Early/earliest HP-41CV halfnut

[AndiGer]

Thank you all for your suggestions. DeOxit red and gold - I read much about it here but never bought.

To clarify ... although there was some battery leakage the calculator has no stuck keys.
I could locate the KR lines on the board and there is no permanent shortage from the keys. Only when I press the appropriate key in rows 1 or 2 the connection is made to that KR line. Today I measured the KR lines to the key in rows 1 and 2 again and all seems to be fine.
The machine sat without batteries since last Sunday.
So I put in batteries, started the machine and after 2 minutes it still behaved normal, showed 0.0000 after MEMORY LOST.
Then I performed the "forensic test": 9 - sin - cos -tan - arc tan - arc cos - arc sin
in FIX-9 mode and let the calculator sit for a while. Still normal.
Switched it off and let the batteries in. Since about 2 hours I switched it on 2 times. Still normal.

The only thing I didn't do this time: CF 28 to get comma instead of point as decimal separator.
I will observe further.