Short long short flashing light

I'm using a Mac Pro and have had the same Apple Cinema Display for five years. Yesterday it suddenly produced a black screen at start up plus a white flashing light on the bezel at the bottom of the display. The sequence of the flashes is short-long-short. According to Apple support documents this seems to have something to do with the power adaptor. I have tried changing the power brick, the power lead and plug, have reset the PRAM several times, and have plugged the display into different power sockets, yet the screen remains black and the white light flashes in the same sequence. I'd appreciate advice before I'm forced to fork out a lot of money for a new display. Thanks. (BTW, I have an older, smaller Cinema Display which works fine as a temporary substitute, and it is having no problems with the power supply.)