Heated bed started shorting power supply, help!

A friend and I have been building our own custom RepRap for the past 2 weeks (it's been a journey...) and we're finally up to the printing stage. However we have a problem that I can't seem to find anywhere else, although I'm sure it's out there somewhere I'm just tired of scouring wikis, forums, youtube and blogs so I thought someone here might have an answer.

The problem is: Whenever I try to turn the heat bed on, the power supply immediately trips and turns off.
Yesterday the heat bed was working perfectly, getting to temp quickly, running at the same time as the extruder or by itself and we wired it such that the LED lit up. This morning we changed the location of the printer from the workshop to the dining room, mostly because of the cold. The move was entirely uneventful, no components bumped or anything like that, we were really careful. After the move we reconnected everything and then tried to turn on the heat bed and the PSU tripped. The LED does not flash at all, if that's useful info and the thermistor is still staying around room temp.

Printer details:
Power Supply: 500W ATX
Heated Bed: MK2B Dual Power Heated Bed
Controller: Arduino Mega 2560/RAMPS 1.4
Firmware: Repetier 0.92

Tried: Lots of replugging, thorough inspection of cables.

Haven't tried: Removing bed (we JUST GOT IT LEVELLED FOR THE FIRST TIME (what a pain) ) for proper inspection of where the cables are soldered on, they were attached very well though, appropriate amount of solder used...

After some research I'm thinking the problem -could- be around the MOSFETs or the Polyfuses, I don't really know how to test them though. One thing, the Polyfuses definitely are not getting hot...

So any ideas what could be causing this? Bearing in mind there were no obvious bumps to the MOSFETs or Polyfuses or anything, it was simply moved from one place to another and it was working perfectly 12 hours before.

Happy to give any other information/do any testing of any components (I have fairly limited electrical knowledge so explicit instructions would most likely be required...)

Huge thanks to anyone who has a look and will be very grateful for any help.

Cheers
Chris.

Do you have an appropriate load connected to the 5V output of the ATX PSU? Without that, you will quite likely get overvoltage on that +5V output when you increase the load on the 12V output, which will trip the PSU.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Also,as a test, have you tried disconnecting the heatbed wires from D8 on the Ramps and then re-connecting the wires directly to the power supply? If the supply shuts down, then you may have a short between PS and bed or on the bed itself. If not, then the Ramps needs looking at (or the PS can't handle it)

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Waitaki 3D Printer

Thanks for the responses guys, greatly appreciated! We figured it out! The solder between the 2 and 3 contacts on the bed was insufficient so I basically slathered the whole thing in solder and now it's working fine!

Thanks again guys!

Do you have one of those mirrored glass plates on top?

I did and the bugger shorted out the heat bed contacts. Fried me 1.4 board