Dirt is the usual offender. The common repair method from the archives (Courtesy of David Smith):
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Each switch dome has a small hole under it that you can clean the switch through. You need to make a tiny wire brush out of three wire brush bristles twisted together in a pin vise, trimmed even. Place a drop of 90%+ isopropyl alcohol on the offending switch hole, dip the brush in more alcohol, and gently "scritch" around in the hole.
Keep the alcohol off the display window. It leaves a white stain.
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I can add to that by saying do not over do things and break a bristle off inside the switch dome.
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The next logical question:
HP41 Disassembly from the "repairs" section:
Remove the battery pack. There are four screws underneath the feet that are held on with adhesive. Peel up the feet and remove the screws. The back will come away easily. This calculator will not work properly if the case halves are not held securely together. Over-tightening the screws can strip the threads and lead to a malfunction.
