RAMPS power issues. [FIXED]

Hi, I am having power issues with my RAMPS board and I am not very seasoned in troubleshooting circuitry. I have looked around the net and this forum but was not finding much in terms of how to generally go about testing what may be wrong with it.

I have tested the Mega and it works fine, I can even connect pronterface to the printer and get accurate temps as long as the 12v power supply is off. As soon as I turn it on, I lose the connection. With the power on there is no way to connect and move axes but the fans get power and the stepper drivers have a normal voltage at the pots. This makes me think it is power supply related (the PSU itself or the power supply circuitry on the RAMPS board) so I have tested that there is still 12v coming from the PSU and nothing on the board looks burnt but beyond that I am not sure where to start.

I was wondering if anyone could give me some pointers as to where to go poking my multimeter. Could it be a fuse? How do I test that? That sort of thing or even links to other guides if you have them. Any help is appreciated.

EDIT: Adding things I have found so i can keep them in one place.

- [forums.reprap.org] - Easily troubleshooting MOSFETs!

Edited 4 time(s). Last edit at 01/27/2015 11:14AM by casht0wn.

I'd start by disconnecting anything external, then slowly add one device with the power off. Turn on power. Does it reset? If not, repeat adding another device until you find the one that causes the short. If it resets with nothing connected, I'd look for bridged solder joints.

It works.

I went through the mosfet tests from the thread I found and I got only about 10.8 volts on step 6 (M106 to turn the fan on and then test the between the +5A input and -D9) and the led was barely on at all. I was not sure if that meant some kind of partial short but it at least stayed on when I powered it up so I plugged it in to my laptop to see if I could keep the serial connection when I powered it with everything else disconnected. This only angered it. It let out a puff of magic smoke and I began to prepare for the worst. I didn't know what else to do so I visually looked it over and didn't see any burnt components then I tried testing the mosfets again and this time I got to step 6 and got 12 volts! All the rest of the steps looked good too so I started adding in piece by piece and everything seemed to work. This left me with more questions than answers but it works. I am printing a test now.

If anyone cares to try a guess at what might have happened here, I'm all ears cause I'd love to know, but I am just happy I can print again.