Thi is a common problem with aging Pioneer machines. There is a rubbery strip inside, just below the display, that applies pressure to keep the mylar keyboard edge connector in contact with pads on the circuit board. As that strip ages, it shrinks, and no longer provides enough pressure.
The simple test is to carefully but firmly pinch the calculator front-to-back, just beneath the display, and roughly above the affected key column. You should notice restored function while applying this "pinch pressure". (It may even temporarily fix the problem, though it will return.)
The fix is to take apart the calculator, detach the PCB, pull out the rubber strip, and bolster it from behind with something to make it "thicker" -- I've use a rolled-up strip of cellophane tape, of all things. (That is, a strip of Scotch tape, as long as the rubber strip, and rolled tightly around itself along its long axis, creating a roughly toothpick-sized tube of tape.) Whatever you use, put it in the rubbery strip's slot (molded into the front half of the case (again, just below the display), beneath the rubber strip itself. Upon proper reassembly, the thing should work fine.
The whole process is non-trivial, but very doable. It's documented in some articles in the Articles Forum. (Look for "Pioneer Internals" or whatever -- I don't remember now.) Read whatever you can find, and ask advice before proceeding.
Or, as suggested above, keep it simple and let the expert handle it. (Randy at fixthatcalc.com, I think.)
Good luck!
