Hello, I am having problems with my electronic (mega+ramps 1.4). You can see what happens when plug into the 12 volts ATX PSU https://www.youtube.com/watch?v=RTvhzJCqcD8&feature=youtu.be . I have been printing several hours with my rostock mini, and yesterday afternoon, I replaced a pololu (it began to fry). If I connect the arduino by the usb, the lcd turns on, and every led is on, but when I unplug it, and connect by 12 volts, nothing wake. I try with the ramps plugged in the arduino, and in the ramps alone. I have two sets of electronic, the left one is a very nice and well done ramps, from a spanish guy, and the right one is a chinese ramps. The arduino is the same model from the same shop. By usb, I can go up, down and push into the menus of the lcd. The psu is ok, it gives me 13.5A. Would apreciaty any kind of help 
would check the D1 diode and see if it is there.
@Wayne_Friedt how could I check it? With the continuity mode? It does not seems to be damaged. Thank you!
@Wayne_Friedt It does not beep, also, the 5v (vcc) terminals give 0 volts, and 12 (auxiliar) gives 12 volts (both ramps)
It happened to me too. After googling some folks said the voltage converter on the ramps is not working.
@BoonKuey_Lee some people told me that, but I can not find the converter on the RAMPs wiki. Will try again. Did you repaired it? Is it complicated? (I am not a very handy man with soldering) Again: THANK YOUUUUUUU!!!
do you know guys where the voltage converter is and what model? there is not any mention about it in the wiki mounting guide
the issue is in the Arduino voltage regulator. see the link image
but it happens to me without the arduino. only the ramps alone, arduino is several meters far away
I changed the mosfets, and it run ok, until a flame appeared and in my wrath, put in the trash all the arduinos and ramps. But it was the hot end’s mosfet (only the one I have used, not the other two)