Kenwood FM Transceiver TK-481 Service Manual page 19

TK-480/481
6-2. Low battery warning
The battery voltage is monitored by the microprocessor
(IC15). When the battery voltage falls below the voltage set
by the Low Battery Warning adjustment, the red LED
flashes to notify the operator that it is time to replace the
battery. If the battery voltage falls even more (approx. 5.8V),
a beep sounds and transmission is stopped.
Low battery warning
The red LED flashes during
transmission
The red LED flashes and
continuous beep sounds
while PTT pressed
6-3. Key input
If the clock is supplied to CLK terminal when the RES
terminal (CPU pin 53) of the decade counter (IC301) is set to
Low, Q0 to Q7 become High sequentially. Normally, KI1 and
KI2 are Low (pulled down). When any key is pressed, KI1 or
KI2 become High. The CPU detects which key is pressed,
according to the voltage of KI1 and KI2 and clock timing.
16 keys
Fig. 10 Key input
RESET
CLOCK
CLOCK
INHIBIT
Q0
Q1
Q2
Q3
Q4
Q5
Q6
Q7
Q8
Q9
CARRY
OUT
Fig. 11 Decade counter timing chart
18
CIRCUIT DESCRIPTION
Battery condition
The battery voltage is low but
the transceiver is still usable.
The battery voltage is low and
the transceiver is not usable to
make calls.
CK
KI1
IC15
CPU KRST
KI2
IC301
Q5 Vdd
Q1 RES
Q0 CLK
Q2 CL
Q6 CA
Q7 Q9
Q3 Q4
Vss Q8
7. Signalling Circuit
7-1. Encode
• Low-speed data (QT,DQT,LTR)
Low-speed data is output from pin 36 of the CPU. The
signal passes through a low-pass CR filter, and goes to the
summing amplifier (IC3 1/2). The signal is mixed with the
audio signal and goes to the VCO (IC14) and VCXO (X2)
modulation input after passing through the D/A converter
(IC4) for BAL adjustment.
• High-speed data (DTMF)
High-speed data is output from pin 35 of the CPU. The
signal passes through a low-pass filter consisting of IC23,
and provides a TX DTMF tone and a RX DTMF tone including
a beep tone. The TX DTMF tone is passed to the D/A con-
vertor (IC4) for DTMF deviation adjustment, and then ap-
plied to the audio processor (IC12).
The signal is mixed with the audio signal and goes to the
VCO and VCXO. The RX DTMF tone is passed a summing
amplifier (IC3 2/2), the D/A convertor (IC4) for audio control,
audio power amplifier and then to the speaker.
• MSK
MSK signal is output from pin 6 of IC12. The signal
passes through the D/A converter (IC4) for the MSK devia-
tion adjustment, and is routed to the VCO. When encoding
MSK, the microphone input signal is muted.
RX audio
IC15
SUM
CPU
IC3 (2/2)
IC23
35
HSD
LPF
OUT
LSD
O5
OUT
36
R111 R107
SUM
IC3 (1/2)
Fig. 12 Encode
I6 O6
AF AMP
IC12
I3
O3
SUM
IC4
D/A (ADJ)
MIC IN
I5
Q21 IC14
I2
O2 MD
AF
VCO
MUTE
IC1 X2
I1
O1 MB
BUFF
VCXO
AMP