Laptop shorts out my AC adapters

Hi, so I bought a couple of "AS IS" Dell n5050's from ebay and decided to finally see what was wrong with them.

From what the previous owner told me was that one of the laptops shorts out AC adapters while the other had a broken screen.

Alright... So I decided to try something out.

I bought an AC adapter of the required specs for the N5050 model and when I plugged it into the DC jack of laptop, immediately the Voltage regulation indicator LED on my adapter starts going crazy (Almost as if the adapter is being shorted.) - Know that the laptop is turned off, with DC jack securely plugged into the motherboard.

I've tested this out on the other broken laptop and shortage problem doesn't occur. I've also verified that the model of the motherboard which is one that of N5050. (So I know it is not a problem with my adapter.)

Could this be a problem with the DC jack? Or is the motherboard just faulty?

I've also tested with battery packs - The shorted laptop refuses to turn on yet the other one with the broken screen will - noting that the same fully charged battery pack was used on both.