There may be something astray in the light path.

It has been years since I used a Durst M6xx series enlarger, and even longer since I had one opened up.

You probably need to attract the attention of one of the Durst users here on APUG. For future reference, it would probably help if you had included Durst M605 in the thread title. No one would mind if you started another thread for that purpose, and linked to it here.

Have you checked the manual?: