Trintron convergance and geometry

[dr.ido]

I've got a couple of old Trintron monitors. A 21" PVM that serves as my daily watcher at the moment and a 12" (forget the model at the moment) that serves as a test monitor/shop set.

The 21" has a great picture except for a red "shadow" around certain images. Similarly the 12" has an excellent picture except that it is a bit trapezoidal rather than rectangular.

How difficult is it to adjust these back to the way they should be?
Hopefully with a the right adjustments these will provide a great picture for years to come.

I remember years ago I tried to adjust the convergance on an old Kriesler delta gun set. I made it a lot worse before I got it any better and it took ages. Admittedly I was doing the adjustments to an off the air picture, and I probably did them in the wrong order. I don't want to repeat that this time.