Panasonic KX-FT21RS - Manual (Page 131)

6.8.5. LINE AMPLIFIER AND SIDE TONE CIRCUITS
1. Circuit Operation
The reception signal received as output from line transformer T101 is given as input to R104, C107, R112, and IC101 (8).
Then it is input to the reception system at an amplifier gain of 5.9 dB from pin (7).
The transmission signal is input from CN202 pin (1), and output to the TEL line through C106, R103, R102 and T101. Without
a side tone circuit, the transmission signal would return to the reception amplifier via C106. Here, the signal output from CN202
pin (1) passes through R105, R106, R107, C111 and R110, and enters the amplifier IC101 pin (5). This is used to cancel the
return portion of the transmission signal. This is the side tone circuit.
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KX-FT21RS