Kenwood TK-290 Service Manual page 23

TK-290
7-6. Low battery warning
The battery voltage is monitored by the microprocessor
(IC406). 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. 5.5V),
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.
8. Signalling Circuit
8-1. Encode
The CPU (IC406) transmits the encode data selected by
the program.
• Low-speed data (QT, DQT)
Low-speed data is output from pin 34 of the CPU. The
signal passes through a low-pass CR filter, and goes to the
summing amplifier (IC605 1/2). The signal is mixed with the
audio signal and goes to the VCO (IC10) and VCXO (X1)
modulation input after passing through the D/A converter
(IC603) 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 IC413,
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 (IC603) for DTMF deviation adjustment, and then ap-
plied to the audio processor (IC607).
The signal is mixed with the audio signal and goes to the
VCO and VCXO. The RX DTMF tone is passed the D/A con-
vertor (IC603) for audio control, summing amplifier (IC604 2/
2), audio power amplifier and then to the speaker.
• MSK
The MSK signal (1200 bps) is output from pin 6 of IC607.
The signal passes through the D/A converter (IC603) for the
MSK deviation adjustment, and is routed to the VCO.
When encoding MSK, the microphone input signal is
muted.
22
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.
MSK
DIN, DCK,
IC607
IC406
MSKE
CPU
IC413
35
HSD
LPF
OUT
LSD
OUT
34
R422 R421
SUM
IC605
(1/2)
Fig. 15 Encode
8-2. Decode
• Low-speed data (QT, DQT)
The demodulated signal from the FM IF IC (IC300) is am-
plified by IC601 (2/2) and passes through a low-pass filter
(IC602) to remove audio components. The signal is input to
pin 28 of the CPU.
The CPU digitizes this signal, performs processing such
as DC restoration, and decodes the signal.
• High-speed data (DTMF)
The DTMF input signal from the FM IF IC (IC300) is ampli-
fied by IC601 (2/2) and goes to IC600, the DTMF decoder.
The decoded information is then processed by the CPU.
During transmission and standby, the DTMF IC is set to the
power down mode when the PD terminal is High. When the
line is busy, the PD terminal becomes Low, the power down
mode is canceled and decoding si carried out.
• MSK
The MSK input signal from the IF IC is amplified by IC601
(2/2) and gose to pin 5 of IC607. The signal is demoduated
by MSK demodulator in IC607. The demodulated data goes
to the CPU for processing.
IC607
AF IC
IC601
IC602
(2/2)
LPF
AMP
IC600
DTMF
DECODE
5
IC607
OSCI
XOUT
PD
3
Fig. 16 Decode
RX AUDIO
I2
O2
AF
SUM
AMP
IC604 (2/2)
IC603
D/A (ADJ)
IC607
I1
O1
SUM
I4
O4 MIC IN
IC604 (1/2) X1
I3
O3 TO
BUFF
VCXO
AMP
IC10
MOD
VCO
MSK
DIN, DCK,
MSKE
28
LSD IN
IC406
CPU
DCK,SD,STD
7
IC405