Kenwood TH-K2AT Service Manual

Also see for TH-K2AT: Service manual Instruction manual Instruction manual

TH-K2AT/K2E/K2ET7CIRCUIT DESCRIPTION■ IF CircuitThe first IF signal (38.85 MHz) amplified by the IF amplifier (Q24)and the second IF signal (38.4 MHz) generated by tripling the 12.8MHz reference oscillator frequency of the TCXO (X1) by Q23, aremixed in the FM IC to produce a second IF signal (450 kHz) (Lowerheterodyne).The second IF signal passes through a ceramic filter (CF1)to remove unw anted components.The second IF signal passing through the ceramic filter (CF1)passes through the IF amplifier in the FM IC again and isdetected to produced an audio signal.■ Squelch and M ute CircuitA noise component is obtained by passing FM detectionoutput (FM IC pin 9) through an operational amplifier in theCF1450kHzCD1AF OUTAFMSMSQIC8M PU49948SM VSQV16 15 14 13 12 11 10 91 2 3 4 5 6 7 8M IXLocalOSCBufferRSSIQuadraturedetectorLPFNoiseAM PIF AM P InverterAM P1st IF input(38.85M Hz)IC3FM IC2nd local OSC(38.4M Hz)5RQ21AF FILTERQ20AF M UTESWITCHVOL INR135C173 C174R125R126Noisecom pRectifier5RSPAF OUT AFFILTERQ21AF M UTESWITCHQ20IC6AF AM PIC3FM ICVR201AF VOLUM EDTM F/BEEPREMIC8M PUSPKAFOSPGREM /TXD40469J4SP JACKFig. 3 IF, Squelch, M ute and S meter circuitFig. 4 AF amplifierFM IC and band-pass filter consisting of R125, R126, R135,C173, C174. The noise component is rectified in the FM IC toproduce a DC voltage, w hich is output from the N-REC terminal(pin 14) of the FM IC as squelch voltage.The squelch voltage enters the SQ terminal (pin 48) of theM PU (IC8) and is compared w ith the reference voltage presetin the M PU to control audio signal ON/OFF.When "L" is output from the AFM terminal (pin 9) of the MPU,the AF mute sw itch (Q20) is activated to mute the audio signal.■ S M eter CircuitThe S meter voltage is output from the RSSI terminal (pin12) of the FM IC (IC3) and input to the SM terminal (pin 49) ofthe M PU. Then the voltage is converted from analog to digitalin the M PU to control the S meter display on the LCD.■ AF AmplifierThe detected audio signal passes through an AF filter (Q21)and enters an AF mute sw itch (Q20).After passing through the AF mute sw itch (Q20), the signalpasses through an AF volume (VR201) and is amplified to aspecified output level w ith an AF amplifier (IC6).The audio signal amplified w ith the AF amplifier (IC6) isoutput through an internal speaker or an external speakerjack (J4).The beep tone and the DTM F signal are output from theDTM F/BEEP terminal (pin 40) of the M PU (IC8), enters the AFamplifier (IC6) and is output as a monitor tone.