Kenwood TK-885 Service Manual

Also see for TK-885: Instruction manual Service manual

Contents

13TK-885Frequency ConfigurationThe TX-RX unit incorporates a VCO, based on a fractionalN type PLL synthesizer system, that allows a channel stepof 6.25 and 12.5kHz to be selected. The incoming signalfrom the antenna is mixed with a first local oscillation fre-quency to produce a first intermediate frequency of44.85MHz.The signal is then mixed with a second local oscillationfrequency of 44.395MHz to produce a second intermediatefrequency of 455kHz. This is called a double-conversionsystem. The TX-RX unit contains a wide/narrow CFs. Thetransmit signal is produced by the PLL circuit for directionoscillation and division. The signal output from the VCO isamplified by a straight amplifier and transmitted.Fig. 1 Frequency configurationReceiver System OutlineThe incoming signal from the antenna passes through alow-pass filter and a transmission/reception selection diodeswitch (D209) and goes to the front end of the receiver. Thefront-end filter is a variable BPF consisting of two two-polehelical resonators and eight varicap diodes (D203, D204,D205, D206, D212, D213, D214, D215) to eliminate un-wanted out-of-band signal components. The low-noise am-plifier (LNA) (Q201) uses a bipolar transistor to achieve wide-band and low-distortion amplification.Fig. 2 Receiver systemCIRCUIT DESCRIPTIONThe signal passes through a BPF and is down-convertedwith the first local signal by IC200, then converted to thefirst IF signal of 44.85MHz. The first local signal passesthrough an LPF and an attenuator to eliminate unwantedharmonics components and implement the optimum inputlevel to the mixer, then enters IC200. A DBM is used as amixer to achieve a high potential.The signal output from the mixer is amplified by an inter-mediate frequency amplifier and input to two MCFs (XF1).The signal is amplified by another intermediate amplifier andgoes to the FM IF IC (IC11). The first intermediate fre-quency signal is mixed with the second local signal of44.395MHz to produce the second IF signal of 455kHz.The unwanted near-by signal components are then elimi-nated by a ceramic filter (CF2) and the resulting signal goesback to the FM IF IC. The signal is quadrature-detected inthe IC to produce an audio signal, which is amplified by aDET amplifier (IC2) and output to the control unit. AF Signal SystemThe detection signal (DEO) from the TX-RX unit goes tothe audio processor (IC504) of the control unit. The signalpasses through a filter in the audio processor to adjust thegain, and is output to IC502. IC502 sums the AF signal andthe DTMF signal and returns the resulting signal to the TX-RX unit. The signal (AFO) sent to the TX-RX unit is input tothe D/A converter (IC5). The AFO output level is adjusted bythe D/A converter. The signal output from the D/A converteris added with the BEEP signal (BPO) and the resulting signalis input to the audio power amplifier (IC10). The AF signalfrom IC10 switches between the internal speaker andspeaker jack (J1) output.ANTSWRXAMP1stMIXTXAMPPOWERAMPCF1 455kHzCF2 455kHzMCF44.85MHz+ – MIX/IF/DET395.15~425.15MHzPLL/VCOVCXO16.8MHz440~470MHz440~470MHz44.395MHzAFOMDMBLPFANTL203BPFL207BPFQ201AMPQ15AMPIC2AMPIC200MIXXF1BPFD211D209,210ANTSW + –Q203AMPCF1/CF2IC11MIX,IF,DETX22ndlocal OSC1st localOSC (HT)DEOFIg. 3 AF signal systemAUDIOPROCE.SUMAMPD/ACONV.IC504 IC502 IC5AF PAIC10 SPDTMF BPOAFODEOCONTORL UNIT