Icom IC-F24 Service Manual

Also see for IC-F24: Instruction manual Instruction manual Servise manual

Contents

Icom IC-F24 Service Manual Manual pdf 8 page image

4 - 24-1-5 AF AMPLIFIER CIRCUITThe AF amplifier circuit amplifies the demodulated AF sig-nals to drive a speaker.The AF signals from the FM IF IC (IC1, pin 9) pass throughthe high-pass filter (IC6, pins 3 and 1) to suppress unwant-ed harmonic components. The signals pass through theRX mute switch (Q34) which is controlled by “RMUT” sig-nal from the CPU (IC13, pin 56), and are then applied toanother high-pass filter (IC6, pins 13 and 14). The filteredsignals pass through the low-pass filter (IC6, pins 6 and7) via the analog switch (IC10, pins 1 and 2). The signalsare applied to the analog switch (IC10, pin 10) again, andare then applied to the AF power amplifier (IC12, pin 4) viathe AF volume (R226). The amplified AF signals are outputfrom pin 10, and are then applied to the internal speakerwhich is connected to J1 via the [SP] jack (J3).4-1-6 RECEIVE MUTE CIRCUITS• NOISE SQUELCHA squelch circuit cuts out AF signals when no RF signalsare received. By detecting noise components in the AF sig-nals, the squelch circuit switches the AF mute switch.Some noise components in the AF signals from the FM IFIC (IC1, pin 9) are applied to the D/A converter (IC8, pin1) as “DET” signal, and are then output from pin 2. Thesignals are applied to the active filter section in the FM IFIC (IC1, pin 8). The active filter section filters and amplifiesnoise components. The amplified signals are converted intothe pulse-type signals at the noise detector section andoutput from pin 13 as “NOIS” signal.The “NOIS” signal from the FM IF IC is applied to the CPU(IC13, pin 53). Then the CPU analyzes the noise conditionand outputs the AF mute control signal from the CPU (pin56) as “RMUT” signal from pin 56. The signal is applied tothe RX mute switch (Q34) to control the AF signal muting.• CTCSS AND DTCSThe tone squelch circuit detects tone signals and opens thesquelch only when the receiving signal contains matchedsubaudible 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 AF sig-nals even when noise squelch is open.A portion of the “DET” signals from the FM IF IC (IC1, pin9) passes through the low-pass filter (IC7, pins 5 and 7)to remove AF (voice) signals, and are then applied to theamplifier (IC7, pin 3). The amplified signals are applied tothe CTCSS or DTCS decoder inside of the CPU (IC13, pin60) as the “CDEC” signal. The CPU outputs AF mute con-trol signal from pin 56, and is then applied to the RX muteswitch (Q34) and analog switch (IC10, pins 12 and 13) tocontrol AF signals muting as “RMUT” signal.4-2 TRANSMITTER CIRCUITS4-2-1 MICROPHONE AMPLIFIER CIRCUITThe 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 from the microphone are passed throughthe microphone mute switch (Q35), and are then appliedto the amplifier (IC6, pins 9 and 8) via the high-pass filter(IC6, pins 13 and 14). The amplified signals are applied tothe analog switch (IC10, pin 4), and outputs from pin 3. Thesignals pass through the low-pass filter (IC6, pins 6 and 7),then applied to the analog switch (IC10, pin 9) again andoutput from pin 8.The signals are applied to the D/A converter (IC8, pin 4).The converted signals output from pin 3, and applied to themodulation circuit (D18) as “MOD” signal.• ANALOG SWITCHING CIRCUITSAnalog SW(IC10)"DET" AF signalfrom FM IF IC (IC1, pin 9)8AFvolumeAFAMPMICMUTEIC12Q35SpeakerR226IC8 D18RXMUTEQ34LPFIC6 BHPFIC6 DHPFIC6 AMicrophoneIC6 CAMP1432910D/A converter FM mod."CTCSS/DTCS" signal fromD/A conveter IC (IC8, pin 10)"TONE" signal from CPU (IC13) via low-passfilters (IC5 A / B pin 7)to TX VCO circuit(Q13, D17, D21)11