Icom IC-2720H Service Manual

Also see for IC-2720H: Step-by-Step Programming Guide Instruction manual Service manual Operating guide

Contents

4 - 4The 46.05 MHz 1st IF signal from the IF amplifier (Q1041) isapplied to the 2nd mixer section of the FM IF IC (IC1004, pin16), and is mixed with the 2nd LO signal (45.595 MHz) to beconverted to a 455 kHz 2nd IF signal.The 2nd IF signal is applied to the each demodulator circuitsby AM or FM mode.• FM MODEThe 2nd IF signal is output from the FM IF IC (IC1004, pin 3)and passes through the ceramic bandpass filter (FI1002).The filtered signal is fed back to the IC, and amplified at thelimiter amplifier section (pin 5), then demodulated into AF sig-nals at the quadrature detector section (pins 10, 11). Thedetected AF signals are output from pin 9 and are applied tothe AF circuit via the AM/FM selector circuit (IC2016, pins 7,1).• AM MODEThe 2nd IF signal is output from the FM IF IC (IC1001, pin 3)and passes through the ceramic bandpass filter (FI1002).The filtered signal is applied to the AM detector circuit(Q1025) to convert into AF signals, and then amplified at theQ1022 (pins 5, 1). The amplified AF signals are applied to theAF circuit via the AM/FM selector circuit (IC2016, pins 6, 1).4-1-10 AF AMPLIFIER CIRCUIT FOR RIGHT SIDEDISPLAY (MAIN UNIT)The AF amplifier circuit amplifies the demodulated AF signalsto drive a speaker.The AF signals pass through the AF mute switch (Q1011),and are then applied to the electric volume control circuit(IC2011, pin 8) as “UAFO” signal after being passed throughthe low-pass filter (Q1008). The level controlled AF signalsare output from pin 7, and are then applied to the AF poweramplifier (IC2012, pin 5) via the “VOUT2” signal. The poweramplified AF signals are applied to the internal speaker (SP1)via the [EXT SP] jack (J2004).When no plug is connected to the jack, the signals are fedback to the UHF audio input (IC2012, pin 2) and combinedwith the UHF audio. The mixed audio is applied to the otherexternal speaker jack (J2005) and then to the internal speak-er.The electronic volume control circuit controls AF gain, there-fore, the AF output level is according to the [VOL] setting andalso the squelch conditions.4-1-11 NOISE SQUELCH CIRCUIT FOR RIGHT SIDEDISPLAY (MAIN UNIT)• NOISE SQUELCHA noise squelch circuit cuts out AF signals when no RF sig-nal is received. By detecting noise components in the AF sig-nal, the squelch circuit switches the AF mute switch.A portion of the AF signals from the FM IF IC (IC1004, pin 9)are applied to the active filter section (IC1004, pin 8). Theactive filter section amplifies and filters noise components.The filtered signals are applied to the noise detector sectionand output from the IC1004 (pin 14) as the “R_SQL” signal.The “R_SQL” signal from IC1004 (pin 14) is applied to theCPU (IC2013, pin 38). The CPU analyzes the noise conditionand outputs the “R_DET_MUTE” signal (pin 58) to the AFmute switch (Q1011).• TONE SQUELCHThe tone squelch circuit detects AF signals and opens thesquelch only when receiving a signal containing a matchingsubaudible tone (CTCSS). When tone squelch is in use, anda signal with a mismatched or no subaudible tone isreceived, the tone squelch circuit mutes the AF signals evenwhen noise squelch is open.A portion of the AF signals from the FM IF IC (IC1004, pin 9)passes through the low-pass filter (Q1004) to remove AF(voice) signals. The filtered signal is applied to the CTCSSdecoder which is inside the CPU (IC2013, pin 39) via the“R_DTCS_IN” line to control the AF mute switch (Q1011).16Limiteramp.2nd IF filter455 kHzX100245.595 MHzRSSI IC1004 TA31136F141st IF (46.05 MHz)from Q1041"R_RSSI" signal to the CPU(IC2013, pin 32111098 7 5 3 2ActivefilterFI1002NoisedetectorFMdetector Noisecomp."R_SQL" signal to the CPU(IC2013, pin 38)12R1187C1148C1145R1174C1131C1035 C1034Q1022,Q1025C1044AMDET.C1122R1164R1168R1175R1179R_R5X1002FM AF signalto AM/FM selectorAM AF signalto AM/FM selectorMixer2nd• 2ND IF AND DEMODULATOR CIRCUIT FOR RIGHT SIDE DISPLAY