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Ok there are 3 parts that are used to cut off retrace.
You need to look at two resistors (100k,68k) and a .01 disk.
You need to check your schematic and find them, they go from the plate/transformer vert lead to the plate of the video out tube.
Its unlikely to be the problem, but its where you should look. I will do a video later showing where they are (one resistor and the cap is on the chroma board, one resistor is on the deflection board).
The more likely problem is just a CRT that is getting soft to the point where the setup bias is just beyond the point of the blanking pulse ability to blank it out (the CRT is blanked by the reg blanking pulse from the video signal AND the blanking pulse from the vert out).
It "may be" possible to overcome a weak CRT (or one with poor cut off) being the issue by the mod noted in the patent. I say "may be" since I am only speculating that the mod noted would generate enough extra cut off voltage to resolve this.
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