This is a general question about the operation of the Tektronix 'Shutter' system for producing a color display. I have a TDS684A that is operating as it should, but the display has an area that is not the correct color. This area can be seen even when the scope is powered down, and is shown in the first and second photo. It is an area in the lower left of the display that is a near perfect circle, and has a greenish halo around it. With the scope powered up and a signal that occupies that portion of the screen, this area displays the signal perfectly, just in the wrong color, as can be seen in the third photo.
My question is this; I have heard about the infamous 'Gel' between the CRT and the Shutter, but I do not see how the problem described here could be that kind of problem. On the other hand, I do not see how the Shutter could come up with an almost round area of defect. Can anyone venture a guess as to whether the problem is in the Gel, or in the Shutter? If I were convinced it is a Gel problem, I would be tempted to try to remove the gel as described elsewhere in this blog. If a Shutter problem, what mechanism can cause such a failure? Any ideas will be appreciated.