Icom IC-R75 Service Manual

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SECTION 3 CIRCUIT DESCRIPTION3 - 13-1 RECEIVER CIRCUITS3-1-1 RF SWITCHING CIRCUIT (MAIN UNIT)The IC-R75 has two antenna connectors. RF signals entereither the [50 Ω ANT.] or [450 Ω ANT.] connector.RF signals from the [50 Ω ANT.] connector are applied to theantenna switching circuit (RL121), and then pass through thelow-pass filter (L131, L132, C131–C136).RF signals from the [450 Ω ANT.] connector are passedthrough the L101 to exchange the impeadance value, andare then applied to the antenna switching circuit (RL121).The signals are applied to the low-pass filter (L131, L132,C131–C136).Each RF signals from the [50 Ω ANT.] connector or [450 ΩANT.] connector are chosen by the antenna switching circuit(RL121).3-1-2 RF FILTER CIRCUIT (MAIN UNIT)The filtered signals are applied to the RX attenuator switch-ing circuit (RL141). Either the signals bypass or pass throughthe attenuator circuit. The signals are attenuated at 20 dBwhen passing through the attenuators. The attenuator sys-tem excludes non-linear components between an antennaconnector and an attenuator to prevent strong signals fromcausing distortion. The signals are then applied to the RF fil-ters. The MAIN UNIT has 8 RF bandpass fileters for signalsabove 2.0 MHz and 2 low-pass filters for signals below 2.0MHz.(1) Below 1.6 MHzThe signals are applied to the low-pass filter consisting ofC170–C175, L171–L173 via the limitter circuit (D141, D142).A diode is removed at the entrance of the low-pass filter. Thisdevice prevents the diode from causing distortion whenreceiving very strong signals. A switching diode (D172) isturned on when the “B0” line is “HIGH”.(2) Above 1.6 MHzThe signals are applied to the high-pass filter consisting ofC161–C163, L161–L164. This filter suppresses strong sig-nals below 1.6 MHz such as broadcasting stations.The filtered signal between 1.6 MHz and 2.0 MHz are appliedto the low-pass filter (C182–C187, L182, L183) via theswitching diode (D181). The switching diodes (D181, D182)are turned ON when the “B1” line is “HIGH”.The filtered signals above 2.0 MHz are applied to one of 8bandpass filters depending on the receive frequencies.After passing through a bandpass or low-pass filter, the sig-nals are applied to the pre-amplifier circuit (Q381, Q382,IC391).(3) FILTER SWITCHING CIRCUITThe RF bandpass filter corresponds to the BPF switchingvoltage (B0–B9) based on the CPU via the shifit registor(IC551, IC552) and driver (IC561, IC562). The switching volt-age of the BPF exit ot improve multi-signal and strong signalcharacteristics.• RF bandpass and preamplifier circuitANT1 (50 Ω)ANT2 (450 Ω)ATT (20 dB)HPFLPF0.5 – 1.6 MHz1.6 – 54 MHzATT (10 dB)LPFB01.6 – 2.0 MHzB1BPF2.0 – 4.0 MHzB2BPF50 – 60 MHzB9PREAMPQ381, Q382PREAMPIC391LPF60 MHzto 1st mixer(Q441, Q442)preamplifier circuit