Panasonic KX-TCD725GM Service Manual ▷ View online
6.2.
THE LINE INTERFACE SECTION (SEE BLOCK DIAGRAM Fig. 17)
6.2.1.
INTRODUCTION
This section consists of the telephone line interface, bell detector, charge-pulse detector, CLIP detector hook switch, pulse
dialing circuits, audio circuits, DC mask & line impedance circuits, power supplies, and battery charger circuits.
dialing circuits, audio circuits, DC mask & line impedance circuits, power supplies, and battery charger circuits.
6.2.2.
TAX DETECT (SEE Fig. 19)
The tax detection will detect the 16kHz burst which is picked up on the CHARGE_PULSE line. The LC filter (L3 and C5) block
the signal so that it does not interfere with speech signals in the line circuit. D5 provides overvoltage protection for the
CHARGE_PULSE input port of the BBIC. The RC filter components provide high pass filtering which attenuates the speech
(below 4kHz) while allowing the charge pulse burst (16kHz) to pass.
the signal so that it does not interfere with speech signals in the line circuit. D5 provides overvoltage protection for the
CHARGE_PULSE input port of the BBIC. The RC filter components provide high pass filtering which attenuates the speech
(below 4kHz) while allowing the charge pulse burst (16kHz) to pass.
6.2.3.
TELEPHONE LINE INTERFACE (SEE Fig. 19)
The telephone line is connected to a bridge rectifier D8. Surge suppressor SA3 protects against excessive line voltages. Test
points are TP40 (A), TP26 (B). A 16 kHz notch filter L3 and L5 block the 16 kHz “charge pulse” signal from the rest of the line
input circuitry. Bridge rectifier D8 provides for lines of either polarity. The output of D8 is “Line +” (TP50) and “Line -” which is
ground.
points are TP40 (A), TP26 (B). A 16 kHz notch filter L3 and L5 block the 16 kHz “charge pulse” signal from the rest of the line
input circuitry. Bridge rectifier D8 provides for lines of either polarity. The output of D8 is “Line +” (TP50) and “Line -” which is
ground.
6.2.4.
EARTH RECALL SERVICE OPTION (SEE Fig. 19)
The circuit can be configured for connection to signalling earth by a service engineer as an optional feature.
In order to facilitate this the components listed below are to be fitted by a Service Engineer/Technician.
This modification connects the ’E’ and ’b’ contacts of the plug when the relay is switched on.
The relay is controlled by the ’EARTH’ signal from the BBIC, IC6 pin 68. It can be programmed to operate in Earth or Flash
mode. The factory preset is Flash mode.
mode. The factory preset is Flash mode.
* If you want to change the time of EARTH RECALL, please change the value of EEPROM (ADR, 0F0E, 0F0F, 0F10).
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KX-TCD725GM
6.2.5.
BELL DETECTOR(SEE Fig. 19)
The AC ringing signal is detected by phoptocoupler IC2, using its internal diode in conjunction with D4. DC from the line is
blocked by C2. 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 will be dragged to a low voltage.
blocked by C2. 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 will be dragged to a low voltage.
6.2.6.
CLIP CIRCUITS (SEE Fig. 19)
The CLIP signal is detected by IC3 and sent to the BBIC through the /TXAF line as a FSK waveform. The CLIP_STATE signal
from the BBIC is used to provide a CLIP impedance through a combination of components selected from R85, R86, C93, C37,
and R1. The combination depends upon the CLIP requirements of the specific country - often there is no requirements for the
CLIP_STATE impedance.
from the BBIC is used to provide a CLIP impedance through a combination of components selected from R85, R86, C93, C37,
and R1. The combination depends upon the CLIP requirements of the specific country - often there is no requirements for the
CLIP_STATE impedance.
T10 disables the CLIP signal during the off-hook condition.
Circuit Diagram
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KX-TCD725GM
6.2.7.
HOOKSWITCH (SEE Fig. 20)
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
(+2.8V), and both transistors will be on, thus T8 will “loop” the line. The zenner diode D10 protects transistors T11 to T13 against
transient line voltages.
(+2.8V), and both transistors will be on, thus T8 will “loop” the line. The zenner diode D10 protects transistors T11 to T13 against
transient line voltages.
6.2.8.
PULSE DIALING (SEE Fig. 20)
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 PULSE-DIAL signal turns
on T11.
during pulses. To force the line impedance low during the “pause” intervals between dial pulses, the PULSE-DIAL signal turns
on T11.
6.2.9.
AUDIO CIRCUITS (SEE Fig. 20)
The loop current (typically 40mA) passes through T13, R57 and R56. R54 and R55 provide dc biasing for the base of T13.
The line output signal from the BBIC RXAF is DC decoupled by C47 and amplified by T13. The emitter load (R57, R58 and C46)
is complex in order to achieve the right frequency response with the complex line impedance.
is complex in order to achieve the right frequency response with the complex line impedance.
The line input signal TXAF is taken from the junction of R41 and R49. Phase cancellation is provided at this point by R70 so
that only the incoming line audio should be passed to the BBIC on TXAF.
that only the incoming line audio should be passed to the BBIC on TXAF.
Circuit Diagram
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KX-TCD725GM
6.2.10. POWERSUPPLIES (SEE Fig. 21)
The AC adaptor for the KX-TCD725 provides unregulated DC through J2 for the handset charge terminal, the 3.8V regulator
(IC7) and the Relay coil (RL1) where fitted.
(IC7) and the Relay coil (RL1) where fitted.
The 3.8V supply from IC7 is used for the RF module, and is further reduced by IC1 to 2.80V for the BBIC supply.
R17, 18 and 19 provide short circuit/over current protection at the handset charge terminal.
Circuit Diagram
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KX-TCD725GM
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