RAMPS 1.2 problem

Hello,
I have just finished building my Prusa Mendel, bought the RAMPS 1.2 electronics assembled, but have the following problem: When i connect the Arduino board without the RAMPS power supply, the software connects, temp readout from the termistor is ok, but when I switch on the main power supply I get disconnected, even with everything (motors, endstops, heating, termistor) disconnected... the current flowing from the main power supply with or without everything connected is 0,75A... I tried using another ATX power source and the problem is still there. Using a lab power source the current gets to 1.5A...
Please does anyone have a clue what could be wrong?
Thank you!

sorry - newbie here... please delete this, I haven't noticed the electronics folder, now posted there...

ok, anti spam won't let me post in the electronics folder (even a very changed post)... could anyone move this there? sorry again for the inconvenience...

Sounds like a short to me. Recheck your connections. Also the green wire on an ATX ps has to be grounded for the ps to work.

a short is possible, but I dunno where to look... the behaviour is still there, when I connect the board without anything connected using a lab power source...