Icom IC-F4021T Service Manual

Also see for IC-F4021S: Instruction manual Instruction manual Instruction manual

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Icom IC-F4021T Service Manual Manual pdf 9 page image

5 - 2The AF signals from the analog switch (IC3, pin 11) areapplied to the volume buffer amplifi er (IC6, pin 9). The buffer-amplified AF signals are adjusted its level (= audio level)by volume control pot (R315), then applied to the AF poweramplifier (IC15, pin 4) and amplified to the 0.5 W of audiooutput power (max., at 8 Ω load).The power-amplified AF signals are output from pin 10, thenapplied to the internal speaker (CHASSIS; SP1) or an externalspeaker via [SP] jack (J2).5-1-6 SQUELCH CIRCUITS (MAIN UNIT)5-1-6-1 NOISE SQUELCHThe squelch mutes the AF output signals when no RF signalis received. By detecting noise components (around 30 kHzsignals) in the demodulated AF signals, the squelch circuittoggles the mute switch and AF power amplifi er ON and OFF.A portion of the demodulated AF signals from the FM IF IC(IC9, pin 9) is applied to the D/A converter (IC12, pin 24) forlevel (= squelch threshold) adjustment. The level-adjustedAF signals are output from pin 23 and passed through thenoise filter (IC9, pins 8, 7, R42, R44–R46, R382, C69, C70,C413, C438). The fi ltered noise signals are amplifi ed the noisecomponents only at the noise amplifi er.The amplifi ed noise components are converted into the pulse-type signal at the noise detector section, and output from pin13 as the “NOIS” signal. The converted signal is applied to theCPU (IC22, pin 75). Then the “RMUTE” signal from the CPU(IC22, pin 96) to the RX mute switch (Q32) and analog switch(IC3, pins 12, 13) becomes “Low” according to the “NOIS”signal level to cut off the AF line.At the same time, the “AFON” signal from the CPU (IC22,pin 70) to the AF amplifier controller (Q41, Q42, D21, D23)becomes “Low” and the controller turns the AF power amplifi er(IC15) OFF.5-1-6-2 TONE SQUELCH• CTCSS/DTCSThe tone squelch circuit detects tone signals and opens thesquelch only when receiving a signal containing a matchedsub audible tone. When the tone squelch is in use, and asignal with a mismatched or no sub audible tone is received,the tone squelch circuit mutes the AF signals even when thenoise squelch is open.A portion of the demodulated AF signals is passed through theactive LPF (Q39) to filters CTCSS/DTCS signal. The filteredsignal is applied to the CPU (IC22, pin 46). The CPU comparesthe applied signal and the set CTCSS/DTCS, then the CPUcontrols the status (“Low” or “High”) of “RMUTE” and “AFON”signals as same as “NOISE SQUELCH”.• DTMFDTMF signals in the demodulated AF signals are passedthrough the LPF (IC6, pins 5, 7) to remove unwantedcomponents (voice signals), then applied to the CPU (IC22,pin 45) and decoded.5-2 TRANSMITTER CIRCUITS5-2-1 MICROPHONE AMPLIFIER CIRCUITS(MAIN UNIT)The AF signals from the microphone (MIC signals) are fi lteredand level-adjusted at microphone amplifi er circuits.• MIC SIGNALSMIC signals from the microphone are applied to the A/D switch(IC25, pins 7, 1), then applied to the D/A converter (IC12, pin1).The level-adjusted MIC signals are output from pin 2, andpassed through the MIC mute switch (Q31), HPF (IC5, pins13, 14) and pre-emphasis circuit (R137, R138, C260), thenapplied to the MIC amplifier (IC5, pin 9). The amplified MICsignals are output from pin 8, and passed through the analogswitch (IC3, pins 4, 3), AF mixer (IC5, pins 6, 7) where the MICsignals and tone signals are mixed with.• TONE SIGNALSThe CTCSS/DTCS signals are generated by the CPU (IC22)and output from pins 19–21. The output signals are passedthrough the 3 resistors (R222–R224) to change its waveform.The waveform changed CTCSS/DTCS signals are passedthrough the LPF (IC7, pins 10, 8), tone fi lter switch (Q40) andD/A converter (IC12, pins 12, 11) for level adjustment. Thelevel adjusted CTCSS/DTCS signals are then applied to theAF mixer (IC5, pin 6).DTMF signals are generated by the CPU (IC22) and outputfrom pin 43. The output DTMF signals are passed through twoLPFs (IC6, pins 3, 1 and pins 12, 14), then applied to the AFmixer (IC5, pin 6).The mixed AF signals are output from pin 7 of the AF mixer(IC5) and passed through the analog switch (IC3, pins 9, 8),then applied to the AF amplifi er (IC7, pin 6). The amplifi ed AFsignals are output from pin 7, and applied to the D/A converter(IC12, pin 9) to be adjusted its level (= deviation). The level-adjusted MIC signals are then applied to the modulationcircuits as the modulation signals.5-2-2 MODULATION CIRCUITS(MAIN UNIT)The modulation circuits modulate the VCO oscillating signalusing the modulation signals.The modulation signals from the D/A converter (IC12, pin 10)are applied to the D12 at the TX VCO (Q16, D10, D13) tomodulate the VCO oscillating signal by changing the reactanceof D12.The modulation signals are also applied to the referencefrequency oscillator (X2) via D/A converter (IC12, pins 16, 15)and the buffer (IC7, pins 12, 14), to ensure the modulation oflower frequency components of the modulation signals.The modulated VCO output is buffer-amplified by Q15 andQ29, then applied to the transmit amplifiers as the transmitsignal via TX/RX switches (D16 is ON, D17 is OFF).