Kenwood FM Transceiver TK-481 Service Manual

Also see for FM Transceiver TK-481: User manual Instruction manual Specifications Instruction manual Service manual

16TK-480/481 CIRCUIT DESCRIPTIONFig. 6 Microphone amplifierFig. 7 Drive and final amplifier and APC circuitHPF ALC COMP LPF HPF15 16 18SW19DTMF9 8IC1212LIMITMICSWMICEXT.MICQ300 D116I5 O5D/AIC4SUMAMPIC3 (2/2)I2 O2D/AIC4AFMUTE5RQ21I1 O1D/AIC4BUFFAMPIC1IC14VCOX2VCXOQ301MSWPRE EMP IDCLSDMUTEQ7Pre-DRIVEAMPDRIVEAMPQ12Q11 IC30RFPOWER AMPFromT/R SW(D5)LPF ANTSWD7ANT+BR127R128R129REFVOL(IC4)IC21(1/2) IC21(2/2)VDD VGGThe signal from microphone passes through the limittercircuit in D11, 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 IC12. When encoding DTMF, mute switch(Q7) is turned OFF for muting the microphone input signal.The signal passes through the D/A converter (IC4) for themaximum deviation adjustment, and enters the summingamplifier consisting of IC3 (2/2), and is mixed with the lowspeed data from the CPU (IC15).The output signal from the summing amplifier passesthrough the D/A converter (IC4) again for the TA maximumdeviation adjustment,and the AF switch (Q21 is off in TX),and goes to the VCO modulation input.The other output signal from the summing amplifierpasses through the D/A converter (IC4) again for the BALadjustment,and the buffer amplifier (IC1 (2/2)), and goes tothe VCXO modulation input.4-2. Drive and Final amplifierThe signal from the T/R switch (D5 is on) is amplified bythe pre-drive (Q11) and drive amplifier (Q12) to 50mW.The output of the drive amplifier is amplified by the RFpower amplifier (IC30) to 2.5W (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 harmonic filter (LPF) and antenna switch(D7 is on) and applied to the antenna terminal.4-3. APC circuitThe APC circuit always monitors the current flowingthrough the RF power amplifier (IC30) and keeps a constantcurrent. The voltage drop at R127, R128 and R129 is causedby the current flowing through the RF power amplifier andthis voltage is applied to the differential amplifier (IC21 1/2).IC21 (2/2) compares the output voltage of IC21 (1/2) withthe reference voltage from IC4, and the output of IC21 (2/2)controls the VGG of the RF power amplifier to make theboth voltages to same voltage.The change of power high/low is carried out by thechange of the reference voltage. Q14,15 and 18 are turnedon in transmit and the APC circuit is active.