Activating PSU kills serial connection??!

Dear RAMPS gurus,
I've got my Prusa i3 set up with RAMPS 1.4, test firmware works Ok all the motors and heaters are fine. But when I connect to Pronterface or Repetier host it only connects when the PSU is switched off.
I can connect, read from the SD and even start a print, but as soon as I fire up the PSU the connection to my laptop dies. Same thing happens in the Arduino IDE, any ideas why this could be before I splash out on another Arduino and RAMPS board?
MarkR

Do you have the D2 (I think) diode installed? It's the one under one of the steeper drivers. It feeds oped from the psu through the arduino voltage regulator. If you do, you could try removing it.

Before you do though you could try and check if the 5v pins on the ramps have 5v not something else. If it's not 5v it will likely confuse the laptop usb forcing it to shutdown.

You probably have a ground loop issue between your power supply and your computer. Check the ground voltages with a DVM.

Thanks for the feedback! I inspected the arduino and there's a little chip next to the USB port that looked fried. Replaced the board and now it all works fine!