Panasonic KX-FM131AL, Service Manual

Page: 181 / 232

6.8.5. LINE AMPLIFIER AND SIDE TONE CIRCUIT
1. Circuit Operation
The reception signal output from the line transformer T101 is input to pin (6) of IC101 via C126 and R131, and then the signal
is amplified at pin (7) of IC101 and sent to the reception system at 12dB.
The transmission signal goes through C131 and R137 and enters IC101-pin (2), where the signal is amplified to about 18dB.
Then, it is output from pin (1) of IC101 and transmitted to T101 via C133, R219, R125 and C213. If the side tone circuit is not
applied, the transmission signal will return to the reception amplifier via C126, R131, R222 and C229. When the side tone circuit
is active, the signal output from IC101 pin (1) passes through C133, R221, R216, R127, C123, R223 and C219 and goes into
the amplifier IC101 pin (5). This circuit is used to cancel the transmission return signal.
6.8.6. AUTO DISCONNECT CIRCUIT
1. Function
This circuit is used to detect that the telephone connected in parallel to the same line is OFF-hook while the unit picks up the
line. If this detection circuit is activated when TAM is being recorded, the recording stops and the circuit is automatically
released.
2. Circuit Operation
If the line is picked up, C105 is charged by following the path shown below.
D6 (+)
→
R103
→
D203
→
C105
When the electric potential difference between the base and emitter of Q206 becomes less than about 0.3V, Q206 and Q102
and PC102 turn off, then the IC501 pin 153 (Digital Board) becomes a high level.
In this condition, if a telephone connected in parallel goes into OFF-hook status, the base of Q206 becomes low.
On the other hand, the emitter of Q206 goes down because the capacitor (C105) is charged. Q206 turns on when the electric
potential difference between the base and emitter of Q206 becomes more than 0.6 V while being charged.
When Q206 turns on, Q102 and PC102 also turn on, then the IC501 pin 153 (Digital Board) becomes a low level.
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KX-FP121AL / KX-FM131AL