Try these steps...

• Open your calculator and remove the logic board
• Inspect the logic board, especially around the connectors and look for corrosion.
• Inspect both sides of the board.
• Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
• Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
• Rinse in alcohol and repeat.
• Let it air dry for a day.
• Re-assemble and hope for the best.