On the Omega 760 lamphouse while facing the front panel, you can unscrew the thumbscrew in the top cover at the rear. Lift the rear edge of the top panel. It will pivot from the front, allowing you to remove the cover. This gives you access to the lamp cover (held by two thumbscrews to the left), the heat-absorbing-glass filter in front of the lamp, the filter mechanism in the center, and the Styrofoam mixing chamber & plastic diffuser assembly at the right.
This will allow you to study the filter mechanism and how the parts interact as you turn the dials. The top knob for the cyan filter is the one most-easily visible. All three filters work the same. There is a plastic or nylon cam fixed to the dial shaft of each adjustment knob. The filters are fixed into a thin aluminum carrier plate. The plate has a black cam follower attached whose cylindrical contact rod rides on the periphery of the cam.
The filter carrier plates are guided and retained by two black shafts that fit through vertical elongated slots in tht plates. The shafts maintain the position of the plates from left to right & front to back, while allowing free movement of the plates vertically.
Each plate has a small-diameter normally-compressed pull-type coil spring attached to the bottom of the plate at the top of the spring. The bottom of the spring is anchored to the bottom of the housing. Each spring exerts a constant downward force on the plate. The filter is fully into the light path when the plate is in its lowest position. The filter is completely out of the light path when the plate is in its highest position. The position of the cam (controlled by the knob) determines the vertical position of the plate and it its filter (how much the filter enters the light path).
Some of the filter mechanism can be seen in the bottom right photo on page 16 in the following PDF for the Omega 760 Dichroic Lamphouse.
http://www.jollinger.com/photo/cam-coll/manuals/enlargers/omega/C760 Modular Lamphouse Systems.pdf
To the left is the cyan filter knob. The lager diameter wheel (shown almost edgewise) is the calibrated filter dial. The numerals on the dial are visible through the filer-setting windows in the front panel.
Immediately to the right of the filter dial is the cam (covered by the black cam follower of the cyan filter carrier plate). The filter dial and cam appear to be a single piece. Each cam has a travel-limiting “shoulder” at the 0 and 200 positions that shoulders against the side of the cam-follower rod. As with cameras, lenses, and any other delicate mechanism, if something doesn’t move in a relatively easy manner, don’t attempt to force it, as doing so will damage some part-for which replacements are no longer available.
The end of the cam follower rod just covers the periphery of the cam. The cam is only about 2 mm in thickness. If the assembly is a bit out of position, then the cam follower might drop off of the edge of the cam, I have seen this in a C700 dichroic head. An adjustment brought the lamphouse back into proper working condition.
Without examining your unit I can only speculate on the problem. By studying the mechanism patiently, you should be able to diagnose the cause. Hopefully, this will allow you to remedy the problem. If the problem is too difficult, you could find another C760 dichroic head in good order via eBay and keep the current one for spare parts.
