We have received many units for repair. They suffer from all possible damages so we have a quite good overview of what the problems are. But we have identified a damage mode that will surprise you - and it is due to the dust that is generated by old HP41c case lining disintegration.
If you see black dust on your calculator when you take it off the case, then you suffer from that problem. This dust is incredibly thin and it is a pain to use the calculator with it. As the grain is very small, it can enter the keys and also into the screen. It can be improved by blowing on the calculator with a hair dryer - but this alleviates the problem only. And in already many cases, it can damage your calculator!
The mechanism is always the same: when I open the calculator, I see dust all around, but it concentrates in particular on the +5v contact between the board and the battery contacts. There it increases the resistance and it is eventually burn, thereon stopping the passing of current.
The solution is not too difficult and in most cases you just need to disassemble the calculator and clean both contacts thoroughly with Caig Deoxit until they shine again (and by the way the rest of the innards too). But the problem with the case remains!!
As soon as you detect dust on a case, just wash it with your hands and liquid hand soap. Use your hand vigorously in the inside until there is a lot of foam. Rinse thoroughly and marvel at what is coming out to the sink! Do the process a couple of times until there is no more dust coming out. Let it dry and you're ready to go. The case will still look good in the inside.
Wash also the sponge in the bottom of the case separately. You will see that it is actually yellow, not dark green!!
Now your calculator will be clean always. Now it is a pleasure to use it!