Icom IC-R6 Service Manual

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Icom IC-R6 Service Manual Manual pdf 11 page image

4-2 AF CIRCUITS (LOGIC UNIT)FM modeThe demodulated AF signal is passed through thede-emphasis circuit (R323 and C316) and AF filter (Q301),and then applied to the AF AMP (Q300) through the AF lineSW (D300).Wide FM/AM modeThe demodulated AF signal is passed through the AF filter(R316, R320 and C314) and the AF line SW (Q302), andthen applied to the AF AMP (Q300).The amplified AF signal is passed through the LPF (R350and C404), whose frequency response is changed by Q508according to the user setting (The "AF FILTER"). The filteredAF signal is passed through the AF mute SW (Q350), andthen adjusted in level by the electric VR (IC451).The level-adjusted AF signal is applied to the AF powerAMP (IC450).AUDIO POWER AMPLIFIERIC450 is a BTL (Bridge-Tied Load) audio power amplifierwhich provides high AF output power.The amplified AF signal is passed through the speaker muteSW (Q507), and then applied to the internal or externalspeaker.• AF CIRCUITS4 - 3AFAMPAFMUTEAFAMPSP1J2LOGIC UNITWFM/AMFM AFFILTERAFFILTERAFFILTERSWMODESWMODEElectricVRFrom the demodulator circuitsIC450IC451Q350Q300D300Q301Q302SP1(CHASSIS)EXT SPLPFCTRLQ508SPEAKERQ507,D505MUTE4-3 FREQUENCY SYNTHESIZER CIRCUITS(RF UNIT)1st VCOThe 1st VCO is composed of Q528, Q529 and D105,and generates the 1st LO signal, ranging from 266.8 to550 MHz. This VCO has a shift switch which changes theoscillating frequency range. The output signal is amplifiedby Q525, and then applied to both the buffer (Q43) and thedoubler circuit.DOUBLER CIRCUITS FOR THE 1ST LO SIGNALIC-R6 has two doubler circuits, "DBL1" and "DBL2". Theydouble the frequency of the VCO output signal to obtain the1st LO signal with a wide frequency range.The "DBL1 (Q31, D43 and D50)" doubles the VCO outputsignal ranging from 274.85 to 400 MHz, and the "DBL2 (Q31,D44 and D47)" doubles the signal ranging from 380.45 to549.85 MHz. When the fundamental frequency signal isdirectly used as the 1st LO signal, the VCO output signalby-passes both doubler circuits through D42 and D52.2nd VCOThe 2nd VCO is composed of Q6, D17, L45, C80, C207and C208, and generates 247.05/286.35 MHz (for FM andAM modes) and 246.750/286.650 MHz (for WFM mode) 2ndLO signals.PLL (Phase Locked Loop) CIRCUITA portion of signal from each VCO is fed back to the PLL IC(IC3). The output signal of the 1st VCO is applied to pin 11of the IC, through the buffer (Q43), and that of the 2nd VCOis applied to pin 19.The applied VCO output signal is divided and phase-compared with a 19.2 MHz reference frequency signal fromthe TCXO (X1), which is also divided. The resulting signal ofthe phase comparison is output from pin 5 (to the 1st VCO)or pin 2 (to the 2nd VCO) through the internal charge pump,and DC-converted by the loop filter, and then applied to theVCO as the lock voltage.When the oscillation frequency drifts, its phase changes fromthat of the reference frequency, causing a lock voltage changeto compensate for the drift in the VCO oscillating frequency.• FREQUENCY SYNTHESIZER CIRCUITSX2 X2BUFFBUFFPLL ICPrescalerPhasecomparatorChargepumpDividerPrescalerPhasecomparatorChargepumpDividerLPFFILLOOPFILLOOPX1TCXOIF ICLOSWSWLOIC1 Q523 IC12D47,D50,D52Q31 Q31D42-D44Q43Q525Q528,Q529,D105Q2,Q45Q6,D172ND LO VCO1st IF MIXER 2nd IF MIXER (3rd IF MIXER)1ST LO VCO266.8-1099.695 MHzFM,AM: 247.050/286.350 MHz19.200 MHzIC31st Lo 2nd Lo3rd Lo:19.2 MHzWFM : 246.750/286.650 MHz