Thanks for the link. I did play around with the focus, and it did appear to be working. Not sure whether its the kind of thing that just works or doesn't, or if it slowly degrades over time.
I'll try one thing at a time, and give the cap kit a go first.
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Next question: I noticed that the color dials on the back of the tube have two settings for each color. A gain, and a cutoff. What is the difference between the two?
I seem to notice more effect on the picture when I adjust the gain.