Read Panasonic KX-TCD951EB / KX-TCD955EC Service Manual online
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KX-TCD951EB/KX-TCD955EC
3. THE LINE INTERFACE SECTION (SEE BLOCK DIAGRAM Fig. 19)
3.1 INTRODUCTION
This section consists of the telephone line interface, bell detector, hookswitch, pulse dialing circuits, audio circuits, DC
mask & line impedance circuits, power supplies, and battery charger circuits.
3.2 TELEPHONE LINE INTERFACE (SEE Fig. 21)
The telephone line is connected (via 2 or 3 jumpers selected for country of destination) to a bridge rectifier D8. Surge
suppressor SA3 protects against excessive line voltages. Test points are TP14 (A), TP13 (B), TP21 (S) and TP15 (E).
Bridge rectifier D8 provides for lines of either polarity.
The output of D8 is “Line +” (TP50) and “Line –” which is ground.
3.3 EARTH RECALL (SEE Fig. 21)
For countries that require Earth Recall facilities, relay RLY1 is provided to short the E line to the A or B lines. The relay
is energised when transistor T2 is switched on by a high level on the E A R T H control line (TP77) from the BB-IC IC101.
D1 will quench the large back-emf voltage that would otherwise occur across the relay coil when T2 turns off
.
3.4 BELL DETECTOR (SEE Fig. 21)
The AC ringing signal is detected by optocoupler IC2, using its internal diode in conjunction with D4. DC from the line
is blocked by the wall plug inside. The other components D2, D3, and R3 reduce current and increase the circuit
impedance in line with national requirements. When ringing is detected IC2 will turn on, and the RING line (TP76) will be
dragged to a low voltage.
Circuit Diagram
Fig. 21
LINE-SENSE
REV-POL
S
B
A
E
PULSE_DIAL
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