HP 41 not working

[AndiGer]

I use 5V power source from an USB hub giving 1A. 5V because the rechargable battery HP82120A also gives about 5V - fully loaded maybe 5.2V.
Always use that to test powering up of HP-41's also with the card reader that is enough to operate the calc as it is from the HP82120A.
That calc had one broken lower post that I repaired (or tried to repair). The case is closed well now.
Measuring resistance from battery plus (anode there) to the lower hole in the tunnel (kathode there) gives 1880 ohms which is comparable to another CV (1820 ohms).
Battery minus to upper hole is near 0 ohms as in another CV.
What makes me wonder is the display behaviour when I put and especially remove power for several times in a fast sequence. If I hold power to the battery ports for a longer time (some seconds) and remove most of the time only right-hand side upper parts of starburst appear. No idea ...
Andi