Kenwood TK-390 Service Manual

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Contents

18TK-3904. Transmitter System4-1. Microphone amplifierThe signal from IC3 (control unit) goes through the muteswitch (Q403).When the SP-MIC is not attached, the microphoneswitching terminal (MSW) on the universal connector be-comes High, and mute switch (Q403) is turned on. Whenthe SP-MIC is attached, MSW is connected to GND at insideof SP-MIC. For this reason, Q403 is turned off, the internalmicrophone is muted, and only the input of the external mi-crophone is supplied to the microphone amplifier of the TX-RX unit.The signal from microphone passes through the limitercircuit in D601, and through the high-pass filter, the ALC cir-cuit, the low-pass filter, the high-pass filter, and pre-empha-sis/IDC circuit in IC607. When encoding DTMF, muteswitch (Q601) is turned off for muting the microphone inputsignal.The signal passes through the D/A converter (IC603) forthe maximum deviation adjustment, and enters the sum-ming amplifier consisting of IC605 (1/2), and is mixed withthe low speed data from the CPU (IC406).The output signal from the summing amplifier goes tothe VCO modulation input.The other output signal from the summing amplifierpasses through the D/A converter (IC603) again for the BALadjustment, and the buffer amplifier (IC604 1/2), and goes tothe VCXO modulation input.4-2. Noise cancelling microphone circuitThe two signals from INT MIC (Main & Sub) are input tothe positive (+) input (Sub) and to the negative (–) input(Main) of the IC3. If the same signal is input to both Mainand Sub, the Main signal is canceled at the output of IC3 (pin7). In other words, noise from nearby sources not directlyconnected to the transceiver enters the Main and Sub in-puts at the same signal and is therefore canceled out.When a signal is only input to Main and there is no signalat Sub, the Main signal is output as is, from IC3 (pin 7). Inother words, only the voice audio of the operator inextermely close proximity to the Main MIC is input to Mainso that the signal is output as is from IC3 (pin 7). Also, whenthe "N/C" switch is set to "L", transistor Q14 turns off so Submicrophone turns off and operation is the same as above.12 HPF LPF HPF IDCPREEMPALC COMPSWLIMITSWN/CAMP MICQ403IC3(CONT)D601MICMAINSUBEXT.MICQ404MSWIC607 15 16 18 19Q601MM TONE9 86D/AD/AIC603IC603IC604(1/2)I4 O4I3 O3LSDIC605 (1/2)SUMAMPBUFFAMP VCXOVCOIC10X14-3. Drive and Final amplifierThe signal from the T/R switch (D7 is on) is amplified bythe pre-drive (Q6) and drive amplifier (Q8) to 20mW. Theoutput of the drive amplifier is amplified by the RF poweramplifier (IC501) to 4W (1W when the power is low).The RF power amplifier consists of two stages MOS FETtransistor. The output of the RF power amplifier is thenpassed through the Transmit-Receive (TX-RX) antennaswitching (D10 is on) and the harmonic filter (LPF) and theInternal-External (INT-EXT) antennal switching (in the univer-sal connector) and applied to the antenna terminal.FromT/R SW(D7)Pre-DRIVEAMPDRIVEAMPRFPOWERAMPTX-RXANTSWLPFEXT-INTANTSWIn theuniversalconnectorANTON→EXTQ6 Q8 IC501 D10VDD VGGR35R37R39+BIC7(1/2)IC7(2/2)REFVOL(IC3)SWSWSWQ7Q9Q13Fig. 8 Drive and final amplifier and APC circuitsFig. 7 Microphoen circuit4-4. Internal-External (INT-EXT) antenna switchingThe INT-EXT antenna switch housed inside the universalconnector only switches to the EXT ANT side when an an-tenna speaker-microphone has been installed.This INT-EXT antenna switch works mechanically andswitches based on the operation shown in Figure 9.OFFFromLPFINTANTEXT ANTONFromLPFINTANTEXT ANTRFRFSwitch OFF↓InternalantennaSwitch ON↓ExternalantennaFig. 9 Internal-Externalantenna switchingCIRCUIT DESCRIPTION