That symptom does sound like it could be related to the vertical deflection section. I?d also take a close look at any electrolytic capacitors around that area, even if they don?t show obvious signs of damage. On older CRTs, dried-out caps can cause some really strange display issues before they fail completely.
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For full context, visit
https://www.electrondepot.com/repair/crt-display-fault-4381624-.htm
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