Need help with Sanguinololu trouble shooting

I thought I would improve my soldering skills by purchasing a Sanguinololu 1.3a kit and assembling it myself using the Reprap.org page as a guide. The assembly went pretty well until I soldered the voltage regulator 7805 on backwards and had to take it off and reverse it. After completion, I hooked it up to my computer and was able to install a Sprinter sketch to run my printer. After disconnecting the computer I hooked to board up to a 30 amp 12 volt power supply, thinking that the LED would light up on the Sanguinololu to let me know I soldered everything OK.
Well the LED did not light up and it appeared that the power supply turned off (light on supply went out) like there might be a short on the Sanguinololu.. No magic blue smoke from the Sanguinololu or the motor drivers. I disconnected the power supply to make sure it was OK and its Led lit up and I checked to make sure it was putting out 12 volts and it was.

I have since checked and re-checked every pad that I soldered and they all seem to be fine, except the 7805 voltage regulator which I am not so sure of. I have order some new 7805s and when they arrive, I will replace the one on the board.

My question is, how would you troubleshoot this problem and try to get the Sanguinololu working?
Do you think I fried the board or some component on it? Everything looks fine, no burn makrs.

Thanks in advance for your help.

... with the 7805 off you can attach 5V from an external PS to test the electronics ...

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Viktor
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I replaced the 7805, but also found a couple cold joints, most notably the negative lead attached to the board. Still not sure what was causing the short if I wasn't even getting 12 volts where it was supposed to be. I now have 12 volts where it is supposed to be and 5 volts at least as far as the LED. I will check more joints today.

Over the weekend I successfully released quite a bit of the magic blue smoke contained in the Heat bed Mosfet. Not just once but twice. I stupidly did not check the maximum amperage draw (20 amps) on the QU-BD silicone 300 x 300 mm bed heater. I think the mosfet is now stuck permanently "on" but that doesn't matter much since I have to find a different way to control the heated bed. Live and learn from my mistakes. I know how to replace the mosfet if it is stuck on.

I was also able to get all 4 of the pololu motors drivers adjusted and working on the Y, Z axes and the extruder but not on the X axis. With a driver properly adjusted on the Y and then placed on the X all the motor would do was make a clicking sound when pronterface was connected to the board and then again when disconnected. I traced and retraced and re-soldered all connections to the X axis a couple times and re-check them all with a multimeter as best I could to no avail. i also re-installed sprinter and that did not help. I am hoping the problem is something with Sprinter that I just haven't figured out.

In the mean time, I ordered another (assembled this time) Sanguinololu and I should have a rumba board coming one of the these days from Reprapdiscount.

Suggest that you change the Mosfets to IRF2804. These have a very low on resistance and I have them switching 22 amps with just a small heatsink. Watch that your power supply can provide sufficient current without the voltage dropping.