The only other thing I can think of is to try a lamp specifically designed for use in an LPL enlarger, if there isn't already one installed. I can't recall whether there was something out of the ordinary about the bulb in mine.
Edit: I lied, I thought of another possibility: If voltage is running higher than expected, I'd expect the lamp's color temperature to rise, and hence, it's spectrum to shift more towards blue. If you have the means to do so, try measuring to ensure that voltage with lamp installed really is 12 volts or close to it.
Print #3 looks like what you might get if the cyan filter itself is stuck in the light path. When I look at the first photos you posted of the light within the light source, that light is also surprisingly cyan.
Perhaps the cyan filter itself has come loose from its control.
Edit: I lied, I thought of another possibility: If voltage is running higher than expected, I'd expect the lamp's color temperature to rise, and hence, it's spectrum to shift more towards blue.
I don't even know where to get a multimeter to measure the output from the enlarger's adaptor.
1) M = 170 (max), Y = 200 (max), C = 0. PLUS: 10M gel and 1/2 of a 10 Y gel (see pic) taped to the mixing box entry aparture.
2) Exactly as above, but with the M, and Y filter settings decreased to M = 160, and Y = 190. Gels unchanged. Not kidding.
So, the only difference between 1 and 2 is -10M and -10Y and you get a massive shift from reddish to cyan on paper? And then further decreasing the filtration (albeit much more) shifts it back to full red?!
Have you established that all differences you get on paper are the result of changing the filtration on the enlarger? Like, you can do 5 prints at M170 Y200 and all will be reddish and then decrease both by 10 and next 5 prints will be cyan?
I have LPL7700 (I think that's basically the same enlarger as you have), my typical filtration with this enlarger is in the range 70-100. That's higher than you typically see in other enlargers, but I have an IR filter installed (as per manufacturer's recommendation for colour printing).
IR filter huh? Never heard of using one when making a print. Is the difference noticeable?
For clarity, the IR filter is used to reduce how much heat reaches the film, thus reducing the problems with negative movement during exposure. A properly positioned IR filter should have either no affect on the colour balance of a print, or a minimal but consistent affect.
Possible solution found.
I noticed that a tiny notch at the top margin of the bulb was not fully seated in the bracket (as seen in the attached pic). Not at all obvious even on close visual inspection - only discovered when I attempted to switch out the bulb as a last resort. So I secured the bulb, and did 2 test strips: #1 with Y/M both set to 100, then #2 with Y/M maxed out. No gels, and C=0 for both test strips. Both showed a heavy cyan shift. I'm going to try making some more test strips tonight with more reasonable Y/M settings around 70/60, etc and see what that shows. If this solves the issue, then everything may have stemmed from only a slightly malpositioned bulb, possibly loose enough to allow some movement between prints, resulting in unpredictable results. But we'll see tonight.
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