Icom IC-F3GT Service Manual

Also see for IC-F3GS: Instruction manual Instruction manual Instruction manual User manual Service manual

Contents

Icom IC-F3GT Service Manual Manual pdf 9 page image

4 - 24-1-5 AF CIRCUITAF signals from the FM IF IC (IC2, pin 9) are applied to themute switch (IC4, pin 1) via the AF filter circuit (IC3b, pins 6,7). The output signals from pin 11 are applied to the AFpower amplifier (IC5, pin 4) after being passed through the[VOL] control (R143).The applied AF signals are amplified at the AF power ampli-fier circuit (IC5, pin 4) to obtain the specified audio level. Theamplified AF signals, output from pin 10, are applied to theinternal speaker (SP1) as the “SP” signal via the [SP] jackwhen 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 (IC2, pin 9) areapplied to the active filter section (IC2, pin 8) where noisecomponents are amplified and detected with an internalnoise 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 is applied to the CPU(IC8, pin 19).The CPU detects the receiving signal strength from thenumber of the pulses, and outputs an “RMUT” signal frompin 49. This signal controls the mute switch (IC4) to cut theAF signal 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 (IC3c, pin 10). The amplified AFsignals are passed through the low-pass filter circuit (IC3d,pins 13, 14) via the mute switch (IC4, pins 4, 3). The filteredAF signals are applied to the modulator circuit after beingpassed through the mute switch (IC4, pins 9, 8).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 VCO circuit (Q7, Q8).The oscillated signal is amplified at the buffer-amplifiers (Q4,Q6), 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 buffer (Q3),drive (Q2) and power amplifier (Q1) to obtain 5 W of RFpower (at 7.2 V DC). The amplified signal passes throughthe antenna switching circuit (D1), and low-pass filter and isthen applied to the antenna connector.The bias current of the drive (Q2) and the power amplifier(Q1) is controlled by the APC circuit.4-2-5 APC CIRCUITThe APC circuit (IC3a, Q37) protects the drive and thepower amplifiers from excessive current drive, and selectsHIGH or LOW output power.The signal output from the power detector circuit (D32, D33)is applied to the differential amplifier (IC3a, pin 2), and the“T4” signal from the expander (IC10, pin 11), controlled bythe CPU (IC8), is applied to the other input for reference.• APC circuitQ1Poweramp.Q2Driveramp.IC3a+–VCCRF signalfrom PLL to antennaT4TXCQ37S5APC control circuitPower detectorcircuit (D32, D33)D33 D32L4, C278, C287LPF