Four 3-wire strings of Christmas mini lights. Incandescent. Likely identical and purchased same year. Bulbs physically interchangeable.
Each string has three circuits. The first and last bulbs in each circuit have larger bases that fit in to larger sockets.
Its bveen several years since they were last used as none of the 12 circuits light. All sockets are full, no broken bulbs. The male/female ends pass electricity, and the female distal ends passes electricity. So fuses are all good and cords are basically good.
I removed all bulbs and chose one proximal circuit as the test circuit. Starting at the proximal end of the test circuit, I added bulbs one by one, checking for continuity in the series wire between the male/female plug and the next (empty) socket using a free multimeter from Harbor Freight. If a bulb did not provide continuity, I set it aside. If it did, I left it in and proceeded to the next empty socket. Eventually the entire socket was full and it lit.
But some of the installed bulbs did not light. I then replaced those bulbs with ones that did light.
I then removed one of lit bulbs, which caused the circuit to un-light. One by one I went through all the bulbs, 100+++, using the empty socket as a testing rig. A few bulbs did not cause the circuit to re-light. These few were set aside as duds. Quite a few caused re-light and themselves lite up. These were set aside as good'uns. However, the vast majority caused the circuit to re-light, but did not themselves light. These were set aside as anomalies.
I filled the other two ciruits with the good'uns. The entire string lit. Jiggling the string to test for bad connections did not reveal any.
What's the deal with the anomalies? shorted out? higher voltage requirements? How can they and the other three strings be salvaged?