There’s a way to test (and fix) the entire string of miniature lights without testing each bulb individually. Try the LightKeeper Pro (www.lightkeeperpro.com). It’ll identify and oftentimes fix most problems in miniature and icicle light sets (but not the sets of larger lights or LED lights) with a few squeezes of the trigger (Photo 1).
The trigger sends an electric charge through the circuit to repair internal bulb failures. (The “shunt” in the bulb is supposed to act as a bypass if a filament fails to complete the circuit. If the shunt fails, it knocks out the lights in the section. The electricity sent by pulling the trigger locates and fixes the defective shunt.)
If that doesn’t work, hold down the black button on top of the tester and move the tip along the string so it beeps (Photo 2). Note where the beeping stops, then replace the previous bulb.
The tester is available at home centers and some hardware stores for about $20 (find retailers on the company’s Web site).