Here is the denouement to Angry Kitty’s perplexing filter setting problem with her Omega C760 Dichroic Lamp House that behaved as if it were haunted.
I sent her the following on December 27, 2010 after examining a C760 dichro head:
“The top cover comes off easily by removing the single knob screw at the rear of the top panel. Lift the rear edge of the top panel to remove it. That exposes the Styrofoam mixing chamber and the filter mechanism. This is exactly what you’d do as a first step in gaining access to the bulb to change it. The Chromega C760 is a much easier to disassemble to this point than the Chromega B head, as no small screws are involved.
As you turn the filter knobs you can see how the rotary cams lift the individual filter panels upward against their hold-down springs. You may need a flashlight at this point to illuminate the mechanism so that you can see it.
This is almost identical to the Chromega B mechanism, except that the C760 has white plastic or nylon cams instead of the Chromega B’s aluminum cams. The other difference is the white-light cam plate parallel to and just inside of the front panel. The Chromega B has no such feature.
If you examine the action of the cams as you rotate the operating knobs, I think that you’ll see that the return springs couldn’t possibly make the filter knobs rotate.
Here is what I think is happening with yours. The white-light mechanism that lifts the 3 cam plates upward moving the filters out of the light path is a clever design, but the design is crudely executed.
By experimenting, I found that I could engage the white-light lever and then return it to the “normal” position—except that the mechanism binds in the slightly raised position. This holds the filters’s cam followers at least partly off of their cams, and, therefore, not fully engaged as the numbers on the panel indicate.
Then with a slight touch or vibration to of one of the knobs, the lever, or even no touch at all, the springs can unexpectedly move the white light cam plate fully back to the neutral position. The springs on the filter plates then snap the filter plates back down until the follower rods seat back onto their respective cams. Of course there is an abrupt change in filtration when this happens.
If this is the case—and I’m hoping that it is, as it’s the only solution that makes sense—then all you need do to prevent this from happening again is to make sure that the white-light lever is fully into its neutral (horizontal) position, and from then on, use only the filter knobs to move the filters. Don’t touch the white-light lever, as it’s unreliable and can spoil the settings by hanging up as I’ve described.
There is nothing to be gained by installing coil springs or elliptical spring washers and attempting to do so might wreck something. For example, you might have to apply a great deal of force to get a knob off and break it, bend the front panel, or break one of the cams. I don’t think that replacements are available. Even if they are, they’d be difficult to install and expensive. Any such tampering will gain you nothing.
Other than this annoying foible, the Chromega C760 is a decently made dichroic head and should give good results for many years. If you get really attached to this machine as I have become attached to my B66XL Dichro, then it might be prudent to look around eBay to find another clean C760 dicho head as a spare.
Angry Kitty replied by PM on Saturday, February 5
“So it turns out you were right about the white light lever.... I didn't use it for a bit and nothing happened, then when I started again I noticed the dials were moving up. No ghost after all!”