Yeah, Cris, that is a classic portrait of a powerful magnet near the CRT. I had a friend who had placed a home speaker near a monitor and it produced exactly that pattern.

At first, we verified that the speaker was the culprit by temporarily removing it from the area. Knowing that the cause of the problem was the speaker, but not having a better place to put the speaker, he put it back. A year later he discovered that the "stain" was there for good even after he removed the speaker.

Earlier in the thread, you asked how arcade owners handle this problem. Well, haven't you ever noticed, in most arcades, how the pinball machines are in their own "row", set aside (more or less) from the video games?

Of course, I'm sure there are such things as a "well made" pinball machines that are magnetically shielded. As well as arcade video machines that are similarly shielded to protect them from outside sources of magnetism. It's your unfortunate luck that you've got two machines, neither of which is shielded.