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  #11  
Old 27th March 2021, 09:15 AM
Keith Haithwa Keith Haithwa is offline
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I suppose it's possible that manufacturing tolerences could mean that the filters are not of exactly the same standard between the two enlargers, or that there is a possible difference in the effectiveness of the mixing chambers.?
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  #12  
Old 27th March 2021, 11:37 AM
John King John King is offline
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Default Filtration

Quote:
Originally Posted by Keith Haithwa View Post
I suppose it's possible that manufacturing tolerences could mean that the filters are not of exactly the same standard between the two enlargers, or that there is a possible difference in the effectiveness of the mixing chambers.?
To be honest Keith I simply have no idea! My immediate thoughts were that this would make a mockery of the settings required for multigrade B&W printing where the filtration values are stipulated on the leaflet enclosed with Ilford papers and if they don't correspond with the values with the enlarger that makes it very difficult.

One very obvious difference that I can see is the 6700 gives exposure times for B&W very much shorter than the 7700. Now there is a thought, I have just remembered that I have another 7700 head wrapped up and stored in the loft, I wonder if that is the same as the one I am using. I will swap the heads over and see if that makes any difference. It is not a difficult job - about 10 mins work.

(No I am not an enlarger magnet, I just acquire them from different places )
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  #13  
Old 27th March 2021, 12:38 PM
John King John King is offline
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Default Head swap

That took a bit longer than I thought but that was down to the stored head being in the depths of the loft and I had to get a bigger ladder to reach it.

I may have solved some of the problem. When I took the old head off I found that the mixing box was not fully in place and tilted at an angle. Also the heat filter was not fitted properly and was blocking some of the light path, hence the brighter light from the smaller enlarger. Hmmmmm!

I have the NOVA switched on now so in a couple of hours I should be able to see if the problem is with the 7700 or the 6700
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  #14  
Old 27th March 2021, 03:10 PM
John King John King is offline
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Default Replacement LPL Head

The printing of this image required even MORE filtration than the old head, which puts it further away from the 6700.
The brightness has certainly improved with this print only requi

The printing of this image required even MORE filtration than the old head, which puts it further away from the 6700.
The brightness has certainly improved with this print only requiring 9.5 seconds @ F11 for a print that is slightly wider than 10x12 sheet of paper It was cropped to A4 to fit my scanner.

It wasn't my sharpest negative, but when I printed it last time I had a lot of problems getting the colour balance correct so I thought it a good one for a test.ring 9.5 seconds @ F11 for a print that is slightly wider than 10x12 sheet of paper It was cropped to A4 to fit my scanner.
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  #15  
Old 28th March 2021, 05:14 AM
MattKing MattKing is offline
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I think this is my first post here. Some of you might recognize me from APUG/Photrio. I hope I can add something that might help.
Some possibilities.
While it is true that dichroic filters don't degrade, they do get dirty, and that can affect their behaviour.
What can degrade is the light mixing boxes in the enlargers. If one is quite dis-coloured, the other might require substantially different filtration.
It would be worthwhile to switch the two bulbs between the enlargers - there can be considerable variation between bulbs.
There also could be differences between the power supplies that might result in a voltage difference which would in turn affect the colour temperature of the bulb.
By the way, as far as I'm aware, John King and I aren't related
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  #16  
Old 28th March 2021, 09:37 AM
John King John King is offline
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Default Different heads

I have had the mixing boxes out and looked at them under a good light and they all appear to be clean and unmarked. I have seen one with scorch marks which could have been a problem for someone because they seem to be shaped expanded polystyrene (I think he had the candle too close!!!!)

The heat filters are clean and being in what is in effect a sealed box I cannot see the filters being any different but I will check. They don't seem to be the easiest things to remove.

The thing is both enlargers work and so does the spare head, it is just the vast difference between filtration values from each which would make a mockery of filter values when using B&W multigrade.

Oh yes I am not related to Matt King either he should be thankful for that!
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  #17  
Old 29th March 2021, 10:41 AM
John King John King is offline
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Default Possible Solution

I have spoken to the manufacturer of DeVere enlargers today (LPL no longer make them) and the manufacture of the dichroic filters can be a bit hit or miss - it is not an exact science.

I was also told to check the power output of the transformer with a voltmeter as they can vary. The one attached to the 7700 was indicating 11.7v at the bulb socket which is just within tolerances at the lower end of the scale, but the one used for the 6700 was indicating 12.8v which is right at the upper end of the scale, hence the shorter exposures. This will also alter the colour of the light passing through the dichroic filters.

This is not a definitive result because the two pin ceramic bulb holder on an LPL has a reputation for corroding slightly inside which will also alter the voltage reaching the bulb.

Whatever the reason, this makes the values given by Ilford for their MG paper to be more of a rough guide than an exact science. I am going to speak to Ilford to get their views on the filtration values.
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