Icom IC-F43GT Service Manual

Also see for IC-F43GS: Instruction manual Instruction manual Instruction manual Instruction manaul Service manual

Contents

Icom IC-F43GT Service Manual Manual pdf 8 page image

4 - 2The 2nd IF signal from the 2nd mixer (IC9, pin 3) passesthrough the ceramic filter (FI2) to remove unwanted hetero-dyned frequencies. It is then amplified at the limiter amplifi-er section (IC9, pin 5) and applied to the quadrature detec-tor section (IC9, pins 10, 11) to demodulate the 2nd IF sig-nal into AF signals.The demodulated AF signals are output from pin 9 (IC9) andapplied to the base band IC (IC14).4-1-5 AF AMPLIFIER CIRCUIT (MAIN UNIT)The AF amplifier circuit amplifies the demodulated AF sig-nals to drive a speaker. This transceiver employs the baseband IC which is composed of pre-amplifier, expander,scrambler, MSK de-modulator, etc. at the AF amplifier sec-tion.The AF signals from the FM IF IC (IC9, pin 9) are amplifiedat the AF amplifier section in the base band IC (IC14, pin23), and are then applied to the high-pass filier and low-passfilter section of it.The filtered signals pass through the high-pass filter to sup-press unwanted harmonic components. The signals passthrough (or bypass) scrambler and expander sections. Thesignals are amplified at the amplifier section in the baseband IC (IC14).The output siganls from IC14 (pin 20) pass through the low-pass filter sector (IC23), and are then applied to the AF vol-ume (R315), and AF power amplifier (IC15).4-1-6 SQUELCH CIRCUITS (MAIN UNIT)• NOISE SQUELCHA squelch circuit cuts out AF signals when no RF signals arereceived. By detecting noise components in the AF signals,the squelch circuit switches the AF amplifier contrler.Some noise components in the AF signals from the FM IF IC(IC9, pin 9) are passed through the D/A converter (IC12, pin1). The signals are applied to the active filter section in theFM IF IC (IC9, pin 8). The active filter section filters andamplifies noise components. The amplified signals are con-verted into the pulse-type signals at the noise detector sec-tion. The detected signals output from pin 13 (NOIS) via thenoise comparator section.The “NOIS” signal from the FM IF IC is applied to the CPU(IC22, pin 75). Then the CPU analyzes the noise conditionand outputs the AF mute signal as “AFON” from the pin 70to the AF power controller (Q41, Q42).• CTCSS AND DTCSThe tone squelch circuit detects tone signals and opens thesquelch only when receiving a signal containing a matchingsubaudible tone (CTCSS or DTCS). When tone squelch is inuse, and a signal with a mismatched or no subaudible toneis received, the tone squelch circuit mutes the AF signalseven when noise squelch is open.A portion of the “DET” AF signals from the FM IF IC (IC9, pin9) pass through the low-pass filter (IC19, pin 5) to removeAF (voice) signals, and are then applied to the amplifier(IC19, pin 3). The amplified signals are applied to theCTCSS or DTCS decoder in the CPU (IC22, pin 46) via the“CDEC” line. The CPU outputs AF mute control signal andtone mute circuit (Q502) controls via the “MUTE” line.4-2 TRANSMITTER CIRCUITS4-2-1 MICROPHONE AMPLIFIER CIRCUIT(MAIN UNIT)The microphone amplifier circuit amplifies audio signalswithin +6 dB/octave pre-emphasis characteristics from themicrophone to a level needed for the modulation circuit.This transceiver employs the base band IC which is com-Base band IC(IC14)"DET" AF signalfrom FM IF IC (IC9, pin 9) 23 20 LPFAFvolumeAFAMPIC15SpeakerIC23IC23IC13IC6 IC12 D12MicrophoneAMP374FM/PM switchD/A converter FM mod.LPF"CTCSS/DTCS" signal fromD/A conveter IC (IC12, pin 11)"TONE" signal from CPU via low-passfilter (IC22, pin 43)to TX VCO circuit(Q16, D13)3• AF AND MIC AMPLIFIER CIRCUIT