This hopefully is a rather simple repair, caps or adjustments. 

But in some cases, the pincussion amplifier transistor shorts, and will do that. 

There is a separate circuit that generates the pincussion drive signal, and it gets mixed/injected onto the horizontal drive waveform to the horiz output, it gets amplified by the horizontal output transistor which drives the yoke and flyback. 

So, if the pin amp is bad, you wont get any pincussion drive for the H out. 

I have only seen this happen 1 time, and that was with an Apple multiplescan 15", when its pin-amp shorts, it causes only a half-raster with a single bright wiggly line from the center down. and it flashes/pulsates. The pin-amp is a separate board bolted just behind the flyback. 

But I digress....