Solved: Enlarger mixing box was causing mark on prints

I managed to solve an issue with my LPL 4550XL today. I figured I would post here in case someone has the same problem in the future.

I was getting an inch long mark on some of my prints as shown in the photo. It was only happening with 4x5 enlargements so at first I thought it might be something with my 135mm lens but I couldn't see anything.

I decided to pull apart the head and there was a black mark on the white plastic on the bottom of the mixing box. Maybe a scuff mark of some kind. It was near the edge so not affecting smaller negatives. I buffed it for 30s with a cloth and it went away. Just finished some prints and all is good.

Thank you for that... from my point of view checking the mixing box as a possible cause for marks on prints it not something I would have done - not even thought about either!

I agree with Fred, thank you. I don't think I would have ever thought a mixing box with a black line somewhere would have caused that.

Always learning.

Mick.

I'm glad you solved the issue!

One of my first enlargers, a Durst M605 with a colour head, was bought used on craigslist. I was just learning to print, and wasn't perhaps as critical of my prints as I am these days. I eventually starting noticing these dark circular marks right in the middle of my prints. Lots of frustrating time later, I traced it back to the opaque white plastic at the bottom of the mixing box - it was a burn mark from a halogen bulb! Someone who had used it in the past had seemingly set the hot bulb temporarily on the mixing box, and it melted the plastic.

I guess I could have replaced the opaque plastic, but - being Durst - it wasn't a simple piece. With the vast majority of darkroom equipment being used these days, I suspect these sorts of problems will be more common, and this sort of problem solving and jerry rigging is almost par for the course now.

Hi Marco, was that plastic diffuser shaped like plan concave lens with flat part towards negative? Thicker at the centre and thinner at the edges. Maybe in future part of photography craft will be designing and running 3D printers too

I have to somewhat sheepishly confess that this had been an issue for a while now that I had been kind of ignoring. Given it's location it only affected certain prints: basically 4x5, not much cropping, where there was a significant amount of featureless tone like a dark sky for example. Once I did seriously take a look for the problem it only took me 5 minutes to solve.

Sometimes the foam inside would heat up and release and fall inward causing real problems, but don't feel bad we all have been there before.

the column that the head runs up and down can sometime wear to raw which sometimes will reflect reflective light and cause nightmares to figure out where the plus density mark on the print is coming from.

Bob Carnie said:

Sometimes the foam inside would heat up and release and fall inward causing real problems, but don't feel bad we all have been there before.

the column that the head runs up and down can sometime wear to raw which sometimes will reflect reflective light and cause nightmares to figure out where the plus density mark on the print is coming from.

Click to expand...

I got so frustrated with the styrofoam flaking I removed it from my Beseler mixing box.

I too am very surprised for a fault at a diffuser to yield such result.

I also had a problem with a Diffusion Chamber.

https://www.photrio.com/forum/threads/beseler-diffusion-chamber.155695/

With a new enlarger I always do some test prints with no negative. Small apertures and proximity to the negative will allow mixing box diffuser material defects to show in prints.

LPL 4x5 enlargers' mixing boxes have LDPE diffusers that are molded thicker in their centers to even out illumination on the negative, i.e. compensate for edge fall off. When my 4500II, purchased new (nearly 40 years ago), caused several small, out of focus white spots on prints, I traced the source to opaque inclusions within the molded diffuser. Replacing the diffuser solved that problem, but it required rejecting several defective replacement diffusers that also contained inclusions before finding a 'clean' one. Hopefully LPL's quality control has improved since then.