Kenwood TK-3140 Service Manual

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TK-314083-5. Squelch circuitThe output from IC200 enters FM IC again, then passedthrough a band-pass filter. And passed through a band-passfilter IC202 (2/2). The noise component output from IC202 (2/2) is amplified by Q208 and rectified by D201 to produce a DCvoltage corresponding to the noise level. The DC voltage issent to the analog port of the CPU (IC309). And IC200 outputsa DC voltage (RSSI) corresponding to the input of the IFamplifier. The CPU reads the RSSI signal via pin 93.IC309 determines whether to output sounds from thespeaker by comparing the input voltage of pin 91 and pin 93with the preset value.Fig. 5 Squelch circuit12DETBPFAMP BPFAMPDETRSSI NOISEAMPIC309CPUIF AMP79193IC200 : FM IF ICQ208IC202(2/2) D201Fig. 6 Squelch and RSSI voltage vs ANT input levelSQ closeSQ openPresetvalueANT input levelSQ voltageANT input levelRSSI voltagePreset balue4. Transmitter System4-1. Microphone amplifierThe signal from the internal microphone goes through themute switch (Q301).When the SP-MIC is not attached, the microphone switchingterminal (MSW) on the universal connector becomes High, andmute switch (Q301) is turned ON. When the SP-MIC isattached, MSW is connected to GND at inside of SP-MIC. Forthis reason, Q301 is turned OFF, the internal microphone ismuted, and only the input of the external microphone is suppliedto the microphone amplifier.The signal from microphone passes through the limittercircuit in D508 and amplified by IC501 (1/2), Mic mute switch(Q502 is off in TX) and through the high-pass filter, the ALCcircuit, the low-pass filter, the high-pass filter, and pre-emphasis/IDC circuit in IC500 on the small board. Whenencoding DTMF, mute switch (Q500) is turned OFF for mutingthe microphone input signal.The signal passes through the D/A converter (IC307) forthe maximum deviation adjustment, and enters the summingamplifier consisting of IC305 (1/2), and is mixed with the lowspeed data from the CPU (IC309).The output signal from the summing amplifier passesthrough the D/A converter (IC307) again and goes to the VCOmodulation input.The other output signal from the summing amplifier passesthrough the D/A converter (IC307) again for the BALadjustment, and the buffer amplifier (IC302 (2/2)), and goes tothe VCXO modulation input.Fig. 7 Microphone amplifier12 HPF LPF HPF IDCPREEMPALC COMPSWLIMITSWMICQ301 D501AMPIC501(1/2)MICEXT.MICQ305IC500 15 16 18 19Q13MUTE DTMF9 86D/AD/AIC307IC307 IC303(1/2)D/AIC307LSD DI9IC305 (1/2)SUMAMPBUFFAMP VCXOVCOMICMUTEQ502PTTMSW4-2. Drive and Final amplifierThe signal from the T/R switch (D101 is on) is amplified bythe pre-drive (Q100 and 101) drive amplifier (Q103) to+15~17dBm.The output of the drive amplifier is amplified by the RF poweramplifier (Q106) to 4.0W (1W when the power is low). The RFpower amplifier is MOS FET transistor. The output of the RFpower amplifier is then passed through the harmonic filter (LPF)and antenna switch (D102,103 are on) and applied to theantenna terminal.Fig. 8 Drive and final amplifier and APC circuitFromT/R SW(D9)Pre-DRIVEAMP DRIVEAMPRFPOWER AMP LPFANTSWD102,103ANTVGGQ100Pre-DRIVEAMPQ101 Q103 Q106VDDR119R120R123+BIC100(1/2)IC100(2/2)REFVOL(IC8)4-3. APC circuitThe APC circuit always monitors the current flowing throughthe drive amplifier (Q103) and the RF power amplifier (Q106)and keeps a constant current. The voltage drop at R119, R120and R123 is caused by the current flowing through the RFpower amplifier and this voltage is applied to the differentialamplifier (IC100 1/2).IC100(2/2) compares the output voltage of IC100(1/2) withthe reference voltage from IC307, and the output of IC100(2/2) controls the VGG of the RF power amplifier to make theboth voltages to same voltage.The change of power high/low is carried out by the changeof the reference voltage. Q105,107 and 108 are turned on intransmit and the APC circuit is active.CIRCUIT DESCRIPTION