Lately I have been finding new power supplies for XL and XEs on eBay that looked a lot like a 5 volt power supply with just a new 7 pin head. As anyone who has ever had an Atari XL knows the power supplies for the XL weren't all that reliable and even though the power supply I have for my 600XL was working I thought I would build another on using current technology like the ones I have been seeing on eBay and make the old one a spare. SO that is what I did and this is how I did it.

Legal: I am not a professional but have had years of experience with computers. Still, use these instructions at your own peril. I assume no responsibility. 

Note: use this information only if you know what you are doing. Get the polarity wrong and you could BLOW YOUR COMPUTER.  So always, check, check and check.

The power supply plug on the XL line attaches by a 7 pin DIN plug. looking down at the plug the left 3 pins are negative and on the right 3 pins are positive. the bottom pin is nul and not connected to anything.

But, on my 600XL, only 1 pin on each side need power. So the 2nd pin from top on left got negative and the 2nd from the top right got positive. All the other pins don't need to be powered even though the Atari power supply does power them. (I assume the 800XL is the same but since I have not tested one test, test, test. )

7 pin DINS are hard to find and expensive. So, since only 1 pin on each side needs power I used a 5 pin DIN which is easier to find and cheaper and it fits. 
NOTE: It was pointed out that the monitor port ALSO uses a 5 pin on the 800XL which I had forgot about since the US 600XL only has a RF connector. IF you use a 5 PIN for power be sure NOT to plug the power into the monitor port by mistake. Bad things will happen if you do, very bad.

Here are the specs for my power supply OUT is 5 volt, 2 amp. The Atari power supply was 1.5 amp so I wouldn't go any lower than that. The Actual 600XL draws 800mA but you might need the extra amps for SIO port devices (such as Fuqinet) that draw their power from the computer.

Oh, and if you are wondering where the power supply came from it was an old power supply for a powered USB hub. Using an old USB powered hub I knew the power was steady and filtered. Using just an old phone charger might not work as well as it could introduce interference or power fluctuations. So check it out before you commit.   

Here is what the plug end looks like. Pin 2nd from top left is the negative, pin 2nd from top right is the positive. all other pins are un-powered.

Here is the finished item.

As I said before, test, test, test and double check yourself before you attach it to the Atari. If the polarity is wrong it could brick your computer. After I attached the power supply I ran a stress test of a little program and then the builtin Atari check program in a loop for quite a while. It held up fine.