Kenwood TH-22A Service Manual

Also see for TH-22A: Instruction manual Instruction manual Instruction manual Instruction manual Instruction manual

TH-22A/AT/ECIRCUIT DESCRIPTIONe S meterThe S-meier signal is output from pin 13 of IC4 as a directcurrent corresponding to the input signal, converted to avoltage by R83, then input to pin 3 of the microprocesser.The DC voltage is digitized to control the LCD S-meter dis-play. The S-meter display level is written into the EEPROM ofeach unit during adjustment in the factory. Thus there is littlevariation in the product level.LCD—TRANSMITTING SYSTEMe Microphone amplifierThe audio band of the signal from the microphone iscorrected by the 6dB/oct preemphasis circuit consisting ofC235 and R244. The 18dB/oct tone frequency is eliminatedby the low-pass filter consisting of C236, R245, C239 andR239 and the preemphasis circuit. The signal is amplifiedand limited by 1C207 (1/2). Distortion components exceedingthe audio band of the resulting signal are then eliminated by18dB/oct frequency correction by a splatter filter consistingof |C207 (2/2) and R235 and C230.¢ Modulation circuitThe signal from the microphone amplifier passes throughthe modulation adjustment variable resister VR202, is ap-plied to VCO varicap diode D3, and modulated by variablereactance.e Drive and Final CircuitThe desired signal is produced by the VCO, and amplifiedto about 75 mVrms by the buffer amplifier. It is then amplifiedFig.4Smetereeetoabout1.4Vrms bythedrive.Theamplifiedsignalisinputtopower moduleIC3.Thepowermoduleconsistsofatwo-stageFETamplifier,andincreasesthepowertoabout5W.* Transmission/reception selector circuitThe transmission output is passed through the transmis-sion/reception selector circuit and low-pass filter to the an-tenna.The transmission/reception selector circuit, which con-sists of D11 and D12, is turned on during transmission andoff during reception to switch the signal.4201EXT MIC MIC tkC235|| + rr 3rte _| # tf Wy ~~ AMA AAR 5 Ne R235C236 — We WY NW WW—< To74 st 2 -F5 ca Cor |al(2/2) 3) g+H Ss Wr ret22)Ns 78 RES > sei" -voo8 = 2) modulation@ |+ > circuitINT.MIG aBEEP °Tigi ; Tm rhMUTEFig.5 Microphone amplifier