Panasonic KX-TCD735AXM Service Manual page 23

6.3.1. Path from Telephone Line to Speaker
Audio from the telephone line passes via the normal signal path of J1 tel-line connector (TP26
and TP40), D8 bridge rectifier, T8 hookswitch (TP50), and R41, to BBIC IC6 pin 133. In the BBIC it
is converted from an analogue audio signal into digital data in the BBIC's internal codec.
Additionally, the speakerphone switching and echo suppression functions are contained within
the BBIC. An eight level speaker volume control is provided, and the functionality for this is also
contained within the BBIC. The analog data is sent from the BBIC IC6 pin 113 to the speaker
amplifier IC14 pin 4, via filter circuit (T14). The audio signal is amplified in IC14, and the push-
pull audio output is sent from IC14 pins 5 and 8 to the speaker (TP9) and (TP10).
6.3.2. Path from Microphone to Telephone Line
DC bias for the microphone is provided from the BBIC IC6 pin 118 (positive bias) and GND
(negative bias). The bias is fed to the microphone via R15 and R23 (positive bias) and R12
(negative bias). The balanced audio signal from the microphone is fed to the BBIC (IC6), via R77,
C71 and R2 (Positive), R78, C72 and R71 (negative).
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