Q50) and pre-drive AMP (Q53).The amplified signal is splitted by the power splitter (L100,L101, R463, C445, C446) and applied to the power AMPs(Q54 and Q55). The amplified signals are combined at thepower combiner (L107, L108, R465, C457, C458) to obtainthe TX output power.The amplified TX signal is passed through the ANT SW(D52) and LPF, and then applied to the antenna.APC CIRCUITA portion of TX output signal is rectified by D91 to producethe DC voltage which corresponds to the TX power level.The APC AMP (IC50) compares the voltage and TX powersetting voltage from the D/A converter (IC190). The resultingvoltage from the APC AMP controls the gain of both driverAMP (Q53) and power AMPs (Q54 and Q55), to keep the TXoutput power stable.• TX AF AND MODULATION CIRCUITSTo LO switchBUFFBUFFLPF LIMITAM PMICAMPMICMUTE MICMUT EQ24Q23Q 21,Q22MODMUT EPREENPHAD/AVCOQ20IC190IC200AIC200DIC200BMICD 20D21D22INMICMODIC260A IC260CMC250LPFANTSWANTENNAJ1PWRAMPDRIVEPRELOSW BUFFFrom VCOAPCCTRLReceive circuitsD52Q53Q54PWRAMPQ55D50 Q50PWRDETD91IC50D90PWRSPLITTERPWRCOMBINERQ357PCON• TX AMPLIFIERS AND APC CIRCUIT