There is a circuit board mounted (with screws) that covers the actual buttons. I have not removed this board, but taken a can of compressed air and sprayed into the cracks around the edge of the board to try to clean any dust or other substance that may be affecting the operation of the ON/OFF key.

It is evident from the picture that crud does get inside the scale (blow this out too, while you are there!

This is all I have needed to do (this is the second time I have had to do this in the 7 years I have had this scale).

To reassemble the scale, follow these steps in reverse. When replacing the weighing plate, you can look in from the side to catch the screw positions on the weigh plate.