The most likely culprit for display issues on these old calculators in my experience is the ribbon cables. You can take a multimeter and check the resistance of

The most likely culprit for display issues on these old calculators in my experience is the ribbon cables. You can take a multimeter and check the resistance of each connection, if any of them read too high you can solder a thin wire in parallel with it. Or it could be the contrast potentiometer giving too high of a resistance, you can try cleaning the internal contacts or replacing it.