Oh, I forgot to say: the trim cap looks like the same kind that was used in some 500-series scopes. With great care they can be disassembled and cleaned.
Here is a video showing the process
If you watch the video (it's a bit slow going) the video creator notes that one of the metallized disks is corroded which limits the upper range of the capacitor. You may have a similar problem.
-- Jeff Dutky
