It's happened to everyone, you accidently plugged something in wrong or pulled too much current through your arduino. Afterwards, you heard a pop, got that wonderful burnt out electronics smell and a whiff of smoke. You probably think you just fried your arduino, but that may not be the case.
This guide specifically applies to an Arduino Mega 2560 (from eBay) but may work with other boards like a Uno etc. It will try to describe in detail how to diagnose a faulty regulator and replace it. It won't even attempt cover all the other possibilities of a dead arduino so if this isn't your problem post in the comments and I'll try to help.
Step 1: What Is a Regulator and Why Did It Fail?
I'm by no means an expert, so I'll do my best to explain what the regulator is and why it may have failed. The main job of a voltage regulator is to regulate, or limit the voltage going to a device. Many components of the arduino board and sensors you might use usually can't be exposed to the same voltage as your power source. (if you aren't using USB)
For example, the ATmega processor on your board is very sensitive to the voltage it receives. If it is exposed to more than 6v it will blow up - literally. Take a look at the datasheet (it is attached, I've also attached a summary screenshot) so you can see the various "absolute maximum ratings". Absolute Maximum means just that, this is what most datasheets will define it as:
"Stresses beyond those listed under 'Absolute Maximum Ratings' may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability."
So now you know what a regulator is, but why did it fail? The most common reason that I know of is you accidently shorted the 5v output to ground. This means a huge amount of current tried to flow through the regulator, burning it out. Another common reason is hooking up one or more devices that draw more current than it's rated for, like a motor. (which is basically the same as the first) You could have also plugged the power source in backwards and exposed the regulator to negative voltage - most things don't like this. There are many other reasons you could have blown up a regulator so don't fret if it wasn't one of those.
Step 2: How to Diagnose a Blown Regulator.
Before you go ordering parts and de-soldering components, it's best to make sure you actually have a blown regulator. Sometimes it will be immediately obvious that the regulator has blown. If it gets very warm when nothing else is connected or has obvious physical damage it's probably dead. Usually you will see a little burnt bubble or two, or in more extreme cases a part of it could be missing. See the photo.
If you can't see any physical damage then get out your multimeter and another arduino mega if you have one. You want to find the regulator - it's between the USB and DC Barrel Jack connectors. (see the photo) Set your multimeter onto continuity and test all the pins. The stock arduino regulator should have no continuity between any pins except for the middle one and the tab sticking out the back. If you get continuity between any other pins, you have a bad regulator.
If you still can't confirm if your regulator is dead, try leaving a comment and I'll see if I can help.
Step 3: Replace Your Regulator
If you've confirmed that your regulator is faulty, it's time to replace it. Check out eBay for some really cheap ones with questionable quality or order from somewhere like RS online. Most 5v regulators should work, just keep in mind that not all pinouts are the same. If it differs, you may need to solder individual wires back to each pad on the arduino.
The exact regulators that I used where bought from RS Online. They have a huge selection of parts and in Australia offer free next day shippinh on everything. (I bought the bag of regulators on a Monday afternoon and had them Tuesday lunch time!) Here is a link. I chose to replace it with the 7805 regulator because it's much more capable (but larger) and can be used for many other projects.
That regulator's pinout is different so you will need to bend some pins in order to solder the right ones onto the board. Take a look at the picture above, and you can see where the new pins have to go. If your interested, Vin is where your power source + goes to and the Vout is where the nice, regulated 5v comes out.
Hold it in place with whatever you have, I find it difficult to solder anything unless I have one of these "third hand" things. It makes it easier to solder if you trim the legs right down, and bend the ends flat with the board. (see the picture)
Step 4: Extras
After you solder it, you may want to bend it down out of the way. You will need to do this if you are using any shields like a RAMPS (what mine is being used for) etc. Be very careful when doing this as the pins are fragile and will break if you bend them back and forth more than once or twice. (or even if you bend them to far!)
I suggest putting a bit of electrical tape on the USB port just in case. For some regulators, it won't end well if the metal part touches it.
You may also want to put a heat sink on the regulator if you intend on drawing a large current, (0.5-1.5A) or if you have spare ones lying around not being used like I did.
Thanks for reading! If I helped save your "dead" arduino please leave a comment and favourite this 'ible. :)