[old_tv_nut]

I skipped quickly through your video, so may have missed if you adjusted the sync level knob. Youmneed not only to use NORMAL sync, but also turn the level knob toward negative a bit until you are triggering only on the sync (if it's there) and not the video pulses.

Refer to the link that zeno posted to see that the sync should be the most negative part of the signal.