IC-GM651 SERVICE MANUAL

4-2 TRANSMITTER CIRCUITS• TX AF CIRCUIT (AF UNIT)The audio signals from the connected microphone arepassed through the analog SW (MIC SW) and pre-emphasiscircuit (R73, C77) to obtain +6 dB/oct of audio response,then amplified by the MIC AMP (IC9b). The amplified MICsignals are amplified and amplitude limited by (IC8a), andpassed through the splatter fi lter to remove unwanted 3 kHzand high signals, then entered to the MAIN UNIT as themodulation signals "MOD."• MODULATION CIRCUIT (MAIN UNIT)The modulation signals from the AF UNIT are passedthrough the DAC (IC15) for deviation adjustment. The leveladjustedmodulation siganls are applied to the D37 of TXVCO for FM modulation.• DRIVER AMPThe output of the TX VCO is passed through the LO SW(D43), and amplifi ed by Q28 and Q30 to obtain input levelof the RF power module (IC13). Q37 controls the APCcontroller (IC14) with “TMUTE” signal to terminate TX, toprevent unexpected transmittion; when the TX power isrising, when the PLL is unlocked, etc.• RF POWER AMPIC13 is an RF power module which provides 35 W of RFoutput power with 50 mW input, and the output power iscontrolled by the voltage applied to pin 2.• APCThe voltage produced by the TX power is appeared on theboth sides of L89 is rectifi ed by D47 and D48, then appliedto the APC controller (IC14). The output voltage of IC14controls the gain of IC13 to keep the TX power steady.• ANT SW, LPF (Harmonic filter)The TX signal are passed through the ANT SW (D1, 46) andLPF. The LPF (L1, 2, 89, C1, 3, 5, 361, 364) is a harmonicfilter and removes the harmonic components in the TXsignal.• APC CIRCUITSTX signalfrom PLLQ28PredriveQ30YGRPower moduleIC13D47L89D48RF detectorcircuitto antennaQ32Q33"TXDET" signal to the CPUH/LT8"TMUTE" signal from the CPUIC14APC control circuit"PCON" signal from the D/A convertor4 - 3