KX-NSX1000, KX-NSX2000 (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 11

11

Provides a description from AC input to the rectifier smoothing circuit.

1. Filter circuit

Consisting of the following parts, removes exogenous noise and noise generated by the power source.
C1/C2 : Normal mode filter (X capacitor) 
L1/L2 : Common mode noise filter
C3/C4 : Common mode filter (Y capacitor) 

2. Rectifier circuit

Consisting of the following parts, the rectifier circuit converts alternating current to direct current. 
D1, D2, D3, D4 : Diode
C8 : Primary smoothing capacitor

3. Surge absorber circuit

Consisting of Z1, the surge absorber circuit protects the equipment from exogenous surge such as lightning surge. 
Z1 : Varistor

4. Rush current prevention

Suppresses rush current to smoothing capacitor immediately following AC input by the following parts. 
NTC1 : Thermistor

5. AC voltage detection circuit

Consisting of the following parts, the AC voltage detection circuit detects presence of AC input voltage input. 
D5, D18, R6-R21, Q2, Q3, Q4, PC3 Rectifies AC voltage by D5 and D18. 
If AC input voltage exceeds threshold voltage, Q2, Q3 and Q4 turn "on" and the secondary side is informed of presence of AC
input through PC3.