HP cm1415fn: White lines printed in black

Hello
I had to reinstall a machine running Ubuntu 10.04×32 and I find myself in the same situation as before: the prints come out with black lines instead of white lines.
The first time I solved the problem by changing (adding) something in a file … but which one?
In the future, I will note these things because I can not lay hands on this information.
If it awakens a memory for someone, let me know!
