The solution I've found is simple.
No need to disassemble computer.
The DC jack will over time crack the solder on the center pin through the pc-board and lose continuity.
First remove the keyboard cover by removing 2 screws (on rear under hinges) and gently prying upward beginning at hinges. Directly under where the power button was and beside the actual power switch find the solder connections for the DC jack. (The jack is on other side of board. You are seeing the bottom of board).
Examine all jack connections for cracked solder around the pins. a magnifying glass helps. Usually it is just the center pin which is toward the rear of the jack, obvious cracking causes intermittent connection or no connection.
If you have no experience soldering I recommend you leave this task to someone who does.
Carefully apply rosen core solder to pin(s) using proper fine tip iron. heat thoroughly til pin moves in hole then apply fresh solder. Be quick. Not too much solder, not too much heat. Be careful not to allow iron's barrel to touch plastic case or tip to build solder bridge to any other connection.
Problem solved.
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