I actually have a IINM 5155 replacement crt module. It's sort of a scratch and dent model, a company in California was selling them damn near 30 years ago, so I bought one. It's powered by 12vdc IINM. Mine is trying to burn a hole in the center of the screen, and has successfully burned away all the phosphor there. I would say about a mm in diameter.
Unfortunately in some industrial CRT monitors , especially for computer & display use, if the power supply voltage falls too rapidly at turnoff, the CRT beam is not cut off. The raster scan collapses, but for a while the full beam current remains, concentrated in the screen center. After a while it burns the phosphor and desensitizes it leaving a dark mark in the center.Its a real shame as it damages CRT's which now have to be regarded as a non renewable resource, nobody is making them anymore.
(this problem was widely known about in the TV industry since the 1930's. Remarks about it have even appeared in popular culture TV, for example the TV series The Young Ones, when somebody asked what the white dot in the center of the screen meant when you turned off the set, Neil replied "It means something really heavy, it means there is no more TV")
In any case as soon as I see this even once on any monitor of TV, I modify the monitor with an additional spot killer circuit. It is easy to do, it normally requires just an added capacitor, resistor or two and a diode. The idea is that when the set is de-powered and the scan collapses the CRT grid is held very negative for a while with respect to its cathode. Another method is to add additional capacitance to the power supply for the video output stage (mostly that drives the cathode) so the cathode voltage is held up high for a while after turn off.
In lieu of a circuit modification like this, the only thing a user can do is to turn the brightness control to zero before switch off, which sometimes doesn't completely solve it.
Since the topic of the turn off spot is a special one related to vintage computers, I have attached a specific example, of a small Panasonic industrial monitor which suffered from it badly. The diagram about the supply fall time was noted by the manufacturer.(mods might want to make this into a new thread)
The simple fix in this case for the turn off spot was to increase the filter capacitor in the G1 grid supply rail from 0.47 to 10uF.
There was another issue, at switch ON. If this monitor is powered by a very low internal resistance 12V power supply, like a car battery, or a supply with a very fast rise turn on output, the initial CRT heater surge currents were very high with a cold heater , causing a bright flash and threatening the heater, so a series resistor was added to limit the initial current.
In the case of a spot killer circuit, the way it should be done is simply dictated by its particular schematic and the easiest way to do it in the individual case.
PS: this Panasonic monitor is very nicely designed in all other respects, one being that it has a proper black level clamp in the base circuit of the video output transistor.