Panasonic KX-FT21BX Service Manual page 7

3-9. Power Supply Board Section (Change from original pages 72~75)
(1) Key components for troubleshooting
Check the following parts first: F101, D101-D104, C106, Q101, PC101 and IC101.
This comes from our experience with experimental tests. For example: power supply and lightning surge voltage test, withstanding
voltage test, intentional short circuit test, etc.
Caution:
If you find a melted fuse in the unit, do not turn on the power until you locate and repair the faulty parts (except for the fuse);
otherwise the fuse will melt again and you cannot pinpoint the faulty point.
In most cases, the symptom is that nothing is output. It is more likely that the fault is in the primary side rather than the secondary
side. Check the primary side first.
AC
Input
Input
Rectifier
Circuit
Circuit
POWER SUPPLY BLOCK DIAGRAM
A
+
C106
R102
-
Kick-on
R103
B
Voltage
Circuit
DUMMY LOAD to check voltage
24V
1
2
3
CN301
GND
4
5
6
5V
7
8
(Model KX-FT21BX/KX-FT21BX-W)
KX-FT21BX/KX-FT21BX-W
Surge
Absorber
Converter Circuit
Circuit
C
Surge
Absorber
D
Circuit
Control
Circuit
E
IC101
F
Error Detecting
Circuit
4.7k
V
2W
24.5V±0.5V
33
5.0V±0.25V
V
2W
-7-
G
24V
Output
Circuit
5V
Output
Circuit
H
When you check voltage in
a power supply unit alone,
use this dummy load.
If the dummy load is not
available, connect the main
P.C.B. to the analog P.C.B.
V : Volt Meter
24V
GND
5V