Read Panasonic KX-TCD951GB / KX-TCD955GC Service Manual online
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KX-TCD951GB/KX-TCD955GC
Fig. 22
Circuit Diagram
2.5 CHARGE PULSE DETECTOR (SEE Fig. 21)
The 16 kHz charge pulse (billing pulse) signal is detected by IC3A, which forms a sharp 16 kHz bandpass filter. If a
16 kHz charge pulse is present, a large 16 kHz output from IC3A will switch comparator IC3B, which output a 16 kHz
square wave. This will cause transistor T18 to switch, sending the 16 kHz square wave signal CHARGE-PULSE to the
BBIC. The BBIC will check the frequency and pulse length to determine if it is a valid charge pulse.
2.6 CLIP CIRCUITS (SEE Fig. 21)
The caller ID signal is detected by IC4.
2.7 HOOK SWITCH (AND PULSE DIALING) (SEE Fig. 22)
T8 is the hook switch, driven by T9. When the phone is “off-hook”, the HOOK control signal from the BBIC will be a
high logic level (+3V), and both transistors will be on, thus T8 will “loop” the line. The zener diode D10 protects transis-
tors T11 to T13 against transient line voltages.
2.8 PULSE DIALING (SEE Fig. 22)
During pulse dialing the hook switch (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).
2.9 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 is complex to achieved the correct fre-
quency response, since the line load (for Germany) is also complex.
The line input signal TXAF (TP123) is taken from the junction of R41 and R70. Phase cancellation of the line output
audio occurs at this point, so that only incoming line audio should be passed to the BBIC on TXAF.
RxAF
TxAF
HOOK
PULSE DIAL
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