Oh so close, but now what (new problem)

[DaveWM]

I assume its just a low voltage tap off the tripler.

I don't like to replace a lot of parts an one time, unless its clearly bad. So if the diode checks ok then I would leave it for now. You have spent a lot of time on this set, I want you to find the EXACT problem. With the scope hopefully you can narrow down the issue (lets hope that ringing in the brightness is it) and then checking/replacing one part at a time in the effected area and checking the scope each time will get it fixed. I know you would prob rather just get it done once to avoid all the chassis pulling, but for your own education process I think its better to see the effects of each part (I don't know for sure if the damper has this effect, so its not just for your education ).

good luck, your doing well. The scope is a real asset, but only to a point.