If you're handy with a soldering iron, and have some electro-mechanincal experience, there's a good change the small board that holds the power supply jack has either a cold solder joint, or the board has a hairline crack that runs through the power supply connector. The toughest part about fixing the board, if this is indeed the problem, is opening the keyboard. There are approximately 20 screws holding the keyboard together, some of which are seated deep inside their openings. All are hexhead screws.
Once the screws are removed, the next step is to remove the Smart Media Card reader board, which is held in place by a few screws. Once the board is unscrewed from the keyboard's body, the keyboard can be opened from front to back like a book. The power supply board is situtated in the back of the keyboard and held in place by a couple screws.
Carefully examine the power supply plug's printed circuit board with a magnifying glass, and look for obvious cracks that run through the board's copper circuit tracks. Check each solder connection using the magnifying glass as well. Some of the boards had connections that were never soldered. It only takes a light touch with rosen-core solder and a PC board soldering iron to correct the problem.
Good Luck,
Gary
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Travlin' Easy