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Hi, Giovanni;
you may want to repeat Hakan´s procedure a couple of times again (as many as you need) and see if the calculator gets back to normal.
There is one straight procedure that is explained in the HP41 manual: leave the calculator without batteries for a couple of days (the C/CV may need about four to five days) in order to discharge all remaining electrical charges in all capacitors (it is known that the HP41 has two 'big' electrolytic capacitors). Because it uses low-power C-MOS technology IC´s as load, these capacitors take too long to get rid of remaining charge. The HP41C/CV (not the CX) may retain RAM contents for hours without batteries. The CX has the internal clock and extra RAM circuitry so discharging is faster.
There is also the electrostatic charge, different of the one found in capacitors due to regular operation. I had once to disassemble a coconut and 'resurrect' it after discharging the capacitors and leaving it over an aluminum foil, carefully pressed in the side without components so all terminals were shorted wit each other. After about an hour it was singing and dancing again.
Hope you succeed.
Edited: 19 Feb 2010, 1:20 p.m.