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I dont think there is anything wrong at this point with the HV, if you can get a decent looking raster and 25kv and it stays 25kv as you adjust the brightness up and down, then its working as designed.
The issue I think is a CRT bias prob, maybe a gassy CRT, something causing the problems getting the correct voltages to correctly drive the crt.
The HV holding steady from fully dark to full raster is key. The next issue is why the CRT cathodes (yellow) have to be so low to get the crt to conduct. That brightness range sure seems excessive I think you had -19 to +17 on the grid of the video out tube (based on the brightness pot). I don't recall checking that but it sure seems like a lot.
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