Black Screen iMac 27

Hy ,

I have an Apple iMac "Core i5" 2.8 27-Inch (Mid-2010)MC511D/A.

When I press the power button i have a black screen, the fans are spinning, backlight working, you can hear the gong. How can I diagnostic a faulty motherboard or a faulty video card? I have already tried with an external monitor, but no signal.

Any ideas ??

Caps Lock LED on wired keyboard doesn’t light when pressed.

I have checked the Diagnostic LED's and all 4 are on. LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found. If the fourth LED is not visible after power on:

• Verify cable connections between LCD panel and logic board
• Inspected LCD display cables for cable damage
• Verify external video functionality, and according to result check the following items:
  • If external display works then verify/replace the LED backlight board
  • If external display works then verify/replace the LCD panel
  • If external display does not work verify/replace the logic board

Please, any suggestion what should I check before changing the logic board. I still suspect the graphic card .