lcd doesn't work without USB

Hi,I have an I3 clone with ramps 1.4/arduino mega2560.Reprapdiscount lcd screen via smart connector.
My lcd screen works fine with the printer connected via USB,but if I unplug the Usb cord,the screen stops working.
I have read all the posts i could find,and didn't find a solution that worked.
The D1 diode is present,Uglib8 is installed in the library,the smart connector cables are the right way round.
Just for the record,I tried both reprapdiscount types of screen,and they both give this same problem.
I thought the smart connector supplied the power to the screen from ramps,but obviously this is not so.
The whole point of the screen is not having to connect your computer,so does anyone have any ideas to fix this situation?
Thanks.

Does the rest of the printer work without USB connection? If not, the 5V regulator on the Mega board is dead.

... or, if not dead, the LCD draws more current, than the 5V-regulator can support ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

See #1 at [reprap.org].

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].