One Bright Corner

Forgive me if I'm discussing a beat up old subject here. I have little time to dig through old threads...
My enlarger is probably way out of alignment. Up until now I don't really care, because the print quality has been satisfactory. But now, enlarging with a 135 (or 150) mm lens, from 4x5 negs, as close to full frame as I can, one single corner of the print is underexposed, gradually getting lighter the closer to the corner it gets. Is this a lens coverage / misalignment issue?
If it is, enter problem two - how do you fix it? I have no clue how to adjust my enlarger. It's an old, beat up Omege Pro-Lab 4x5.
Interestingly, I've had this happen before on a 35mm/MF enlarger from the same company... So for now I crop a little bit, and it seems to help. Since I can rotate the neg holder I'm not too worried about it, but feel I should probably fix this somehow.
Any leads? Cheap and easy is good. Time consuming and expensive is bad.

Attachment shows what I mean (upper left hand corner). Sorry for the small attachment, the original file was too large for the server to accept it as an upload, so I had to shrink it.

- Thomas

Have you changed bulbs recently? Looks like an offcenter bulb to me. Possibly a problem with bulb height too if centering it results in a darker spot in the center with the corners lighter.

Hey Gary, it's a brand new bulb, unfortunately... But it could well be misaligned! I'll look into that immediately.
- Thomas

Any chance you have a filter holder attached to or under the lens? If it (or any filter frame) is slightly in the light path it will cause a lightening in one corner.

Joe

Joe,
it has an 'under the lens' filter holder. It's a great suggestion. It's the kind that swings under the lens and is therefore mobile. When I print variable contrast, I try to remember to check that the filter holder does not obscure.
More and more I'm liking the bulb socket problem. Here's another socket wrench thrown into it. When I take the neg holder out, I get a perfect circle with even lighting. I'm thinking perhaps it has to do with the neg holder and how it's held in place. Perhaps it is time to get an alignment tool after all...
Thanks for helping out!
- Thomas

Maybe check with Harry at classic-enlargers.com if nothing here proves out.

Does the perfect circle of light you get without the negative holder shine directly perpendicularly down on your baseboard? If not, it's pretty certain to be alignment trouble. I had pretty much all the same problems you're describing with my Omega DII, and I thought it might be the bulb, the condensers, lens coverage, all the things you're looking at, but eventually it was the off-center circle of illumination that clued me in. Everything else was fine.

DIIs are fairly simple machines, and I imagine your enlarger isn't too different. To figure out how to realign it took me an almost complete tear-down (actually, I had to tear-down my entire darkroom to get at the enlarger first even), the whole project accomplished with a screwdriver, a set of hex-wrenches and a torpedo level, in maybe four or five hours. But I was starting from scratch without too much native mechanical aptitude- now that I know how it works it'd probably only take me 10-15 minutes. Essentially, the lamp-house has to sit nicely on the negative-stage, and the negative-stage must be parallel to the lens-board and the base-board.

Very worthwhile project, though. Omegas are notorious for going out of alignment, so even if this isn't the problem at hand you ought to check it out. All parts of my prints look better since, not just the corner I don't have to burn in anymore.