Every single year I buy at least 10-12 boxes of of those white Christmas lights form the Borg. And really, I do not mind. At $1.99, who gives a rip. One of the reasons is my bushes keep getting bigger. The other is attrition. But, invariably, 10-20 of my year old strings will not light. And, at $1.99, who wants to spend the time debugging them? Last year, I bought this handy tool from a local Borg store. It seemed worth a try at $20. If I can relight a 1-2 year old string, then it pays for itself in about 2 years. Get this. It works!!! Modern light strings use light bulbs that close a circuit when they burn out so the rest of the string stays lit. But, this does not always work as intended. Pull any light out of the unlit circuit (normally two circuits per 100 lights) and plug the socket into the gun's recepticle. Pull the piezo trigger until the lights turn back on. The high voltage surge is what causes the faulty bulb to activate the designed short circuit. I am able to salvage more than 90% of my strings doing this.
This tool also checks bulbs & fuses and has a live AC signal detector as well. NFI.
Merry Christmas!
Mike