HP-67 not working anymore

[christophe guerin]

Hi,
I recently had a broken HP-67: no batteries, and a rubber wheel.
I changed the batteries and the wheel and everything worked fine for a week.
I was able to use the cards, do any operation, etc.
And all of a sudden, things started to go wrong: the display didn't match what I put on (ex: just after OFF/On, key 1 gave -266665616540 01)
Sometimes funny things happen on the screen : CoC-E or 0C-E.
Sometimes nothing appears at all, although the current is flowing (about 150 mA).
Can someone give me advice on how to repair this machine?
Thank you for your help