I have a Durst M-805, condenser type for Black and White Photography. I have noticed that my enlarger is not able to keep the focus for a long time. Let me explain a little bit. I put the negative in its place and then I focus easily the image on the paper surface located on the easel. However, after some minutes, the negative is out of focus. I have investigated what is going on, and I have noticed that the enlargers head goes down a little bit, even though I lock any nut or similar thing that should be locked. It seems to be a problem with the counterweight of the enlarger. Can you tell me what I can do? Thank you for your comments, Henry.