Icom IC-F21 Series Service Manual

Also see for IC-F21: Instruction manual Instruction manual Service manual Service manual

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Icom IC-F21 Series Service Manual Manual pdf 8 page image

4 - 2The applied AF signals are amplified at the AF power ampli-fier circuit (IC4, pin 4) to obtain the specified audio level. Theamplified AF signals output from pin 10 as “AFOUT”signalare applied to the internal speaker (SP1) as the “SP” signalvia the [SP] jack when no plug is connected to the jack.4-1-6 SQUELCH CIRCUITA squelch circuit cuts out AF signals when no RF signals arereceived. By detecting noise components in the AF signals,the squelch switches the AF mute switch.A portion of the AF signals from the FM IF IC (IC3, pin 9) as“DET” signal are applied to the D/A convertor IC (IC9, pin24). The signals from the D/A convertor (IC9, pin 23) as“SQLIN” signals are applied to the active filter section (IC3,pin 8) where noise components are amplified and detectedwith an internal noise detector.The active filter section amplifies noise components. The fil-tered signals are rectified at the noise detector section andconverted into “NOIS” (pulse type) signals at the noise com-parator section. The “NOIS” signal output from IC3, pin 13,and is applied to the CPU (IC7, pin 53).The CPU detects the receiving signal strength from thenumber of the pulses, and outputs “EXST”, “SO”, “SCK” sig-nals. The signals are applied to the expander IC (IC15, pins1, 2, 3), and then outputs “RMUT” signal from pin 4. Thissignal controls the analog switch (IC6, pin 13) to cut the AFsignal line.4-2 TRANSMITTER CIRCUITS4-2-1 MICROPHONE AMPLIFIER CIRCUITThe microphone amplifier circuit amplifies audio signals with+6 dB/octave pre-emphasis characteristics from the micro-phone to a level needed for the modulation circuit.The AF signals from the microphone are applied to themicrophone amplifier circuit (IC5c, pin 10) after being passthrough the high-pass filter (C186, C187). The amplified AFsignals are passed through the low-pass filter circuit (IC5d,pins 13, 14) via the mute switch (IC6, pins 2, 3, 4). The fil-tered AF signals are applied to the modulator circuit afterbeing passed through the mute switch (IC6, pins 8, 9, 10).4-2-2 MODULATION CIRCUITThe modulation circuit modulates the VCO oscillating signal(RF signal) using the microphone audio signal.The audio signals change the reactance of a diode (D6) tomodulate an oscillated signal at the TX-VCO circuit (Q11).The oscillated signal is amplified at the buffer-amplifiers (Q5,Q7), then applied to the T/R switching circuit (D3, D4).4-2-3 DRIVE/POWER AMPLIFIER CIRCUITSThe signal from the VCO circuit passes through the T/Rswitching circuit (D3) and is amplified at the YGR (Q4, Q3),pre-drive (Q2) and power amplifier (Q1) to obtain 4 W of RFpower (at 7.2 V DC). The amplified signal passes throughthe antenna switching circuit (D1) via the power detector(D2), and low-pass filter and is then applied to the antennaconnector.The bias current of the pre-drive (Q2) and the power ampli-fier (Q1) is controlled by the APC circuit.4-2-5 APC CIRCUITThe APC circuit (IC2) protects the drive and the poweramplifiers from excessive current drive, and selects HIGH orLOW output power.The signal output from the power detector circuit (D2) isapplied to the differential amplifier (IC2, pin 3), and the “T4”signal from the expander (IC9, pin 11), controlled by theCPU (IC7), is applied to the other input for reference.When the driving current is increased, input voltage of thedifferential amplifier (pin 3) will be increased. In such cases,the differential amplifier output voltage (pin 4) is decreasedto reduce the driving current.APC CIRCUITQ1Poweramp.Q2Pre-driveramp.IC2+—VCCRF signalfrom PLL to antennaT4T5VQ14S5VAPC control circuit Power detectorcircuit (D2)D2LPFD1ANTSWAFamp.LPFHPF"DET" signalfrom IC3, pin 9"NWC" signalfrom IC15, pin 14Q21AnalogswitchIC6IC5BIC5DAF VOL.R1SW-A/SW-BunitIC4SP1119, 102, 3AF CIRCUIT