This gateway MD7820u laptop came in with a very loose power port and slight burn around the actual dc jack. It's a pretty common model I'm starting to see come in for plug repair.
Now that Acer bought gateway they are using the acer style dc jack. In my opinion it's the same poor quality used by gateway but a different style. It uses a five pin, 1.65mm tip jack that I've been working on for years.
Once disassembled the old dc jack needs to be de-soldered directly off the motherboard and a new power receptacle soldered back on.
After the dc jack is replaced the laptop powers up great and charges the battery.