Latitude E6520 Black Screen (all tests succeed )

Hi everybody.  

The screen of my Latitude E6520 suddenly went black while booting.  The screen receives light (which can even be increased or decreased from keyboard) but it does not print anything: no dell logo on boot, no bios page, no grub dual-boot options, no OS  boot screen, etc.  All these are visible and work well if an external screen is plugged in through VGA. 

I performed some tests: 

  Test 1: Built-In Self-Test (BIST):  press D and power button to boot 

  Result: The laptop screen DOES display multiple flashing colors.  It seems the screen itself is not broken. This is the only context in which the screen prints anything whatsoever. 

Test 2: Run the integrated tests: press Fn + Power Button to boot. 

Result: All Tests succeed. There is one test which prints some colored lines on the screen and asks if the lines where visible. These lines are only visible on the external screen. (if I switch back to laptop screen it will receive light but the lines are not visible).  

Test 3: Run the Diagnostics tests from the  Dell support page.

Result:  All tests succeed.  

Any idea of anything else I could try ? 

Best regards, 

emil