[GrimJosef]

I'd use whatever input arrangement you had when you were measuring 2V AC across the load.

Start with 5ms/cm. One 50Hz cycle will be 4cm long, one 100Hz cycle will be 2cm long. Get it triggering stably then measure the cycle length. Does the waveform look like 2V RMS - roughly 6V peak to peak ?

If all you see is a broad band of light then the cycles may be so short (frequency so high) that you can't make them out. In that case keep speeding the timebase up (smaller and smaller secs/cm) until you can make the cycles out. How many msec or usec is one complete cycle ?

Cheers,

GJ