Thank you for the comments. I have checked the focusing screen, and it is the E3 that corresponds with this camera.
I have also put new batteries. However, there is still the same problem. I think I will go to Nikon service to repair it.
However, I would like to ask you a new question: If you compare the FM3a (or another manual camera) photometer value with an electronic camera (like F100, F80, etc) photometer, both with central valuation and the same light situation, Do you get always the same exposure? It is supposed to be the same; however, I find strange measures with my F100 and FM3a.
Henry.