Kenwood TH-77A Service Manual page 7

IA-f fA/E
CIRCUIT DESCRIPTION
2-4 Audio Circuit
® Outline
The AFV and AFU signals that are deemphasized in the trans-
mitting-receiving unit are input to control unit IC3 (TC4066BF)
and separated into the main signal and subsignal. The separat-
ed signals are passed through high-pass filters and input to
IC4, which performs speaker selection and mix/separate se-
lection. The resulting signals are amplified by IC5 and IC6
(NJM386BM)}, then output.
2-4-1 CTCSS and DTSS selection
Input of the AFV and AFU signals to the CTCSS unit and DTMF
decoder is switched by the CBC and DTB signals. The CBC and
DTB signals are unstable when the CTCSS, DTSS, and paging
functions are off. Figure 3 shows the port status.
2-4-2 Received audio signal selection
The audio signals for the VHF and UHF bands are separated in-
to the main signal and subsignal by IC3. IC3 opens all switches
for muting when it outputs a BEEP pulse. IC4 performs speaker
selection and mix/separate selection. Figure 4 shows the port
status.
AFV
(VHF AF)O
AFU
(UHF AF} O
ane
a
Fig. 4 RX audio switching circuit
Sub VOL
Main VOL
CBC DTB
(CTCSS SW) (DTSS SW)
CTCSS Unit
AFV Decode input
(VHF AFIO
To DTMF Decode IC
AFU
(UHF AFIO
DTB
Fig. 3 CTCSS, DTSS switching circuit
AF CONTROL
Ic 4
© EXT. SP
* When the normal separate mode is set, the
subband is heard on the internal speaker and
the main band, on the external speaker.