Kenwood TK-780 Service Manual

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

27TK-780 AF Signal SystemThe detection signal (DEO) from the TX-RX unit (A/2)goes to the audio processor (IC504) of the TX-RX unit (B/2).The signal passes through a filter in the audio processor toadjust the gain, and is output to IC502. IC502 sums the AFsignal and the DTMF signal and returns the resulting signalto the TX-RX unit (A/2). The signal (AFO) sent to the TX-RXunit (A/2) is input to the D/A converter (IC5). The AFO out-put level is adjusted by the D/A converter. The signal outputfrom the D/A converter is added with the BEEP signal (BPO)and the resulting signal is input to the audio power amplifier(IC10). The AF signal from IC10 switches between the inter-nal speaker and speaker jack (J1) output.AUDIOPROCE.SUMAMPD/ACONV.IC504 IC502 IC5AF PAIC10 SPDTMF BPOAFODEOTX-RX UNIT (B/2) : Control section Squelch CircuitThe detection output from the FM IF IC (IC11) is ampli-fied by IC2 and the signal (DEO) is sent to the TX-RX unit (B/2). The signal passes through a high-pass filter and a noiseamplifier (Q503) in the TX-RX unit (B/2) to detect noise. Avoltage is applied to the CPU (IC511). The CPU controlssquelch according to the voltage (ASQ) level. The signalfrom the RSSI pin of IC11 is monitored. The electric fieldstrength of the receive signal can be known before the ASQvoltage is input to the CPU, and the scan stop speed is im-proved.Transmitter System OutlineThe transmitter circuit produces and amplifies the de-sired frequency directly. It FM-modulates the carrier signalby means of a varicap diode. VCO/PLL CircuitThe TK-780 has a VCO for the transmitter and a VCO forthe receiver in a sub-unit (A1). They are housed in a solidshielded case and connected to the TX-RX unit (A/2) throughCN101. One of the VCOs is selected with an ST signal. Afiltered low-noise power supply is used for the VCOs andvaricap diodes.The VCO for the transmitter is described below. It is de-signed so that Q103 turns on with a prescribed frequencywhen a reverse bias is applied to D102 and D107 by usingthe control voltage (CV) through CN101. The control voltageis changed by turning the trimmer capacitor (IC102). Theoutput from Q103 is applied to the buffer amplifier (Q106) togenerate a VCO output signal. This signal is used as a driveinput signal or a local signal of the first mixer. Since a signaloutput from Q160 is input to the PLL IC, it passes throughCN101 and buffer amplifier (Q300) and goes to the PLL IC(IC300). The modulation signal from CN101 is applied toD109 and passes through C125 and C126 to modulate thecarrier.The PLL IC uses a fractional N type synthesizer to im-prove the C/N ratio and lock-up speed. The VCO output sig-nal input to the pin 5 of the PLL IC is divided to produce acomparison frequency according to a channel step. This sig-nal is compared with the reference frequency which is out-put from the VCXO (X1). VCXO provides 16.8MHz, 2.5ppm(–30 to +60°C) and guarantees stable performance whenthe temperature changes. The output signal from the phasecomparator passes through a charge pump and an externalactive LPF (Q301, Q302) in the PLL IC to generate a DC VCOcontrol voltage CV. Serial data (DT, CK, EP) are output fromthe CPU (IC511) and shift register (IC8) in the TX-RX unit (B/2) to control the PLL IC. The PLL lock status is always moni-tored by the CPU.FIg. 4 AF signal systemIC2AMPIC503AMPQ503NOISE AMP D509IC11 IC511DEORSSIHPF DETCPUIFSYSTEMTX-RX UNIT (B/2) : Control sectionFig. 5 Squelch circuitIC504 IC3 Q103IC711MICAF AMPTA75S01FMIC KEYINPUTAF AMP,IDC, LPFTC35453FIC511CPU30620M8A-2K9GPSUM AMPTA75W558FUX1VCXO16.8MHzVCO2SK508NV(K53)IC300PLLSA7025DKQ106RF AMP2SC4215(Y)Q300BUFFER2SC4215(Y)Q203RF AMP2SC3357Q204ANTRF AMP2SC2954IC400POWER AMPM67741H-32Q7BUFFER2SC4215(Y)Fig. 6 Transmitter systemCIRCUIT DESCRIPTION