Miniature Christmas light sets are inexpensive, so the first impulse is to throw them away when they stop working. Even though they are cheap, it still costs less to buy replacement bulbs and fuses than to buy a new set of lights. Miniature Christmas lights sets in good working order use less power and create less heat, helping prevent holiday home fires.
Untangle last year's light sets, and stretch them out. Check for frayed cords, exposed copper wire, blackened or melted bases, broken or loose bulbs, or staples that have gone through the outer plastic coating of the wiring. Do not plug them in. Discard light strings if they have anything wrong other than broken or loose bulbs.
Remove loose bulbs from their sockets. Tighten them by bending the two copper wires on the sides of the lamp base so they are straight, pulling the bulb further into the lamp base, then folding the wires flat along the sides of the lamp base again. Replace tightened bulbs with bulbs with the same voltage as the original bulbs. Do not replace with higher voltage bulbs or your light string could overheat and cause a fire.
Remove broken bulbs from their lamp bases. Carefully insert the two copper wires of a replacement bulb of the same color and voltage through the small holes in the lamp base so the bulb settles into the base and the two thin copper wires are poking out of the base. Bend these wires up the two sides of the outside of the base.
Plug your light string into the wall outlet. Check for any additional blown bulbs. Unplug light string and replace these as well. If your entire light string does not light up, you will have to replace the fuse.
Find the small door at the plug end of each light string, and move it aside to expose the fuses. Remove and replace with new fuses of the same voltage.