another 1920x1200 LCD problem

I had a Samsung 2443 sitting in a cupboard for ages. I plugged it in to a
laptop using VGA cable, and screen is blank. If I shine a torch onto the
screen, I can see the image, meaning the backlight is shirking its duty.
However, if cable unplugged, you get the moving box telling you there is
no input, check cable. So the backlight is working.
I can't see anything in VGA pinout that would tell monitor to turn out
the backlight.
It works using the DVI for about 10 seconds, then backlight goes out,
but the power indicator LED stays blue.