Need help with purity adjustments

[matt_s78mn]

Ok, so i'm trying to tackle some half finished projects that have been sitting around for a while. One of the projects is to do convergence and purity on my CTC-31. I spent over 3 hours on it yesterday and got the convergence aligned pretty good, but I've got a real problem with the purity. No matter what I do with the purity rings, I cannot get rid of the color blotches on the screen (they move around when the rings are turned, and are not permanent, so i know there isn't an issue with the shadow mask) I've followed the correct procedure in the service manual, where you slide the yoke back, and adjust for a red spot on the center of the screen, but when I slide the yoke back into position, it the color blotches come back. I've even degaussed it several times.

Do any of you have any suggestions, or tips and tricks that I could try? Also, the blue lateral magnet and purity rings are not original, but are off a newer set (ctc-68.) Is it possible that perhaps the magnetic rings are either not strong enough or too strong?