Kenwood TK-250 Service Manual page 23

TK-250
CIRCUIT DESCRIPTION
8. DISPLAY CIRCUIT
9. DTMF
The display circuit consists of the CPU, LCD, and periph-
eral circuits. The CPU (IC451) mainly controls the following
functions :
1) Transmitting the ON/OFF serial data signal from the
SCAN switch, DIA switch, TA switch, LO switch and
rotary encoder (channel selector) to the control unit.
2) Receiving the serial data signal from the control unit
and displaying the data on the LCD. The LCD contains
the 13-segment display and BUSY, CALL, SCAN, and
other indicators. Controls the LEDs (TX, BUSY LAMP).
3) Wh en the power is initially turned on, the reset circuit
(IC452) resets both the display CPU (IC451) and the
main CPU (lC221).
9.1 DTMF encode
Once a signal is passed from the DTMF keypad to the
microprocessor. The microprocessor passes this informa-
tion to the DTMF encoder (IC219) for encoding. The en-
coded signal is then passed to IC206
(DI
A convertor) for
DTMF deviation adjustment. This signal provides a TX DTMF
tone and a RX DTMF tone. The TX DTMF tone is passed to
the pre-emphasis circuit (mie. amplifier) and then to the VCO.
The RX DTMF tone is passed to the de-emphasis circuit,
audio power amplifier and then to the speaker.
9.2 DTMF decode
The DTMF input signal from the DET line passes through
the low pass filter (IC205
2/2)
and is then passed to IC218,
DTMF decoder. The decoded information is th en processed
by the microprocessor.
f rom
IC20S (1/2)
'"
N
a:
to D~ EMPHASIS(RX TONE)
R2l61R212
;:;.1. ~
~T ~
IC205
(2/2)
B
24. v I B
21. v I?
to MICAM~ (TX TONE)
IC206
C2H
SOA
IC219 ~
OCO
3
TON OC I
?ISCL
KEY PAO(OPTION)
DJ
m
D J
m
[TI
w
[ IJ
w
w
m
m
w
[i]
[Q]
m
[ Q]
IC221
~
'"
M
a.
POl
Fig.10 DTMF. encode, decode
23