Kenwood TH-77A Service Manual page 14

IA-ff7A/E
14
CIRCUIT DESCRIPTION
7-2 DTMF and DTSS/PAGING
7-2-1 DTMF encode
A DTMF signal is directly produced using the TOR and TOC
signals of IC12. The frequency-response characteristic of the
DTMF signal is compensated for by C58, R48, and VR5. The re-
sultant DTMF signal is level-adjusted, then input to pin 3 of C1.
The DTMF monitor signal is sent to the input pins of the main
and sub high-pass filters. The audio frequency is muted by IC3.
7-2-2 DTMF decode
The AFV and AFU signals are split up and sent to the CTCSS
unit and DTMF decoder by IC2, then sent to C11. C11 outputs
the code corresponding to the DTMF signal from pins 11
through 14. The output code is compared with the DTSS and
PAGING codes by IC10, which determines whether they
match.
7-2-3 DTMF decode timing
The main band and sub-band are activated by one DTMF de-
coder. For a single band, only the main band is checked. For a
dual band, the DTMF decoder is switched into the band carry-
ing a BUSY signal to check the band. The other band may not
be checked when a BUSY signal is carried in the main band and
sub-band at the same time.
5M
ENCODER
ENDP ENCP
MPU IC10
DTMF DECODE
Fig. 11 Key, Encode input circuit
7-3 LED Drive Circuit
The LMP pin of IC12 is made high when the lamp switch is
pressed. The constant-current circuit consisting of D7, Q21,
and R116 is then turned on. As a result, a constant current (40
mA) flows through the LED even if the supply voltage fluctu-
ates.
Fig. 12 Lamp circuit
MPU !C305