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The gun bias seems normal. The HV sag is interesting...I don't recall seeing HV sag due to anything but high gun current. High gun current should produce a bright picture unless the ion trap is misdirecting the current.
There are 2 things I would focus: on the trap adjustment and the 1M series HV resistor...those old carbon comp resistors can get flake in HV circuits from internal arcing and the like and despite measuring fine at 9V from a DMM not behave right at HV...I would either replace it or short it temporarily.
Have you adjusted the ion trap for max brightness? If you have you may want try to flipping the trap backwards and readjust...By backwards I mean reversing it such that the open end of cylinder the trap forms that usually faces the screen instead faces the CRT base. Sometimes these traps don't work optimally depending on which way they're facing on the CRT.
BTW what is your room lighting level and brightness expectation of this CRT? Early non-aluminized CRTs (which an ion trap usually indicates) were not meant to produce a visible raster with sun shining on the screen. Non-aluminized CRTs were meant to be used in subdued/romantic room light levels (ie no light shining directly on screen and enough light that your not tripping on the furniture, but not enough that you are comfortable reading paper media.
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