nbohemen: Rcu Gcu en Bcu (cut off levels). What do they do?
Thought I'd answer the question for future reference.
The "cut-off" points for a CRT refer to the grid control voltage (and thus electron flow) where the phosphor dots cease to emit any light. This sometimes changes when the tube ages - material from the cathode evaporates and contaminates the grid surfaces, or some similar process.
As the red, green and blue circuits are mostly separate, what this setting effects in a colour CRT is more intuitively called "black balance" by camera control operators - quite simply put, whether black is black.
Aging components can also put this setting between R, G and B out of balance. This is obviously originally set at the factory and you can't just copy values between sets or use some "correct" values, they depend on the particular tube and the component tolerances in the set.
The adjustment should be set with a fully warmed TV, colour saturation set to 0, using a gray scale test image. If you see any tint in the darkest areas, adjust the cut-off levels to make it go away. This will not be easy at all if you don't know how the RGB colour system in television works.
If - and only if - you see any discolouration in the brightest areas AFTER setting the cut-off levels, should you use the respective drive level settings to arrive at correct white. It is very easy to make a mess here!
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ALWAYS make notes of original settings before messing wth the service menus! Better yet, don't even go there unless you have the skills and the test equipment needed to set them correctly!