What's great about this is that it's actually a universal USB charger since it's equivalent to existing USB ports. If your MP3 player requires a USB cable to charge the battery, then this will certainly work with it.
Step 1: Note: Not Compatible With 2nd Gen Zunes
Do not attempt to try this with the 2nd Gen slim model Zunes (4gb, 8gb, 30gb, 80gb 120 gb slim zunes).
I found out that Microsoft designed the 2nd Gen Zunes to be incompatible with the third-party AC Adapters. Even the Official AC Adapters that were designed for the 1st Gen Zunes are incompatible with the 2nd Gen Zunes.
Step 2: Tools
You can easily find all this stuff at Radioshack.
The materials you need for modifying are:
One 5 Volt AC Adapter
One USB input plug (a.k.a female USB)
You can acquire the USB plug from an old USB extension cable you don't use. I pulled mine off of a dead MP3 player. Find the Pinouts and make sure you know what each of the four pins actions are. I included a Pinout diagram with this Instructable. Your USB cable should have pins sticking out the back where you will solder it to the AC Adapter plug.
Step 3: Finding the Right AC Adapter
The reason for this is that we need it to be equivalent to the USB port of a computer since that's how a lot of these MP3 players are charged. Standard USB ports have a rating of 5 Volts and 0.5 Amps (500 milliAmps). I know most computers and even the Xbox 360 follows these standards. The Playstation 2 also has USB ports rated at 5 Volts but I think it's current is a lot lower. The rate of the current can vary for some MP3 players, I'm not entirely sure if all of them can withstand 1.0 Amps of current. So your best bet is to find an AC Adapter at that's rated at 0.5 Amps. However, I am positive that the Zune can use a current of 1.0 Amps, it actually charges faster too.
The 5 Volt AC Adapter I used was actually for an old cellphone. I heard that you can find some AC Adapters at Radioshack but you can also check eBay and other sites.
To clarify, find an AC Adapter that's 5 volts and 0.5 Amps (500 milliamps). The Zune can handle a 1.0 Amp current, but if you are not sure what your MP3 is rated at, don't exceed 0.5 Amps.
Step 4: Connecting the AC Adapter to the USB Plug
Second, use you wire cutter and clip off the male plug at the end. Don't try to modify the end with the huge black box (transformer) that connects to the wall outlet. Then strip off the black insulation to expose the red and black wires on the inside.
Before you solder those wires to your USB plug, make sure you know the Pinouts. If you hold the USB so the pins are on the bottom, then the pin on the far right will be the Positive. The pin on the far left will be the negative. You won't need the two pins in the middle since those are for data. Refer to the Pinout picture attached to this Instructable for more info.
Solder the red wire to the Positive pin in the USB, then solder the black wire to the Negative pin. That's all you really need to do but it is highly recommended to insulate any exposed wiring with a heat sink or electrical tape. I didn't do that in any of my pics so anyone who reads this can clearly see what the wiring should look like.
Step 5: All Done !
For me, I just saved $50 by not buying the Zune AC Adapter from Microsoft, and you can too.
BTW, this is my first Instructable so if you have any tips or questions, just PM me or email me at aDimWit@hotmail.com
Step 6: Warning !
The Zune Voltage/Current rating is 5 Volts, 1.4 Amps. So any AC Adapter that has a voltage of 5 volts, and a current less than 1.4 Amps should work. If you use one with a Current higher than 1.4 Amps you risk frying your Zune. I have confirmed that an AC Adapter that's rated at 5 Volts, 1.0 Amps works well without any problems on the Zune.
Note: If you plan to make an Adapter for a different MP3 player, you can usually find the Voltage/Current Rating either on the box package or on the player itself. You can also Google it.