Icom IC-F50 Service Manual Addendum

Also see for VHF IC-F50: Price list Instruction manual Insrtuction manual Instruction manual Service manual

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Icom IC-F50 Service Manual Addendum Manual pdf 27 page image

4 - 24-1-5 AF AMPLIFIER CIRCUIT(MAIN AND FRONT UNITS)The AF amplifier circuit amplifies the demodulated AFsignals to drive a speaker. This transceiver employsthe base band IC which is composed of pre-amplifier,expander, scrambler, MSK de-modulator, etc. at the AFamplifier section.The AF signals from the FM IF IC (IC1, pin 9) are amplifiedat the AF amplifier section of the base band IC (IC10, pin 5)and are then applied to the low-pass filter section of it.The filtered signals passes through the high-pass filter tosuppress unwanted harmonic components. The signalspass through (or bypass) scrambler and expander sections,and are then applied to (or bypass) the scrambler IC (IC14)via the analog switch (IC13). The signals are amplified atthe amplifier section of the base band IC (IC10), and passthrough the AF mute switch (IC406) and low-pass filter(IC403). The filtered signals pass through the AF volume,and are then applied to the AF power amplifier (IC405) todrive the speaker.4-1-6 RECEIVE MUTE CIRCUITS(MAIN AND FRONT UNITS)• NOISE SQUELCHA squelch circuit cuts out AF signals when no RF signalsare received. By detecting noise components in the AFsignals, the squelch circuit switches the AF mute switch.Some noise components in the AF signals from the FM IFIC (IC1, pin 9) are passed through the D/A converter (IC6,pin 1). The signals are applied to the active filter section inthe FM IF IC (IC1, pin 8). Noise components about 10 kHzare amplified and output from pin 7.The filtered signals are converted into the pulse-typesignals at the noise detector section and output from pin 13(NOIS).The “NOIS” signal from the FM IF IC is applied to the CPU(FRONT unit; IC401, pin 41). Then the CPU analyzes thenoise condition and controls the AF mute signal via “AFON”line from expander IC (FRONT unit; IC410, pin 7) to the AFpower controller (FRONT unit; Q401, Q402).• CTCSS AND DTCSThe tone squelch circuit detects AF signals and opens thesquelch only when receiving a signal containing a matchingsubaudible tone (CTCSS or DTCS). When tone squelch isin use, and a signal with a mismatched or no subaudibletone is received, the tone squelch circuit mutes the AFsignals even when noise squelch is open.A portion of the “DET” AF signals from the FM IF IC (IC1,pin 9) passes through the low-pass filter (IC5, pin 5) toremove AF (voice) signals, and are then applied to theamplifier (MAIN unit; IC5, pin 10). The amplified signalsare applied to the CTCSS or DTCS decoder inside of theCPU (FRONT; IC401, pin 44) via the “CDEC” line. The CPUoutputs AF mute control signal, and is then applied to theI/O expander IC (IC410). The IC outputs AF mute circuit(IC406) and AF power supply circuits (Q401, Q402) controlsignals via the “AFON” line.4-2 TRANSMITTER CIRCUITS4-2-1 MICROPHONE AMPLIFIER CIRCUIT(FRONT AND MAIN UNITS)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 iscomposed of microphone amplifier, compressor, scrambler,limiter, splatter filter, MSK modulator, etc. at the microphoneamplifier section.The AF signals (MIC) from the microphone (MC401) arepassed through the microphone mute switch (IC406, pins2, 1), and are then applied to the amplifier (IC407, pins2, 6). The amplified signals pass through (or bypass) thescrambler IC (IC14) via the analog switch (IC13), and arethen applied to the microphone amplifier section of thebase band IC (MAIN unit; IC10, pins 3, 4). The amplifiedsignals are passed through or bypass the compressor,scrambler sections of IC10 (MAIN unit), and are thenpassed through the high-pass, limiter amplifier, splatterfilter sections of IC10 (MAIN unit).The filtered AF signals are applied to the FM/PM switch(MAIN unit; IC11, pin 6), and pass through the low-passfilter (MAIN unit; IC5, pin 1). The amplified signals areapplied to the D/A converter (MAIN unit; IC6, pin 4)The output signals from the D/A converter (MAIN unit; IC6,pin 3) are applied to the modulation circuit (MAIN unit;D18).AF mute(IC406)Base band IC(IC10)"DET" AF signalfrom FM IF IC (IC1, pin 9) 520Scrambler IC(IC14)LPFAFvolumeAFIC405Speaker(SP1)IC40318 18Analog switch(IC13)8111091915• AF AMPLIFIER AND MICROPHONE AMPLIFIER CIRCUITS