Colour head question

SteD · May 20, 2019

Hi all.
Have just put my colour head onto my durst m670 for the first time and when the lamp is lit I’m getting very uneven colour across the base board (please see attached pic) Haven’t made any prints yet as new darkroom isn’t yet ready. Have never noticed this on other colour enlargers I have used, but then never cared to look. Is there something wrong with the filters on the head? Or am I being stupid?! By the way, in the attached pic the filter pack is set to 70/70/0.
Thanks in advance.

koraks · May 20, 2019

I'd inspect the inside of the head and particularly the construction around the dichroic filters and the light mixer. Seems like something is very wrong there. Check for loose or missing parts; this looks like a fairly fundamental defect.

RalphLambrecht · May 20, 2019

koraks said:
I'd inspect the inside of the head and particularly the construction around the dichroic filters and the light mixer. Seems like something is very wrong there. Check for loose or missing parts; this looks like a fairly fundamental defect.

I agree,something seems misaligned. This will lead to uneven loighting and contrast.

Martin Rickards · May 20, 2019

Mine's a Meopta colour head, but that looks like the mixing chamber is totally absent. There's no way that I can see to get that effect with the mixing chamber in place.

Chan Tran · May 20, 2019

Martin Rickards said:
Mine's a Meopta colour head, but that looks like the mixing chamber is totally absent. There's no way that I can see to get that effect with the mixing chamber in place.

Yes I think so. The mixing chamber is missing. The dichroic filters would partly cover the lamp depending on your setting. So without the mixing chamber that what you would have.

SteD · May 20, 2019

I’m not entirely sure what the mixing chamber looks like. Looking at the head from the bottom, light passes from the lamp, through a piece of glass, through the filters then passes through a 90 degree bend of course reflective material, through the condenser to the lens.
Is something missing here?

Martin Rickards · May 20, 2019

https://www.thedarkroom.co.uk/used-durst-setobox-66-mixing-chamber.html shows the 6x6 model which would make do for 35 mm. There's probably a special 35 mm model as well.

ic-racer · May 20, 2019

SteD said:
I’m not entirely sure what the mixing chamber looks like. Looking at the head from the bottom, light passes from the lamp, through a piece of glass, through the filters then passes through a 90 degree bend of course reflective material, through the condenser to the lens.
Is something missing here?

Through the condenser! This won't work, as you see.

mshchem · May 21, 2019

Not sure if this helps.

SteD · May 21, 2019

mshchem said:
Not sure if this helps.

It does kind. That’s basically just what I’ve done.

SteD · May 21, 2019

So is the mixing box the siriocon 50 or the 90 degree coarse reflective chamber? Or both?!

AgX · May 21, 2019

SteD said:
I’m not entirely sure what the mixing chamber looks like. Looking at the head from the bottom, light passes from the lamp, through a piece of glass, through the filters then passes through a 90 degree bend of course reflective material, through the condenser to the lens.
Is something missing here?

The beam coming from the lamp is only partially filtered at the filter stage. And that actually is the idea behind that stage. By partially letting it pass a filter to achieve only a degree of filtration. However when employing this kind of filtration technique the parts of the beam must be blended again to be able to illuminate the film at the intended filtration. This blending typically is done by reflection of aluminium or white surfaces. These surfaces form the so called mixing chamber.
At some models of enlarger the the size of such chamber differs depending on the film format and the chamber can be substituted respectively. That again yields the chance to run the enlarger without any mixing chamber.

SteD · May 21, 2019

Ahh. Ok that makes sense.
Well the mixing chamber is definitely there and intact.

SteD · May 21, 2019

Here’s a shot of the mixing box.

AgX · May 21, 2019

I admit I am puzzled... Actually I meanwhile think that with a mixing chamber missing likely hardly any light would enter the lens at all anyway.

We must have overlooked something. I guess I better sleep over it...

cowanw · May 21, 2019

Is that box really that dark?

mshchem · May 21, 2019

This is what you should have

Martin Rickards · May 22, 2019

Is there a dedicated mixing chamber for 24x36 mm format or just the 6x7 one? Whoops, I see there's a Vegabox 35 as well for the M670. On either there should be a frosted glass/translucent plastic window immediately above the negative carrier. https://www.thedarkroom.co.uk/used-durst-vegabox-35-diffuser-box.html

AgX · May 22, 2019

A frosted pane at that position would not blend the light sufficiently to overcome the OP's issue.

SteD · May 22, 2019

I see. I have one of these..

Which appears to be something else altogether. Whoops. I thought it would turn out to be something ridiculously obvious.

Martin Rickards · May 22, 2019

AgX said:
A frosted pane at that position would not blend the light sufficiently to overcome the OP's issue.

As in inegral part of the mixing chamber, it would. Light is mixed in the chamber and exits through the window,

Martin Rickards · May 22, 2019

SteD said:
I see. I have one of these..

Which appears to be something else altogether. Whoops. I thought it would turn out to be something ridiculously obvious.

That is the lower condenser for the B/W version with a 50 mm lens. It isn't used, I think, with the colour head and the mixing chamber provides a diffuse light source. Using just the Siriocon would produce the effect shown in the opening post.
P.S. You can use either the Vegabox 67 or the 35 for 35 mm with some change in exposure, but only the 67 for 120 film.

SteD · May 22, 2019

Got it. Problem solved. Thanks everyone!
Going to order a Vegabox now.

AgX · May 22, 2019

Martin Rickards said:
As in inegral part of the mixing chamber, it would. Light is mixed in the chamber and exits through the window,

The mixing chambers I have in mind all have a 45° diffuse reflector, for reflecting the unblended light back into the chamber. Its absence may keep any light from entering the lens

Martin Rickards · May 22, 2019

AgX said:
The mixing chambers I have in mind all have a 45° diffuse reflector, for reflecting the unblended light back into the chamber. Its absence may keep any light from entering the lens

On my Meopta head, the filtered light enters a semi-enclosed expanded polystyrene box at the bottom of which there is a translucent plastic window that sits right above the negative. From what I've seen, the Durst enlargers use a similar system.

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