As I need to create HP41CL calculators, in some occasions I end up buying defective calculators for parts. A surprising number of them can be repaired; but until now, I had been able to repair only the mechanical failures.
There are many calculators where the previous owners were not too careful, or too clean, and sticky liquids were spilled on the calculator. Some times, the key behavior is inconsistent, and you need to press harder on some key to register. And in some cases, the keys stay down because there is coffee, tea or other dirt keeping them down. Until now, I have not been successful in solving that problem.
The person that I personally know that has more repairing experience in HP calculators (and a fantastic collection thereof) recommended to me 3-in–1 contacts as the cleaner method to sort this out, and WD–40 in harder cases.
But then, Geoff White also recommends to use Caig Pro-Gold contact cleaner to do the same job. I happen to have Caig contact cleaner myself! I use it when I do my audio system maintenance once a year. It is used to clean all electrical connections in the system - to very audible effect. It protects all gold and silver plated surfaces, and avoids staining. It keeps the connection gas-tight. When a key stops working or requires a much harder action to register, he recommends to put Caig Pro-Gold through the pin-hole of the circuit corresponding to the key that fails. This is a more expensive product, but nevertheless a cleaner option, since it doesn’t leave residues.