KENWOOD TK785
Service Manual

TK-785
CIRCUIT DESCRIPTION
s Display Circuit
The CPU (IC511) controls the shift register (IC508) and display LEDs. When the LG line goes high when the transceiver is busy, Q508 turns on and the green LED on D511 lights. In transmit mode, the LR line goes high, Q509 turns on and the red light lights. Backlighting LEDs for the key operation unit (D512~D517) and LCD are provided. When the KBLC line goes high, Q512 turns on, then Q513 turns on, and the key illumination LED lights. A voltage is applied to the LEDA line to turn on the LCD backlight.
KBLC Q512 SW IC508 LG Shift register LR Q508 SW Q509 SW Q513 SW LEDA

s Encode
The QT, DQT signals are output from LSDO of the CPU (IC511) and go to the D/A converter (IC5) of the TX-RX unit. The DTMF and single tone signals are output from HSDO of the CPU and goes to the audio processor (IC504). An MSK signal 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 resulting signal passes through an analog switch (IC506) and goes to the TX-RX unit (MO). MO is summed with the external pin DI line by the summing amplifier (IC3) and the resulting signal goes to the D/A converter (IC5). The D/A converter (IC5) adjusts the MO level and the balance between the MO and TO levels. Part of a TO signal is summed with an output signal from pin 3 (MO) of IC5 and the resulting signal goes to the MD pin of the VCO. This signal is applied to a varicap diode in the VCO for direct FM modulation.
TX-RX UNIT

D511 D512~517 GRN RED

Fig. 10 Display circuit

LSDO MIC IC511 CPU AFDAT HSDO IC504 Audio processor IC506 Analog SW

TO DI IC3 SUM AMP

MB

X1 VCXO

IC300 PLL

MO

IC5 D/A

MD

A1 VCO

HT

s Key Matrix Circuit
The TK-785 front panel has ten keys. Each of them is connected to a cross point of a matrix of the KEY1 to KEY7 ports of the microprocessor. The KEY5 to KEY7 ports are always high, while the KEY1 to KEY4 ports are always low. The microprocessor monitors the status of the KEY1 to KEY7 ports. If the state of one of the ports changes, the microprocessor assumes that the key at the matrix point corresponding to that port has been pressed. Unused points (KEY1 to KEY7) are also used for foot switch (FSW) input.

Fig. 12 Encode s Decode
The signal (DEO) detected by the TX-RX unit passes through two low-pass filters of IC501, goes to LSDI of the CPU (IC511) to decode QT, DQT. The MSK signal passes through IC504. IC504, includes band-pass filter and MSK decoder, converts the input signal into a square waveform (0 and 5V). This square waveform is then fed to AFRDT line of CPU (IC511).
IC501(2/2) IC501(1/2) LPF LPF LSDI IC511 CPU

FSW VOL UP (CLEAR) D VOL DOWN A CALL B SYS DOWN Q511 C SYS UP

DEO
IC507 DTMF DECO. IC504 AUDIO PROC.

KEY7 KEY6 KEY5 IC511 CPU KEY4 KEY3 KEY2 KEY1

AFRDT

STD

Fig. 11 Key matrix circuit

Fig. 13

Decode

18