Another common cause of distorted ("folded over") vertical scans is that the vertical (& audio) output stages are fed with a highish voltage (usually around 150v) derived from an overwind on the Horizontal output/ EHT transformer.
The 15kHz output of this is rectified using a halfwave rectifier, then filtered by a fairly small caoacitance, but high voltage rated Electrolytic capacitor.
(This same system was used in the valve days, when it was called "boost HT")
If the Capacitor fails, the filtering is radically degraded, affecting the vertical scanning.
If the rectifier fails, the same thing happens, but more so, as the vert stage supply reverts to the quite low "start up" voltage, & will be unable to perform its function.
Fix?----Easy, replace the rectifier or capacitor, or both.
But wait! there's more!
A trap for young players exists, in that the capacitor has to be a "high ripple" type.
(Well, that's the old terminology, which I guess, translates to a very low ESR in today's talk!
The rectifier is a "fast recovery" type.
If you fit ordinary old caps or rectifiers, they will work OK for a few minutes then let the magic smoke out!
No prize for guessing how I know this!
Strangely, I first came upon this problem not at work, but on my Mother in Law's TV.
It had the classic "fold over" symptoms, but my old Fluke 77 read the largely unfiltered output as around 103 v DC-----Not the 150v it was supposed to be, but, I felt "not low enough to cause the problem".
When I dragged an old BWD home from work, I was confronted with a "copybook" display of a unfiltered half wave rectifier output.
From this, I learnt another useful lesson.
DMMs can be fooled by strange waveforms, although more modern ones may be able to tell the difference.
I'm not sure, as I don't have a modern DMM.