Matsushita Electric KX-TCD952NLB, Руководство по обслуживанию

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A or B lines. The relay is energised when transistor T2 is switched on by a high level on the
EARTH 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.
8.3.4. 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
8.3.5. CLIP CIRCUITS (SEE Fig. 21)
The caller ID signal is detected by IC4.
8.3.6. HOOKSWITCH (AND PULSE DIALING) (SEE Fig. 22)
T8 is the hookswitch, driven by T9. When the phone is “off-hook”, the HOOK control signal from
the BBIC will be a high logic level (+3V), and all both transistors will be on, thus T8 will “loop”
the line. The zenner diode D10 protects transistors T11 to T13 against transient line voltages.
8.3.7. PULSE DIALING (SEE Fig. 22)
During pulse dialing the hookswitch (T8, T9) is used to generate the pulses using the HOOK
control signal, which is set high during pulses. To force the line impedance low during the
“pause” intervals between dial pulses, the PAUSE-DIAL signal turns on T11, which turns on T12
harder (increases current), thus reducing line impedance (see 8.3.10. BATTERY CHARGER)
8.3.8. AUDIO CIRCUITS (SEE Fig. 22)
The line output signal from the BBIC RXAF (TP120) is amplified by T13. The RXAF line is DC
coupled to T13 thus making it work as a current limiter (typically < 8mA). The emitter load of T13
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