Too much current being drawn through R237. Measure resistance from side of R237 opposite the one that goes to the flyback. If very low, check for solder splashes. If none, likely culprits are shorted T115 or D139.
While you are in that area, list one side of R235 and check value (200kΩ). If excessively high, the 200V video B+ will not bleed down when the monitor is powered off. This will be most noticeable when you get a bit of shock while removing the neck board. Guess how I know!
If you haven't already, check the value of the film caps in the horizontal output circuit - C170, C172, C174, and C175. Shorts are highly unlikely, so need to check value or, being known failure parts, just replace them.