Tip & How-To about Miscellaneous

Test/Find bad XMAS light bulb (Mini-Lights) in series string of lights


Have you ever tried to find a bad bulb/socket in a long series string of 50 or more bulbs??

This simple tester (Pic 1) uses a 9 volt battery to test your series lights 3, 4, 5, 6 or 7 at a time (since bulbs are rated 2.5 or 3.5 volts, more than 7 or 8 bulbs is very dim). Radio Shack sells a 9v battery snap connector for $2.99 (Catalog #: 2700325). Using 2 short pieces of stranded #14 wire connect (solder) one to each lead of the snap connector.Then strip about ½ inch of insulation off the end of each stranded wire and fold the bare wires back on its insulation. (Pic 2) These 2 stranded wires can now be easily inserted into any socket with a good snug contact.

Be careful that the two tester wires don't touch and short out the battery.


To test, start at one end of the string, pull out a bulb and insert one stranded end of the tester.Then move down the string and remove the 4th, 5th or 6th light, and insert the other stranded end. All bulbs between the two test wires should light, if not, one of those lights or socket is bad.To narrow down to an individual light, simply move the tester back one or two lights at a time.Then keep moving the tester up/down the string 4, 5 or 6 at a time until all lights/sockets have been tested.

(Testing the FIRST and LAST bulbs/sockets in the string) To test the FIRST bulb touch the tester wire to one or the other male a/c plug prongs. Testing the LAST bulb will require inserting a small wire into the female a/c end and touching it with the tester wire.

Finally, you need to overlap each group of lights so as to also test the bulbs that were removed to insert the tester wires.

Pic 1 - Completed tester

Pic 2 - Wires folded back to insert in bulb socket

Pic 3 - Wire inserted in socket

Pic 4 - Side view of tester wire inserted in socket

Pic 5 - Fixed Light string using tester

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1 Answer

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Just replace the bad bulb. Finding the bad bulb is a different matter if it is a series string. They sell testers for a few bucks, with a little practice they work good.

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If its a mini fluorescent bulb replace the started, its probably bad. If incandescent bulb make sure you have 110Vac at the bulb socket and the new bulb is actually good! Don't overlook the obvious, you may have a bad switch or blown fuse.

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On the side that is not working, 1 or more bulbs are burnt out or making a bad connection. On incandescent bulbs, typically, every 50 bulb is connected in series (or 35 depending on bulb voltage size).

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Assuming the wires to be good, if the string of lights consists of more than one section (for example, a string of 100 lights is made up of two sections of 50 lights each) and the lights of only one section are out, you have a bad bulb (or possibly more than one) that must be located and replaced. If your light string is only a single section set (a set of 50 or 35 or 10), it's most likely also a bad bulb but it could also be one of the fuses located in the plug. Light sets come with a few replacement bulbs and usually two fuses. Take the plug and slide open the little compartment door and you'll see them. Replace them and try the set. If it still doesn't work, it's a bad bulb. On multiple-section sets, the fuses in the plug power all the sections, so a working section eliminates the fuses as suspect.

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