Panasonic KX-TG6700B Service Manual page 18

KX-TG6700B/KX-TG6702B/KX-TGA670B
4.5.5.2. Line 2
Bell signal detection and OFF HOOK circuit:
In the idle mode, Q241 is open to cut the DC loop current and the ring load. When ring voltage appears at the Tip (T) and Ring
(R) leads (When the telephone rings), the AC ring voltage is transferred as follows:
T → L201 → R211 → PC211 → DSP pin 85 [L2_BELL]
To provide an off-hook condition (active DC current flow through the circuit), CPU turns Q246 on.
Thus Q241 turns on. Following signal flow is the DC current flow.
T → L201 → D201 → Q241 → Q261 → R263 → D201 → L202 → P201 → R
ON HOOK Circuit:
Q241 is open, Q241 is connected as to cut DC loop current and to cut the voice signal. The unit is consequently in on-hook con-
dition.
Pulse Dial Circuit:
Pin86 of DSP turns Q246 ON/OFF to make the pulse dialing.
Side Tone Circuit:
Basically this circuit prevents the TX signal from feeding back to RX signal. As for this unit, TX signal feed back from Q261 is
canceled by the canceller circuit of DSP.
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