It's usually easier to correct geometry issues, than to correct convergence issues. Frequently convergence can only be corrected by way of ferrite or magnetic strips along the tube, can be a real tough nut to crack just the right way. In the western world, where any CRT you find to buy will surely be second-hand, the best advice is to ask to view the unit before purchasing. If you want it for a specific purpose, be it DVD playback, Bluray, gaming or whatever, bring your equipment. The worst any seller can say is "no," and they won't likely say that either as they want to unload it. Just make sure you can take a look at it working to see how it is before buying, and you will be as reasonably safe with your new tube as you can be. And once you have a well-working tube, if you don't subject it to a lot of abuse, it should remain so for a good long while.