Kenwood TK-780 Service Manual

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

27TK-780 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.AUDIOPROCE.SUMAMPD/ACONV.IC504 IC504 IC5AF PAIC10 SPDTMF BPOAFODEOCONTORL UNIT Squelch CircuitThe detection output from the FM IF IC (IC11) is ampli-fied by IC2 and the signal (DEO) is sent to the control unit.The signal passes through a high-pass filter and a noise am-plifier (Q503) in the control unit to detect noise. A voltage isapplied to the CPU (IC511). The CPU controls squelch ac-cording to the voltage (ASQ) level. The signal from the RSSIpin of IC11 is monitored. The electric field strength of thereceive signal can be known before the ASQ voltage is inputto the CPU, and the scan stop speed is improved.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 (A-1). They are housed in a solidshielded case and connected to the TX-RX unit 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 TK-780 changes the VCXOreference voltage (MB) by software and fine-adjusts the ref-erence oscillator frequency to use the set with a 2.5KHz fre-quency step. The output signal from the phase comparatorpasses through a charge pump and an external active LPF(Q301, Q302) in the PLL IC to generate a DC VCO controlvoltage CV. Serial data (DT, CK, EP) are output from theCPU (IC511) and shift register (IC8) in the control unit to con-trol the PLL IC. The PLL lock status is always monitored bythe CPU.FIg. 4 AF signal systemIC2AMPIC503AMPQ503NOISE AMP D509IC11 IC511DEORSSIHPF DETCPUIFSYSTEMCONTROL UNITFig. 5 Squelch circuitIC504 IC3 Q103IC711MICAF AMPTA75S01FMIC KEYINPUTAF AMP,IDC, LPFTC35453FIC511CPU30620M8-394GPSUM AMPTA75W558FUX1VCXO16.8MHzVCO2SK508NV(K53)IC300PLLSA7025DKQ106RF AMP2SC4215(Y)Q300BUFFER2SC4215(Y)Q203RF AMP2SC3357Q204ANTRF AMP2SC2954IC400POWER AMPM67741H-32Q7BUFFER2SC4215(Y)Fig. 6 Transmitter systemCIRCUIT DESCRIPTION