You need a Degaussing Coil, sets of that vintage often needed degaussed whenever they were moved.
http://www.ebay.com/itm/9317-GC-DEGA...item3a82bccfd1
It should have one built in but it may not be powerful enough to degauss a badly magnetized tube.
I would set the purity after degaussing and in the orientation that you intend to use the set in and not by compass points, usually once it's set you can just degauss it when you move it and it'll be OK.
You don't have to get a red dot, just get as much of the screen as possible red and as centered as possible, then slide the yoke forward until the entire thing is red.