iMac screen goes black 2-4 secs after boot

I had a problem with the backlight on my 27" i7 2010 iMac. It was the well known loose connector....

The wires have now been soldered and when I turn it on the white screen apears correctly for a few seconds, but then the screen turns off. If I put the screen to sleep and wake it, then the screens turns on again for 3-4 seconds before turning off. I can see the backlight is working correctly. When switching to secondary monitor (TV) everything works fine.

I am confused about this beeing a hard- or software problem??

I tried CMD+Option+P+R but no help. Also tried a lot of boot menus like holding down C or D or Option and the like during boot, but same result. Screen goes black...

How long ago was it that Apple repaired your iMac? I would take it on back to them and have them diagnose the problem. It appears to be hardware related and possibly connected to the repair they did.

Apple did not repair it. I had a local TV repair guy do it. It may well be hardware error, but it just seems strange that when I put the mac to sleep and wake again the screen turns on fine for about 3-4 seconds with normal brightness.

Ohh dear the local TV guy. Think you will need Apple to take a look at it now.