Kenwood TK-790 Service Manual

Also see for FleetSync TK-790: Specifications Instruction manual Specifications Instruction manual

26TK-790/(B)/H(B)CIRCUIT DESCRIPTION1. Transmitter Circuit1-1. Microphone amplifier section (X57 B/3)The audio input from the microphone is attenuated byVR501 and passes through the active high-pass filter (pre-emphasis circuit) in IC505, the compressor circuit in IC504,the IDC (limiter circuit) in IC505, the summing amplifier cir-cuit in IC510, the active low-pass filter in IC510, the sum-ming amplifier circuit in IC513, and the D/A converter inIC512, and is output from the CN502 to the CN202. Q504 isused as a microphone mute switch.SUMAMP SUMAMPDACLPF IDC COMP HPF MUTESW MICVR501IC513 IC510 IC510 IC505 IC504 IC505 Q504PCMOIC512CONTROL UNIT X57 B/3to TX VCOFig. 11-2. Modulation section (X57 A/3)The signal from the MO terminal of the CN202 goes toD305 in the VCO, and the VCO is directly modulated. Thetransmit signal output from the VCO passes through switchD306, is amplified by Q201 and Q204, and is output fromthe CN203 to the CN1.TX-VCOQ201 Q204AMP AMPSWDOAPCTX-RX UNIT X57 A/3D3068TMOFig. 21-3. Final amplifier section (X57 C/3) : TK-790/(B)The signal from the DO terminal of the CN1 is applied todrive the power module and the output is fed through trans-mit/receive switching diode ; D3,D4, low-pass filter, and CMcoupler to the antenna connector.CM coupler is a line for detecting traveling and reflectedwaves. Traveling waves are detected by D5 and fed throughthe APC control to differential amplifiers Q5, which comparethe signal level with the reference voltage of PC tuned.The output is amplified by DC amplifier Q1 to control APCDC amplifier Q2. Q1 controls the power supply voltage forthe power module, keeping the transmitter output constant.To protect the transmitter power amplifier stages, there isprotection circuit, against abnormal antenna loading.If an abnormal antenna load is connected the reflectedwave level increases. Reflected waves are detected by D6and the output level is fed to the differential amplifier, lead-ing to the transmitter output power being reduced in theway already described.POWERMODULE ANTSWFWDDETREFDETLPF LPFCMcouplerAPCDRIVEDCAMPDCAMPDifferentialAMP8TBDB Q1Q3D3,4D5 D6VR1Q5Q6PC+BDOFINAL UNIT (X57 C/3)Fig. 31-4. Final amplifier section (X45) : TK-790H(B)The signal from the DO terminal of the CN1 is applied todrive the power module, and the output is divided into twosignals which are amplified by Q6 and Q7. The signals aremixed and the resulting signal is fed through CM coupler,transmit/receive switching relay ; K1, and the low-pass filterto the antenna connector.CM coupler is a line for detecting traveling and reflectedwaves. Traveling waves are detected by D1, D3 and fedthrough the APC control to differential amplifiers Q5, whichcompare the signal level with the reference voltage of PCtuned.The output is amplified by DC amplifier Q4 to control APCDC amplifier Q3. Q4 controls the power supply voltage forthe power module, keeping the transmitter output constant.To protect the transmitter power amplifier stages, there aretwo protection circuits which one is against abnormal an-tenna loading and the other is against overheating.If an abnormal antenna load is connected, the reflectedwave level increases. Reflected waves are detected by D2,D3 and the output level is fed to the differential amplifier Q5,leading to the transmitter output power being reduced in theway already described. If an abnormal high temperature isdetected by the thermistor TH1, DC SW Q1 is made to re-duce the reference voltage of the PC tuned. This also leadsto the transmitter output power being reduced. Antennaswitching is done by a relay K1 with 8R.POWERMODULE AMPFWDDET REFDETLPFANTSWCMcouplerAPCDRIVEDCAMPDCAMPDifferentialAMPBDB Q3Q4Q6D1 D2VR1Q5 Q2PC+BFINAL UNIT (X45)AMPQ7RectifiedDiodeK1DCSW DCSWDCSWQ1TH1D3Q9 Q88R8TIC1DOFIg. 4