Answered Power source for the LCD controller ?

Last entry here: http://forums.reprap.org/read.php?219,320347
Which points to here: http://forums.reprap.org/read.php?219,545922,545924#msg-545924

So a bad RAMPS or Arduino (probably the arduino) under warranty get Robo to replace them. Out of warranty the arduino is cheap, the RAMPS you can't use a generic (i.e. cheap one) unless you do this:
http://community.robo3d.com/index.p...and-rewiring-to-make-generic-ramps-work.8356/

 

This all comes down to the way the arduino works, it can power itself from the 12v line or from the USB and it can switch
It appears the 12v side is defective on one of yours (the 5v regulator it uses to step the 12v from the power supply down). When powered from USB it uses THAT for the 5v source and that wotks.

I would suggest out of warranty try an new Arduino Mega 2560 (they are cheap) and you will need to (of course) reload the firmware when you swap the new one in.

Only reason I suggested the RAMPS as a possibility is that is where the LCD plugs into so theoretically something could be wrong there specific to the LCD ... but not likely.