This is a little heads up on an issue I ran into a while back regarding my 3D printer and Ramps 1.4. The symptom where that although the printer worked when connected to the USB and external power the LCD went blank. When using just the external power supply it didn’t function correctly. The most obvious thing was when connected just though the USB to the computer the LCD interface worked but once external power was applied the LCD went blank. After a lot of looking around I found it was all about a 5V power regulator on the mega board. In fact on the board below you can see a three prong power regulator right there on the lower left next to the power plug. I ordered a new regulator “10 Pcs AMS1117 LM1117 1117 5.0 H342TE 5V 1A Voltage Regulator SOT-223 from eBay and was good to go. In all honestly I just bought the new board on amazon for 40 dollars and fixed the old one later for a backup.
Thank you sooooo much! I had exactly the same problem and your post gave me the solution!
Thank you again