[h2]We want to installed a receptacle a couple feet away from a light switch that will be used to power a tv and a Roku.[/h2] Here's a pic of the wiring for the light switch... There is also another light switch in the room that controls the same overhead light (it's on an adjacent wall). Here's the wiring for that one... What needs to be done?

I recently bought a property with a house and shop. Previous owners ran 240v from a 100amp breaker in the main house panel out to a sub panel in the shop. So thats two 120v lines and a neutral for three wires going out there. The problem I’m having is that in the shop, some circuits cannot support any load. They will test at 120v until you turn on a single light and then it goes down to 10-20v. I determined this to be from one of the 120v lines. Circuits on the other 120v line work fine. The bad line will test fine (120v) at the last connection point at the house before it goes underground, and test bad (10-20v) at the first connection point at the shop where it comes above ground. So my thinking is that the line has failed underground somewhere. My questions are, is there any other possible explanation before I start digging? Open neutral? Something else I’m not thinking of? If I were to find the damaged spot in the underground cable, can it be safely repaired or does the whole line need to be replaced? The lines were not run in conduit but they look like the kind of heavy duty plastic encased lines that you’re allowed to do that with. Location: North Carolina, USA