Posts: 883
Threads: 17
Joined: Feb 2006
The problem is most likely one or more failed heat stake pins that hold the front and back case halves together, especially below the LCD. This causes a loss in contact between the keyboard connector and logic board.
It's not something that can be repaired... at least in a reliable or transparent way.
You can peel away the upper 28 logo label on the top half of the calculator and the lower keyboard surround label. Once they are off, you'll see a number of recessed plastic holes that are melted plastic pins that extend from the back half of the case into the front. Several will most likely fall out once the labels are off, those are what caused the problem, they shear off the back of the case. All the remaining heads will need to be drilled off for the case halves to separate.
Once done, there is no known good way to put Humpty Dumpty back together again...