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This is very odd.
The tube(s) and camera produce a good picture apart from the spot, so all tube voltages, scans, etc must be OK.
The newer Bi-Alkali IO works ok, no spot, resolution a bit down but o/wise OK.
The older soda glass target tubes have the spot which brings me back to my first thought that is is a tube fault, even thought several different tubes have been tried.
next step might be to try a known good tube tested in different camera, and to try your tubes in the other camera.
As an aside:-
I am told that IO tubes, and for that matter other high vacuum devices, have a failure mode were natural helium in the air "seeing the vacuum in the tube" goes though the glass as the helium is so small. The helium then accumulates in the tube.
If anyone can confirm or deny this I would be interested to hear what is said.
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