By chance a friend dropped by with a broken SE today, giving me an excuse to open one up. There is indeed a focus control. Have you tried adjusting it? If it has no effect, then follow trag's advice and check R26. The focus circuitry is very simple, so finding and fixing the problem should not be too hard.

It is perhaps useful to note that a defective CRT can also exhibit poor focus, but this symptom is generally accompanied by other artifacts, such as severe ghosting, excessive brightness, or intensity saturation. It's rare, but not unheard of.