Icom IC-R20 Service Manual

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Contents

Icom IC-R20 Service Manual Manual pdf 18 page image

4 - 104-2-7 BFO CIRCUIT (LOGIC UNIT)The oscillated signal at the BFO VCO circuit (IC6, X2) isapplied to the MAIN unit via the J5 (pin 35) as the BFO, andis then amplified at the BFO amplifier (RF unit; Q530). Theamplified signal is mixed with 3rd IF signal to demodulate toAF signal on SSB/CW modes.The carrier frequency is shifted by “BFO_SHIFT” signal fromthe main CPU (IC3, pin 2) while receiving SSB modes.4-3 OTHER CIRCUITS4-3-1 BATTERY CHARGER CIRCUIT (LOGIC UNIT)When charging the installed battery (BP-206), the batterycharger control signal (CHGC) becomes high, and is thenoutput from the CPU (IC3, pin 39). The signal is applied tothe battery charger controller (Q34), and its output controlsthe battery charger circuit (Q31, D28, D29) to output 400 mA(typical) charging current.4-3-2 BAR ANTENNA TUNING CIRCUIT (RF UNIT)When selecting the internal bar antenna while AM band sig-nals are receiving, the interal bar antenna switching signal isoutput from the D/A converter (IC22, pin 7) as “ANTSW” sig-nal. The signal is applied to the antenna switching circuit(Q510, D76), and then switches to the internal bar antenna.The expander IC (IC14) outputs the “A-TRAC” bar antennacontrol signal from pin 8. The signal is applied to the levelconverter (Q513), and is then applied to the AM bar anten-na tune circuit (D100). The circuit tunes to the desire fre-quency to change the D100’s capacity.4-3-3 EARPHONE ANTENNA CIRCUIT(LOGIC AND RF UNITS)When selecting the earphone antenna while WFM bandreceiving, the received RF signal passes through the exter-nal speaker jack (LOGIC unit; J3). The signal is applied tothe RF unit via the J5, pin 4 as “EAR” signal. The signal isapplied to the antenna switch (RF unit; D101), and is thenapplied to the 30–300 MHz RF circuit.4-3-4 RESET CIRCUIT (LOGIC UNIT)When [POWER] switch is ON, CPU3 signal from the +3 reg-ulator (IC8) is applied to the reset IC (IC2, pin 2). The IC out-puts reset signal via the “RESET” line, and the signal isapplied to the main CPU (IC3, pin 10) to reset it.4-3-5 LCD BACKLIGHT CIRCUIT (LOGIC UNIT)When the LCD backlight is ON, the LCD backlight controlsignal becomes high, and is output from the main CPU (IC3,pin 41) as “LIGHT” signal. The signal is applied to the back-light control switch (Q2, Q4 and Q5), and is then applied tothe LCD backlight LEDs (DS1–DS9 and EP12).