Kenwood TKR-820 Service Manual

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TKR-820 TKR-820CIRCUIT DESCRIPTIONTransmitter CircuitThe signal generated at the transmitter frequency syn-thesizer is amplified by RF amplifier transistors Q1 (2SC3357) and Q2 (2SC2954) and amplifier module (C1(M57729) to a level of 25 watts in the power amplifierunit. The signal is then routed to the antenna connectorafter going through a harmonics filter.The transmitter output is detected by D3, 4 (188101)and is converted to a DC form. The DC signal thus detectedis level adjusted by APC control (VR1)} and is applied tothe base of O6 (2SC3326A). The base current of 03 (2SB946Q) is varied according to the difference from the com-parison voltage at Q5, 6 (2SC3326A) so that the collectorvoltages of Q2 (2SC2954) and the IC1 first stage are con-trolled to maintain the transmitter output level constant.In the event an abnormal temperature rise occurs, thetemperature is sensed by a thermister (TH1) and reducesthe output power to a safe level.The harmonics filter is of a fifth order butterworth typelowpass filter having a minimum attenuation of more than65dB at the second harmonics frequency with a passbandinsertion loss of less than 0.5dB. With a characteristics ofthe transmitter final power amplifier module, which hasmore than 30dB of attenuation for the frequency at thesecond harmonics or higher, total attenuation of harmonicsis more than 70dB.TX-AXUNITFINALUNIT|rl>|VANT.TxvcOiaRFEAMPPRIDRIVE FINALHYBRIDLPFaia21c118.9,10wl.\A”vcoATT"4¢9csTa a3REFAMPO5,64+APC78CONT:=pCAMPQsTHI+BFig. 1 Transmitter block diagram19 20CIRCUIT DESCRIPTIONReceiver CircuitAn incoming RF signal from the antenna is fed into abandpass filter (L7). Then eliminated RF signal passa pro-tecter D3 (1SS226).The signal is then amplified by an RF amplifier (Q3,2SC3358K) and filtered again by bandpass filters (LQ andL10), After amplification and filtering, the signal is appli-ed to a double balanced modulator (DMB, D4, ND487C1-3R) for mixing with the first local signal generated at thecommon frequency synthesizer.The heterodyning action of the first mixer produces a21.4MHz first intermediate frequency (first 1F), which isapplied to a Gpole monolithic crystal filter (MCF, XF1)after being amplified by a buffer amplifier (04, 2SK125and Q5, 2SK125 connected in paralle!). The signal out ofthe MCF is further amplified by a first IF amplifier (Q6,2SK302GR)} and sent to the IF unit (Z8).The signal applied to Z8 is mixed with a 20.945MHzsignal at IC1 in 28, which produces a 455kHz second IFsignal. The signal obtained at the second mixer is filteredby a 455kHz ceramic filter (CF1) and amplified by limitingamplifier stages in 1C1. The recovered audio signal from theincoming signal is also obtained at IC1 by a quadrature typeFM detector. This recovered audio signal is then sent to theaudio amplifier circuit and to the noise actuated squelchcircuit.In the receiver audio amplifier section, the recoveredaudio signal from Z8 is first applied to a bandpass filter/voltage controlled amplifier (BPF/VCA, Z9) unit. At thisBPF/VCA unit, the signal is amplified and sent to pin Q ofCN6 as the DET signal. The signal is returned to Z9 by wayof the signaling unit. 1C1 (NJM4558M) forms a lowpassfilter and a highpass filter, and IC2 forms a bandpass filterand lowpass filter in Z9. The frequency components below300Hz and above 3000Hz are attenuated in the above filterCircuits.The filtered audio signal is then applied to an elec-tronic volume control (IC3, M522FP), where the audiosigna! level is controlled by a DC voltage sent from thefront panel volume control. The signal is then de-empha-sized and sent to the audio power amplifier circuit (IC1,pPC1242H) after going through squelch switches (Q9 and010, 2SC3326A).The alert signal is also applied to 1C1, when a specificsignaling board, which requires an audible alert through thespeaker, is installed. The signal, which is amplified by 1C1,drives either the internal speaker or the optional externalspeaker and this selection is done through the accessoryconnector located on the final unit.ANT.TX-RX UNIT —BPF RF AMP BPF BPF Ist MIXL7 a3 Lg L10 Da Ist _Local(#:21,4MHz_)+ pit ATT trom CommonPLL circuitD3 - (_21.4MHz >)2ZB: IF IF AMP MCF ist LF AMPaa) so XF4 04,5}] emsystemic | | Bi ,| ies) Nl| a ee LPFL13x1 ES (455KHz_)20.945MHz7 CERAMICFILTERCF1DISPLAY UNIT (A/4) Z9 : BPF/VCA AF PA“| —_— oo OT Se TOUT}| AF DE- AF |yyave EMP Jf \ VCA F— amp [74 —| -Lj eA Ne lS >ra ra) 9w we 3O} CNG <) cN6 CNG | CN4~ ALERTI F g is 2 ——2. S o cN1' AF VOLJ101 | | | “ |Eee —EEDP101 SIGNALING UNIT DISPLAY UNIT (A/4)sP I [accFig. 2 Receiver block diagram