Kenwood TK-780 Service Manual

Also see for TK-780 series: Programming instructions Instruction manual Service manual supplement Instruction manual Service manual

26TK-780Frequency ConfigurationThe TX-RX unit (A/2) incorporates a VCO, based on a frac-tional N type PLL synthesizer system, that allows a channelstep of 5, 6, and 25kHz 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 (A/2) contains a wide/narrow MCFand CFs. The transmit signal is produced by the PLL circuitfor direction oscillation and division. The signal output fromthe VCO is amplified by a straight amplifier and transmitted.ANTSW RXAMP1stMIXTXAMPPOWERAMPCF 455kHz(Wide)CF 455kHz(Narrow)MCF44.85MHz– + MIX/IF/DET190.85~218.85MHzPLL/VCOVCXO16.8MHz146~174MHz146~174MHz44.395MHzAFOMDMBFig. 1 Frequency configurationReceiver System OutlineThe incoming signal from the antenna passes through alow-pass filter and a transmission/reception selection diodeswitch (D211) and goes to the front end of the receiver. Thefront-end filter is a variable BPF consisting of three coils andthree varicap diodes (D206, D207, D208) to eliminate un-wanted out-of-band signal components. The low-noise am-plifier (LNA) (Q202) uses a bipolar transistor to achieve wide-band and low-distortion amplification.The signal passes through the BPF and is down-con-verted with the first local signal by IC202 to produce the firstIF signal of 44.85 MHz. The first local signal passes throughan LPF and an attenuator to eliminate unwanted harmonicscomponents and implement the optimum input level to themixer, then enters IC202. A DBM is used as a mixer toachieve a high potential.The signal output from the mixer passes through twoMCFs (XF1). The signal is amplified by an intermediate fre-quency amplifier and input to the FM IF IC (IC11).The first intermediate frequency signal is mixed with thesecond local signal of 44.395MHz to produce the second IFsignal of 455kHz.The unwanted near-by signal components are then elimi-nated by a wide ceramic filter (CF1) or a narrow ceramic fil-ter (CF2) and the resulting signal goes back to the FM IF IC.The signal is quadrature-detected in the IC to produce anaudio signal, which is amplified by a DET amplifier (IC2) andoutput to the TX-RX unit (B/2). Wide/Narrow Changeover CircuitThe W/N port (pin 11) of the shift register (IC7) is used toswitch between ceramic filters. When the W/N port is high,Q24 turns on and the ceramic filter SW diode (D22, D23)CF1 turns on to receive a Wide signal. At the same time,Q16 turns on and one of the filters is selected so that thewide and narrow audio output levels are equal.When the W/N port is low, Q23 turns on and the ceramicfilter SW diode (D22, D23) CF2 turns on to receive a Narrowsignal.LPFANTHPF L202~204BPFQ202AMPQ15AMPIC2AMPIC202MIXXF1BPFD211,212ANTSW – +CF1/CF2IC11MIX,IF,DETX22ndlocal OSC1st localOSC (HT)DEOFig. 2 Receiver systemCF1(Wide)CF2(Narrow)IFIMXOIC11IF systemAFODETOUTC107C100C108R114R123R133R126 R125R130R131++Q16DMQ32 R121W/NIC7 11pinWide : HNarrow : LC118D22 D23Q235CQ24Wide : LNarrow : HFig. 3 Wide/Narrow changeover circuitCIRCUIT DESCRIPTION