Kenwood TH-K20A Service Manual

Also see for TH-K20A: Manual Instruction manual

TH-K20A/K20E6CIRCUIT DESCRIPTIONRealign the transceiver after replacing the EEPROM.6. Signaling Circuit6-1. Encode■ CTCSS/ DCSThe CTCSS and DCS signals are output from pin 49(LSDO) of the MCU (IC401). The signal passes through alow-pass CR filter. The signal is mixed with the audio signaland goes to the VCO and TCXO (X1) modulation input aftersignal processing in the DAC IC (IC300).■ DTMFThe DTMF signal is output from pin 50 (HSDO) of theMCU.The signal passes through a low-pass CR filter. TX devia-tion making an adjustment by MCU is applied to the DAC IC(IC300). The signal is mixed with the audio signal and goesto the VCO and TCXO.6-2. Decode■ CTCSS/DCSThe output signal from IF IC (IC200) enters the MCU(IC401) through IC300. IC401 determines whether theCTCSS or DCS matches the preset value, and controls theSPMUT and the speaker output sounds according to thesquelch results.Fig. 6 Encode7. Power SupplyThere are five 5V power supplies for the MCU:5M is always output while the power is on.5C is a common 5V and is output when SAVE is not setto ON.5R is 5V for reception and output during reception.5T is 5V for transmission and output during transmission.5MS is 5V for the SP/MIC connector and the DAC IC(IC300).4. Frequency Synthesizer Circuit4-1. Frequency SynthesizerThe frequency synthesizer consists of the TCXO (X1),VCO, PLL IC (IC1) and buffer amplifiers.The TCXO generates 19.2MHz. The frequency stability is5.0 ppm within the temperature range of –30 to +60°C.The frequency tuning and modulation of the TCXO aredone to apply a voltage to pin 1 of the TCXO. The output ofthe TCXO is applied to pin 1 of the PLL IC.The VCO consists of 1VCO and covers a dual range ofthe 144.00~174.00MHz and the 182.85~212.85MHz. TheVCO generates 182.85~212.85MHz for providing to the firstlocal signal in receive.The PLL IC consists of a prescaler, reference divider,phase comparator, charge pump (The frequency step of thePLL circuit is 5 or 6.25 kHz).PLL data is output from DAT (pin 19), CLK (pin 18) andPLDL (pin 20) of the MCU (IC401). The data are input to thePLL IC when the channel is changed or when transmissionis changed to reception and vice versa. A PLL lock conditionis always monitored by the pin 22 (PLUL) of the MCU. Whenthe PLL is unlocked, the PLUL goes low.Fig. 5 PLL block diagram5. Control CircuitThe control consists of the MCU (IC401) and its peripheralcircuits. It controls the TX-RX unit. IC401 mainly performsthe following;1) Switching between transmission and reception by PTTsignal input.2) Reading channel information, frequency, and programdata from the memory circuit.3) Sending frequency program data to the PLL.4) Controlling squelch on/off via the DC voltage from thesquelch circuit.5) Controlling the audio mute circuit via the decode datainput.6) Transmitting tone and encode data.7) Sending display data to the LCD assembly.Note:The EEPROM stores tuning data (Deviation, Squelch,etc.).HSDOLSDOIC401MCUIC300DACAF POWERAMPTCXO3478FREQVCO12MOD9BEEP624CTCSS/DCSINVVCOPLLMCUIC401 DAT,CLK,PLLEIC1PLLD81514RFSWLPF To mixerLSDOQ4BUFFQ5TCXOX1SW ToPre-AMPSHIFT(TX: Low)D8D7Q6CVAMPQ3LDAMPTXRXTXRX