If the above doesn't work, then you need to look at the contact springs from the batteries to the main board. The board flexes slightly when buttons are pushed and this causes there to be wear between the spring and board contact pad. In my calculator, the spring wore thru the contact pad. To reach the contact pads, you simply remove the 6 screws along the outer part of the cover. Then remove the screw covering the flat backup battery - just above the batteries. Get a small flat blade screwdriver and pry along the edge of the upper and lower covers (not the loose cover). Work along the edge and unsnap the 3 snap joints. There is one at the top center and one on each size approximately centered along the length of the battery. When you get the cover removed, look at the pads. You will probably see a black dot but if you look under a magnifying glass, you will see the bare circuit board. At this point, look at the pads to be sure there is no corrosion on them. It will look like a white or green powder. Remove with alcohol and Q-tip.
Turn over the the battery cover and with a pair of needle nose plyers, bend the two outer spring fingers inward a bit. When you put the covers back together, you will be able to see thru the two square holes by the inner 2 batteries if the springs are making contact with a different location on the pad. A flashlight and magnifying glass is helpful. Best to put the batteries back in place and see if it will power on. If so, then reinstall the screws and your back to a working calulator.