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Old 22nd June 2012, 08:54 AM
Stan160 Stan160 is offline
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Default LPL C7700 uneven illumination

I've not been doing a lot of printing recently, but I've noticed a problem on a number of prints: The corner has an uneven texture that's only noticeable in areas of continuous tone on the negative, such as the sky.

It's taken a while to conclude that the problem lies in the enlarger, but have seen it on negatives from two different camera bodies, one 6x6, one 35mm, and two different enlarger lenses. It's also appeared on paper from different boxes, processed in chemicals from different sources.

The problem was so subtle that it wasn't visible until after the prints had dried and were viewed in daylight, but was a consistent pattern every time. I worked around it mostly by just putting negatives in the carrier to avoid smooth tones in that corner.

Anyway, starting the first printing session for weeks, the light was noticeable dim so I cleaned the lamp contacts. Much better output, but now there is a dark band right across the middle of each print - I missed it when viewing the prints in the darkroom, but very noticeable in daylight.

Has anyone had experience of this - I know there are several C7700 owners on the forum? One thing that occurs to me is that the original lamp only lasted a few weeks after buying the enlarger, but I don't remember there being a problem until I fitted a replacement. Both lamps are marked A1/231, but the new one has a totally smooth reflector, whereas the old lamp had a "panelled" reflector. I wonder if what's happening is that the light isn't being sufficiently diffused as a result? After refitting the new lamp, the uneven pattern has moved, which suggests this might be the case.

Thanks,
Ian
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Old 22nd June 2012, 12:27 PM
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RH Designs RH Designs is offline
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Has a foreign object found its way into the mixing box I wonder? It's removable, just lift it out of the head. The front and top are a single metal panel which just clips onto the rest of the assembly and can be removed by pulling the bottom edge forward and hinging the panel upwards until it disengages. Maybe there's a spider in there or something .

I've not noticed any difference in the results with either lamp type.
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Old 24th June 2012, 07:26 PM
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MartyNL MartyNL is offline
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Ian this can be maddening but you need to pin-point exactly where the problem lies;
Lamp stage
Condensor/diffuser stage
Negative holder stage
Lens stage

Try to eliminate the various possiblities and physically look up and down the light pathway for foreign bodies as suggested by Richard.

In addition, try printing some blank sheets of paper to mid/dark grey without any negative in the holder. And swap different condensors/diffuser boxes to see how cleanly they print.

This is how I discovered a damaged diffuser box! Loads of marks that mirrored the marks on my prints exactly!
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Old 24th June 2012, 10:52 PM
Stan160 Stan160 is offline
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I didn't realise how much of the head could be easily disassembled!

It looks like I've found the culprits: There are two glass discs in the light path, and both had several spots of dirt stuck to them, and also some quite large specs of loose dust. Both now cleaned, and the entire assembly gently air-dusted.

After a printing session this morning - starting with a negative that's mostly low-contrast sky - it looks like the problem is solved

Thanks for your help Richard and Marty
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Old 25th June 2012, 07:53 AM
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Good news Stan, pleased the problem has been resolved.
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