Hello
I was given a Panasonic TV model TX-29AS10F chassis EURO-4H flat tube with a purity problem, a discoloration visible on top left and right. After trying to adjust the purity rings, yoke position and making sure it was not a magnetization problem, I concluded it was a defective mask, the set was likely dropped. I have spent some time playing with magnets around the yoke, the defect can be corrected quite easily with some magnets but after a degaussing cycle, part of the discoloration returns no matter how near the yoke the magnets are placed. Also, too near to the yoke and the geometry and convergence are affected.
But almost casually I found that I can magnetize the tube in a way that the picture is perfect without adding any magnet, the only drawback is after a degauss cycle the problem returns, but I disconnected the degausser to fix that. It's been some days and the picture is still perfect, as long as the set is not moved it looks like a good solution. This set weighs like hell to be moved anyway.
To magnetize the CRT I use a big speaker magnet, put near the discoloration on the back of the tube and then move it away slowly. As I'm moving it away all the funny colours go away including the original discoloration - got it perfect!
I thought I would share this experience as it saved an otherwise bad tube, at least for now.