The negative time feature could be very useful if it's real, but doubtful. I had a scope that did that too, but a very long time ago, so I don't recall the cause.
I'd think the CRT is just fine, and it's almost certainly due to crosstalk somehow between the vertical and horizontal signals. Could be in circuits in close proximity, or coupled through supply lines. Possibly a failed component in an area where X and Y share control circuitry, like say in the beam-find section.
To help in tracking it down, you can try to view the effect by setting it up for zero X deflection. Leave out the time-base plug-in, so you just get a dot near center screen (turn the intensity way down of course) when there's no Y signal applied. Then put in enough AC vertical signal to reach the top and bottom of the screen. Linear crosstalk should make a straight, tilted line. If the line doesn't tilt, then look closely at the time-base plug-in and horizontal control circuits for possible causes.
If the line tilts or is jagged or nonlinear, remove the vertical input signal and use the vertical position controls the move the dot up and down. This will give an idea if it's DC-coupled or transient/AC, a clue to how it's happening.
Go back to the first setup, then exercise all of the controls on the mainframe a bunch of times and see if there are any unexpected results. Especially look at the beam-find, channel switching, and trigger selection. You can also set up and look to see if it's reciprocal, where the X signal disturbs the Y, by swapping everything around. These should provide some clues for where to go next. Good luck.
Ed
