Tree lighting

Two years ago we bought a new Christmas tree for our den. The tree in the living room (yes, we have two trees) is decorated with small lights of green, white and yellow. In the den we were looking for something different and found a tree with the lights already attached, but the unique thing was that the lights were C7s. These are the regular bulbs, not the miniature lights. I thought, cool. I wouldn't have too string lights on at least one tree. First year was a breeze putting it up. Second year a few bulbs were out, which wasn't surprising. But this year I set up the bottom half and before adding the top half I checked to see how many lights were out. There were more than a few so I proceeded to see if tightening up the bulbs would get them to light. I did a few with good results, but in tightening one white one I heard a small pop seemingly on the opposite side of the tree and half the lights went out. After some investigation I saw that there were actually three strings on the bottom half of the tree and one whole string had gone out. I checked the fuse in the plug end of the string. It was okay. I went along and wiggled each and every light but the string stayed out. Then I plugged another string into the end of the faulty string and that one lit. Electricity was going through the dead string, but just not lighting up the bulbs. Totally frustrated I sat there and tried to think what I was missing. Then I took the time to undo all the twisted wires that were holding all the bulbs and wire to the branches. Not an easy task. Once I had the whole string off I examined each and every socket, twisted each bulb, checked the wire for cuts or fraying, and still no lights. At this point I gave up and went to the store and bought a string of C7 lights. We now have a tree that is prestrung with lights except for the bottom string. I fully expect that next year another string will be dead. At that time I just might take all the strings off and have it become a regular tree.