cinema display HD standby light flashes

Hi,

I have an old Cinema HS Display and the standby light flashes short/long/short. Regarding to this document that means that sth. is wrong with the power supply: http://support.apple.com/kb/HT2811. However, I tried it with a power supply for the same screen that my colleague uses at work and it still happens and flashes with the same pattern.

Does anybody have an idea what might be wrong?

Thanks,

Marco