Trouble with SC1224 (black screen / white diagonal stripes)

Hello everyone,

I've bought a used (so-called functioning) Atari SC1224 for my 1040 STE. Before I was using an Atari SM124 (monochrome). I switched it on and the screen got a little brighter and white diagonal stripes were moving from top to bottom but no desktop appears. I didn't modify the switches on the back for sync or size. In addition the monitor is whistling at high frequency.

Any ideas what happened to the screen?

I'll try to make a photo of it.

Blackswan