Icom IC-F620 Service Manual

Also see for IC-F510: Instruction manual Instruction manual Instruction manual Service manual Instruction manual

Contents

Icom IC-F620 Service Manual Manual pdf 8 page image

4 - 24-1-5 AF AMPLIFIER CIRCUIT (MAIN unit)The AF amplifier circuit amplifies the demodulated AF sig-nals to drive a speaker.The AF signals from the FM IF IC (IC1, pin 9) are amplifiedat the AF amplifier section of the compander IC (IC14, pins5, 4) and are then applied to the high-pass filter circuit(IC21b).The high-pass filter characteristics are controlled by theFSW signal from the LCD driver IC (FRONT unit; IC1, pin 6).When FSW signal is high, the cut-off frequency is shiftedhigher to remove CTCSS or DTCS signals.The filtered AF signals from the high-pass filter (IC21b,pin 7) are applied to the de-emphasis section of companderIC (IC14, pin 3) with frequency characteristics of –6dB/octave, and are then passed through the low-pass filter,high-pass filter, expander sections of compander IC (IC14).The output signal from IC14 (pin 38) is applied to the elec-tronic volume controller (IC6, pin 1).The output AF signals from the electronic volume controller(IC6, pin 2) are applied to the AF amplifier (IC18) and AFpower amplifier (IC8) to drive the speaker.4-1-6 RECEIVER MUTE CIRCUITS (MAIN unit)• NOISE SQUELCHThe noise squelch circuit cuts out AF signals when no RFsignals are received. By detecting noise components in theAF signals, the squelch circuit switches the AF mute switch.Some noise components in the AF signals from the FM IF IC(IC1, pin 9) are passed through the level controller (IC6, pins24, 23). The level controlled signals are applied to the activefilter section in the FM IF IC (IC1, pin 8). Noise componentsabout 10 kHz are amplified and output from pin 7.The filtered signals are converted into the pulse-type signalsat the noise detector section and output from pin 13 (NOIS).The NOIS signal from the FM IF IC is applied to the CPU(IC20, pin 37). The CPU then analyzes the noise conditionand controls the AF mute signal via “AFON” line (IC20, pin18) to the AF regulator (Q39, Q40, D31).• 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 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 AF signals from the FM IF IC (IC1, pin 9)passes through the low-pass filter (IC5) to remove AF(voice) signals and is applied to the CTCSS or DTCSdecoder inside the CPU (IC20, pin 46) via the “CDEC” lineto control the AF mute switch.4-2 TRANSMITTER CIRCUIT4-2-1 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.The AF signals (MIC) from the FRONT unit via J2 (pin 1) arepassed through the level controller (IC6, pins 9, 10) to themicrophone amplifier circuit.The AF signals from the level controller (IC6) are applied tothe microphone amplifier section of compander IC (IC14, pin12). The amplified signals are passed through the compres-sor, low-pass filter and high-pass filter sections of IC14.The filtered AF signals are amplified at the buffer amplifier(Q21) and pre-emphasized with +6dB/octave at the pre-emphasis circuit (R122, C187), and are then applied to theIDC amplifier section of IC14 (pin 8).The amplified AF signals are passed through the limitteramplifier, low-pass filter and smoothing filter sections ofIC14 after being passed through the AF mute switch insideof IC14.The output signals from pin 6 are passed through the ana-log switch (IC15), splatter filter (IC21d) and applied to thelevel controller (IC6, pins 21, 22). The deviation level con-trolled signals are then applied to modulation circuit as the“MOD” signal.The narrow/wide switch (Q22) is connected to the input ofthe splatter filter (IC21d) and switched by the “NWC” signalcoming from the CPU (IC20, pin 19). When “NWC” is at ahigh level, the narrow/wide switch (Q22) shifts the filter cut-off frequency for narrow deviation selection.4-2-2 MODULATION CIRCUITThe modulation circuit modulates the VCO oscillating signal(RF signal) using the microphone audio signals.The AF signals from the level controller (IC6, pin 22) changethe reactance of varactor diode (D18) to modulate the oscil-lated signal at the TX VCO circuit (Q14, D17). The modulat-ed VCO signal is amplified at the buffer amplifiers (Q11,Q10) and is then applied to the drive amplifier circuit via theT/R switch (D14).The CTCSS/DTCS signals from the CPU (IC20, pins 89–91)are passed through the low-pass filter (Q37), level controller(IC6, pins 12, 11) and mixer (IC21a), and are then applied tothe VCO circuit via the splatter filter (IC21d).4-2-3 DRIVE AMPLIFIER CIRCUIT (MAIN unit)The drive amplifier circuit amplifies the VCO oscillating sig-nal to the level needed at the power amplifier.The RF signal from the buffer amplifier (Q10) passesthrough the T/R switch (D14) and is amplified at the YGR(Q9) and pre-drive (Q8) amplifiers. The amplified signal isapplied to the power amplifier circuit.