iMac 21.5" EMC 2428 - Black Screen

Hi there

I have a 21.5” mid 2011 Imac, its the 2.8ghz i7 version EMC 2428.

The problem started a few weeks ago. I was away from the computer for 30 minutes and when I came back the screen was grey. I powered it down and on restart the screen remained Black. Checking online I found that the cure for this was a PRAM reset, I did this and sure enough the computer booted up as normal and all was good again.

It was fine for a couple of weeks, but it happened again. I powered down the computer and when restarting the black screen happened again. I powered down again and tried to start it up again with the PRAM reset, however this time the Keyboard isn’t being powered from the USB ports, so there’s no way of doing it.

I have tried an SMC reset and have also tried replacing the powersupply to no avail. The Imac seems to start up, hardrive starts up but there is no chime. I have shone a light on the screen, but cannot make out anything on the screen. The 1st and 2nd diagnostic lights are on inside.

Any ideas on what might be the culprit?

Cheers

Kieran