Panasonic KX-FT21BX Service Manual page 103

(3) Broken parts repair details
(D101, D102, D103, D104)
Check for a short-circuit in terminal 4. If D101, D102, D103 and D104 are short-circuited, F101 will melt (open).
In this case, replace all of the parts (D101, D102, D103, D104, F101).
(Q101)
The worst case of Q101 is a short-circuit between the Drain and Gate because damage expands to the
peripheral circuit of Q101.
This is due to a very high voltage through the Gate circuit which is composed of R105, Q102 and D107.
You should change all of the parts listed as follows.
F101, Q101, R105, Q102, D107
(D201)
If D201 is broken, the oscillation circuit in the power supply cannot operate. Check it with an electric tester.
(D204)
If D204 shorts, the voltage feedback circuit is almost always the cause.
Replace the following parts if this happens.
PC101, Q102, D203, D106, D105, D107
Does D204 short?
YES
Replace D204.
24V output OK?
NO
Replace the whole board.
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NO
YES
A
KX-FT21LA