Troubleshooting Christmas Mini-Light Strings
Here is an excellent article on troubleshooting a bad string of Christmas mini-lights. (I do not know the author and am not connected to him in any way.)
I know of three occurrences of non-working light strings from this year's celebrations. In the first case, the woman who owns the Christmas tree simply bought a new string of lights and hung the new lights over the old string attached at the factory to her synthetic tree. I did not see her tree and cannot say what the cause of the malfunction was. In the second occurrence we put up a synthetic tree with two strings of factory installed lights, one for the bottom half of the tree and one for the top half of the tree. The string on the top half worked some of the time, but then would go out. Typically one suspects a bad bulb. That is also the general suggestion made in the very fine article I linked above. However, that proved not to be the problem. Rather, the problem was the very cheap electrical plug. I cut the plug from the string so I could open it and determine exactly where and how it failed in order to satisfy my own curiosity. Installing a new male plug solved the problem. In the third case, my daughter had a string near the top of her tree that was "out," but not completely at the top. I had limited tools and resources, but plugging the bad string into a different molded female plug brought it back to life. The moral of the story is that while a bad bulb is a frequent source of problems, the molded male and female plugs on these inexpensive light strings are often held together only by a lick and a promise, and fail easily. Giving the molded plugs a hard look is much easier and faster than removing and testing bulb after bulb.
The first photo shows an extension cord with molded plugs from three strings plugged into the extension cord. The second photo shows the bad male plug on our tree, and I am using my multi-meter to test for continuity between the brass plug blade and the load end of one of the tiny fuses inside the plug. That part tests "good." The other fuse tested "good," too. In the third photo I inserted a straight pin into the wire and tested for a circuit from the brass blades to the wire. Both sides failed this test.
A multi-meter can easily test what I needed to find this problem. I would have needed to strip away some insulation from the wire or stick a straight pin through the wire to obtain a reading with my multi-meter. But, by this time I had bought a hum tester with a high and a low range for checking different voltage ranges on an AC circuit. It led me to know the plug was the problem.