Control Circuit; Signalling Circuit; Memory Circuit; Shift Register - Kenwood TK-790 Service Manual

6. Control Circuit

The control unit consists of microprocessor IC and its
peripheral circuits. It controls the TX-RX unit and transfers
data to and from the control unit. The CPU (IC516) mainly
performs the following :
1) Switching between transmission and reception accord-
ing to the PTT signal input.
2) Reading channel, frequency, and program data from the

memory circuit.

3) Sending frequency data to the PLL.
4) Turning the squelch on and off according to the voltage
from the squelch circuit.
5) Controlling the audio mute circuit according to input de-
code data.
6) Sending encode data (QT, DQT).
7) Sending data to the D/A converter.
6-1. Memory circuit
A flash memory (IC519) with a capacity of 2Mbits con-
tains the transceiver control program for the CPU (IC516)
and data such as transceiver channels and operating fea-
tures.
This program can be easily written from an external de-
vices. Data, such as DTMF memories and operating status,
tuning data are programmed into the EEPROM (IC514).

6-2. Shift register

IC517 and IC523 are an interface IC for I/O port expan-
sion. It is used to expand the CPU (IC516) output ports.
6-3. D/A converter
IC512 is used as a conventional semi-fixed-resistor con-
verter. It sets the following :
1) RX sensitivity
2) Transmission power
3) Modulation level
4) Audio power
5) Frequency
6-4. Power supply circuit
D507 is a protection diode for the excess voltage
1) Turning the transceiver on/off with a power switch. Each
time you press the power switch, the IC501 output is al-
ternated. The output is feed through Q501, Q503 to turn
Q519 on. The power source is always supplied to the
IGN terminal. If 24V DC is supplied to the transceiver by
mistake, Q502 is turned on then Q503 and Q519 are
turned off to protect the transceiver (turned off).
2) Turning the transceiver on with a power switch, then
turning the transceiver off with the Timed Power Off
(TPO) function. POF is controlled by CPU's TPO function.
POF is to turn the transceiver off. The POF logic signal is
feed through Q507 and Q506 then 8 pin of IC501 to turn
the transceiver off.
3) Turning the transceiver on with IGN SENS, then turning
the transceiver off with TPO function. If IGN is turned on,
Q543 and Q542 are turned on then the IC501 detects the
change. IC501 outputs the signal to turn Q541 off, then
Q503 and Q519 are turned on. When POF is detected by
6 pin of IC501, Q541 is turned on then Q503 and Q519
are turned off.
CIRCUIT DESCRIPTION

7. Signalling Circuit

7-1. Encode
• QT, DQT
the program. The data items are output from CPU pin 34.
filter and goes to the summing amplifier (IC510) in the mi-
crophone amplifier.
• DTMF
The encode data is passed to the D/A convertor (IC512) for
DTMF deviation adjustment, and goes to IC505 in the IDC
circuit of the microphone amplifier.
7-2. Decode
• QT, DQT
IC521 and passes through a low-pass filter (IC502) to re-
move audio components. The signal is input to pin 25 of the
CPU. The CPU digitizes this signal, and decodes the signal.
• 2 TONE
IC504 and passes through a high-pass filter (IC518) and a
band-elimination filter (IC524) to remove QT and DQT.
CPU.
• DTMF
IC504 and passes through a high-pass filter (IC518) and
band-elimination filter (IC524) to remove QT and DQT.
DET
TK-790/
IGN
+B
D507 Q543
AVR SW
Q507 Q506 Q544
POF
SW SW SW
Q518 IC501
PS
SW D-FF
Q542
SW
Fig. 10 Power supply circuit
The CPU (IC516) transmits the encode data selected by
The signal from this pin passes though the CR low-pass
The DTMF-encoder (IC511) transmits the encode data.
The demodulated signal from CN502 pin 5 is amplified by
The demodulated signal from CN502 pin 5 is amplified by
This signal is amplified by IC521 and inputs pin 91 of the
The demodulated signal from CN502 pin 5 is amplified by
This signal inputs pin 1 of the DTMF decoder (IC525).
IC521 IC502 IC502
CN502
AMP LPF LPF
IC504 IC518 IC524
5
AMP HPF BEF
Fig. 11 Decode
( B ) /H ( B
Q519
SB
SW
D501 Q503
SW SW
Q501
SW
Q502 Q541
SW SW
IC516
CPU
25
QT·DQT
IC521
91
AMP 2TONE
IC525
1
DTMF
SD
decoder
29
)