Kenwood TK-780 Service Manual Supplement

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

31TK-780/H Display CircuitThe CPU (IC511) controls the shift register (IC508) anddisplay LEDs. When the LG line goes high when the trans-ceiver is busy, Q508 turns on and the green LED on D511lights. In transmit mode, the LR line goes high, Q509 turnson and the red light lights. Backlighting LEDs for the keyoperation unit (D512~D517) and LCD are provided.When the KBLC line goes high, Q512 turns on, thenQ513 turns on, and the key illumination LED lights. A volt-age is applied to the LEDA line to turn on the LCD backlight. Key Matrix CircuitThe TK-780/780H front panel has ten keys. Each of themis connected to a cross point of a matrix of the KEY1 to KEY7ports of the microprocessor. The KEY5 to KEY7 ports arealways high, while the KEY1 to KEY4 ports are always low.The microprocessor monitors the status of the KEY1 toKEY7 ports. If the state of one of the ports changes, themicroprocessor assumes that the key at the matrix pointcorresponding to that port has been pressed. Unusedpoints (KEY1 to KEY7) are also used for foot switch (FSW)input.Q512SWQ508SWQ509SWQ513SWKBLCLGLRIC508ShiftregisterD511GRNREDD512~517LEDAFig. 11 Display circuitMO is summed with the external pin DI line by the sum-ming amplifier (IC3) and the resulting signal goes to the D/Aconverter (IC5). The D/A converter (IC5) adjusts the MOlevel and the balance between the MO and TO levels. Partof a TO signal is summed with an output signal from pin 3(MO) of IC5 and the resulting signal goes to the MD pin ofthe VCO. This signal is applied to a varicap diode in the VCOfor direct FM modulation.IC511CPUKEY7KEY6KEY5KEY4KEY3KEY2KEY1MONI AD SCAN SYSDOWN SYSUPB CVOLUP VOLDOWNFSWQ511Fig. 12 Key matrix circuit EncodeThe QT, DQT, and LTR signals are output from LSDO ofthe CPU (IC511) and go to the D/A converter (IC5) of the TX-RX unit (A/2). The DTMF signal is output from HSDO of theCPU and goes to the audio processor (IC504). An MSK sig-nal is output from the audio processor according to the data(AFDAT) from the CPU. The signal is summed with a MIC/MSK signal by the audio processor (IC504), and the resultingsignal passes through an analog switch (IC506) and goes tothe TX-RX unit (A/2) (MO).X1VCXOIC5D/AA1VCOIC3SUMAMPIC506AnalogSWIC504AudioprocessorIC300PLLMBMDMOTOHTDIMICTX-RX UNIT (A/2)AFDATLSDOHSDOIC511CPU DecodeThe signal (DEO) detected by the TX-RX unit (A/2) passesthrough two low-pass filters of IC501, goes to LSDI of theCPU (IC511) to decode QT, DQT, and LTR. The DTMF signalis decoded by a dedicated IC (IC507) and the resulting signalis sent to the CPU (IC511) as serial data (STD).IC511CPUIC501(2/2)LPFIC501(1/2)LPFLSDIIC507DTMFDECO.STDDEO D/A ConverterThe D/A converter (IC5) is used to adjust TONE and MOmodulation, beep, AF volume, TV voltage, FC reference volt-age, and PC POWER CONTROL voltage level.Adjustment values are sent from the CPU as serial data.The D/A converter has a resolution of 256 and the followingrelationship is valid:D/A output = (Vin – VDAref) / 256 x n + VDArefVin: Analog inputVDAref: D/A reference voltagen: Serial data value from the microprocessor (CPU)CIRCUIT DESCRIPTIONFig. 13 EncodeFig. 14 Decode