Andrew,
I noticed in your video that you were changing trigger selections of the B Trigger. Perhaps you have already figured this out, but if you haven¡¯t, the A Trigger controls are below the Time/Div knob. Mess with those a bit. You should find some sensible settings such as Auto mode to get a repeating sweep. You might also select XY on the Time/Div apply sinusoidal signals to both CH1 and CH2. You will need fiddle with Volts/Div and associated position controls.
It might reveal something useful.
