Kenwood TK-780 Service Manual page 29

Display Circuit
The CPU (IC511) controls the shift register (IC508) and
display LEDs. When the LG line goes high when the trans-
ceiver 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 volt-
age is applied to the LEDA line to turn on the LCD backlight.
KBLC
Q512
SW
IC508
LG
Q508
Shift
SW
register
LR
Q509
SW
Fig. 11 Display circuit
Key Matrix Circuit
The TK-780 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.
VOL
UP
KEY7
MONI
KEY6
KEY5
D
IC511 KEY4
CPU
KEY3
KEY2
KEY1
Fig. 12 Key matrix circuit
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
(A/2). The DTMF and single/5-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 (A/2) (MO).
CIRCUIT DESCRIPTION
LEDA
Q513
SW
D512~517
D511
GRN
RED
FSW
VOL
Q511
DOWN
A B C
CALL CH CH
DOWN UP
MO is summed with the external pin DI line by the sum-
ming 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.
LSDO
MIC
IC511
CPU
AFDAT
IC504 IC506
HSDO Audio Analog
processor SW
Fig. 13 Encode
Decode
The signal (DEO) detected by the TX-RX unit (A/2) passes
through two low-pass filters of IC501, goes to LSDI of the
CPU (IC511) to decode QT, DQT. The DTMF signal is de-
coded by a dedicated IC (IC507) and the resulting signal is
sent to the CPU (IC511) as serial data (STD).
The 5-tone signal passes through high-pass filter, IC504
and then through low-pass filter, IC710. After passing
through these filters, only the audio signal between 300Hz
and 3kHz is extracted and input to comparator, IC502. The
comparator converts the input signal into a square wave-
form (0 and 5V). This square waveform is then fed to the
HSDI line of CPU (IC511).
IC501(2/2)
LPF
LSDI
IC511
CPU
HSDI STD
DTMF
DECO.
IC502 IC710
Comparator LPF
Fig. 14 Decode
D/A Converter
The D/A converter (IC5) is used to adjust TONE and MO
modulation, 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 following
relationship is valid:
D/A output = (Vin – VDAref) / 256 x n + VDAref
Vin: Analog input
VDAref: D/A reference voltage
n: Serial data value from the microprocessor (CPU)
TK-780
TX-RX UNIT (A/2)
TO MB
X1 IC300
VCXO PLL
DI
MD
IC5 A1
IC3
D/A
VCO
MO
SUM
AMP
IC501(1/2)
LPF
DEO
IC507
IC504
HPF
HT
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