Read Panasonic KX-TCD951EB / KX-TCD955EC Service Manual online
26
KX-TCD951EB/KX-TCD955EC
Fig. 22
Circuit Diagram
3.5 CLIP CIRCUITS (SEE Fig. 21)
The caller ID signal is detected by IC4.
3.6 HOOKSWITCH (AND PULSE DIALING) (SEE Fig. 22)
T8 is the hookswitch, driven by T9. When the phone is “off-hook”, the H O O K control signal from the BBIC will be a
high logic level (+3V), and all three transistors will be on, thus T8 will “loop” the line. The zenner diode D10 protects
transistors T11 to T13 against transient line voltages.
3.7 PULSE DIALING (SEE Fig. 22)
During pulse dialing the hookswitch (T8, T9) is used to generate the pulses using the H O O K 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 section 3.10)
3.8 AUDIO CIRCUITS (SEE Fig. 22)
The line output signal from the BBIC R X A F (TP120) is amplified by T13. The R X A F 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 UK) 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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