It is possible, if it is a CRT monitor, that there is no digital adjustment left for it. Many CRT displays have knobs inside the cabinet that can be adjusted to calibrate things that go a whole level beyond the digital adjustments that are available to consumers.
Could still be an extended service menu, but since it is not a digital display like LCD or DLP, it may require something a little more physical than pressing a few extra buttons on the remote.
At least, that was the case with my last CRT I used. Yours may very well be different.
