Panasonic KX-TD612NE Service Manual page 17

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3. Expansion Block
Composition: / This is composed of the following circuits: / (a)
Current Supply Circuit / (b) Hook Detection Circuit / (c) Ring Trip
Detection Circuit / (d) Bell ringing section /
Circuit Operation: / (a) Current Supply Circuit / Current Supply
Circuit is constant-current circuit which supplies the call current
to SLT. / With the telephone off hook, a DC loop is formed, and
current is supplied to the telephone. / This circuit is limited to
about 25mA by R610, D601 R614 and D602. / +24V
R610 Q602
Q603 R614
(b) Hook Detection Circuit / When the telephone handset is taken
off, DC loop is formed and collector of Q604 change to L from H.
And Gate Array detects off hook condition. / When the handset is
replaced back on hook, the DC loop is interrupted and collector of
Q604 change to H from L. / And Gate Array detects on hook
RL60 L602
R615 GND /
telephone L603
17
R611
RL60