Note

@jcs As others have said, this is a vertical linearity problem. The vertical deflection waveform should be a linear sawtooth wave, but it this case it's not linear. There may be an adjustment for it in the vertical timebase circuit. It may be a coil with an adjustable ferrite core

@jcs "It's broken"

@jcs Pure awesomeness.

@jcs My Dad, who was a jack of all trades and collected a lot of old hardware in his day had CRT's that did this. It is warming up. The explanation goes something like this: "The [CRT] dot stretches into a thin horizontal line as the vertical deflection kicks in before the horizontal fully syncs. "

My Dad's old Tektronix oscilloscope CRT's would start as a single bright white or green dot right in the center. The electron beam would be firing but the deflection coils had not warmed up enough yet to sweep the dots across the screen.

@jcs @paulrickards I have limited CRT knowledge, but that’s a failure of vertical linearity: lines far too widely spaced at the top, and bunching up towards the bottom. (Actually, on a second look, it doesn’t look like it is bunching up at the bottom; I think the sweep is starting out way high, above the top of the tube, which results in it ending far above the bottom of the screen.)

@jcs bad vertical linearity. This is usually adjustable. Check the back of the CRT.