I am getting a small cyan cast when I print with times longer than 10sec. I originally thought that maybe the cast was a developing problem with the film, but it's not the film. I scanned it and it has no cast. It's not the paper either, Because the cyan cast problem happens on different type of papers in about the same spot. Only a spot.
I think it must be the dichro head. (Beseler 23C-II Dichro head). If I expose any picture at F/11 for 11-12 sec. The cast appears. If I exposed the same picture at F/8 for 7 sec the cast is almost not noticeable , expose same pic at F/5.6 for 3 sec the cast is non existing. (All exposures being density correct). .
( keep in mind the cast is only on a certain spot, like a small blob or irregular circular shape.) always on the same spot. And the colors on the rest of the picture are correct.
Something wrong with the enlarger ? what to do ?
Does your lensboard have an illuminator just to the side of the lens that lights up the ƒstop ring? A lot of beseler boards have this "feature" and I'll bet that this is the cause of your problem. Check this link and you'll see what I'm referring to.