Test and Fix Incandescent Christmas Lights




Introduction: Test and Fix Incandescent Christmas Lights

A very simple tester to find bad bulbs/sockets in incandescent Christmas lights. Instead of pulling out 50 bulbs, you should only have to pull 8 or 10 before finding the bad bulb.

Just be sure to UNPLUG LIGHTS BEFORE TESTING!! There is the potential of getting shocked if you don't.

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Step 1: Needed:

1 each - 9 volt battery snap connector (Pic 2) (Radio Shack Catalog #: 2700325) $2.99

2 each - 4 to 6 inch stranded 16 gauge wire (Pic 3)

1 each – 9 volt battery

Soldering iron and solder

Replacement bulbs - Walmart stocks (usually at Christmas) fuses, 2.5 volt bulbs (for strings w/multiples of 50 bulbs) and 3.5 volt bulbs (for strings w/multiples of 35 bulbs). (Other stores may stock these as well)

Step 2: How To:

Strip about ½ inch off both ends of 2 short pieces of STRANDED 16 gauge wire and solder one end to each lead of a 9 volt battery snap connector. On the other ends of both stranded wires, fold the bare wires back onto their insulation (Pic 4). The ends of the two stranded wires are the tester wires and can now be easily inserted into any bulb socket with a good snug contact (Pic 5)

A Better Tester Probe (Pic 6). If you have 2 extra bulb holders (Pic 6a-center), you can use them by inserting a couple of the tester wires (same as you would replace a bulb) into the holder (Pic 6a-bottom) . Then, using these 2 probes (Pic 6b) for the tester probes results in a much better connection to the socket contacts and aren't as likely to fall out during testing.

Step 3: What It Does:

Don’t know about you but every year when I pull all my Christmas lights out, I hate the hassle of trying to find a bad bulb in strings of 50 or more lights. It usually winds up being the 49th bulb I pull out that is bad.

This year I decided I would attempt to find an easier way (if there is one) to check my light strings. First, I purchased the ‘Light Keeper Pro’ but didn’t have much luck with it. I finally came up with this very simple tester which uses a 9 volt battery to light up small groups of lights. All I have to do is pull out 2 bulbs in a string, insert the tester wires into the 2 empty sockets and all the bulbs between the two light up. If they don’t light up, one or more is bad. Since most bulbs are rated 2.5 volts or 3.5 volts, don't try to test too many at a time as each additional bulb will decrease the brightness of all the bulbs being tested.


Not perfect but cheap, beats pulling and testing each bulb individually, also tests fuse and sockets and lights don't wind up in land fill.

Need to try to improve the tester probes, maybe next Christmas.

Step 4: Testing Procedure:


Be careful that the two tester wires don't touch and short out the battery. Best to remove battery when not in use!!


Starting at one end of the non-working string, pull out a bulb and insert one tester probe (OR as below for FIRST bulb) Then move down the string 5 or 6 bulbs (be sure to follow the same single wire as there might be more than 2 wires in your string of lights), pull out the 2nd bulb and insert the other tester probe. Now, ALL bulbs between the two tester probes should light, if not, one or more of those bulbs or sockets is bad. To narrow down to an individual bulb, simply move the probe back one or two bulbs at a time.

Then move both tester probes up and down the string in groups of 4, 5 or 6 bulbs at a time until all bulbs/sockets have been tested.

Overlap each bulb pulled, so as to also test the bulbs that were removed to insert the tester probes.

Be sure to replace bulbs with the same voltage rating, 2.5 volts for strings with multiples of 50 bulbs and 3.5 volts for multiples of 35 bulbs.

Testing the FIRST and LAST bulbs/sockets in the string

To test the FIRST bulb touch the tester probe to one or the other male a/c plug prongs. Make sure the fuse is good since this test also tests the fuse. (this test will not work if your light string uses a transformer). Testing the LAST bulb will require inserting a small bare wire into the female a/c outlet and touching it with the tester probe.


Step 5: What If Its NOT a Bulb?

If you have narrowed your problem down to one light and replacing the bulb doesn't fix it, then your problem must be the socket. Here are a couple of ways to fix bad sockets:

1) The easiest fix is to simply cut wires on both sides of the socket and splice the 2 (in some cases 3) wires together. This will slightly increase the voltage drop across all remaining bulbs however, this increase should not be enough to cause a problem. But it's not a good idea to do this on more than a couple of sockets in a string of 50 lights. Also, best to solder and use heat shrink tubing on splice (Pic 7).

2) On my bad sockets, I found that the contacts were crimped to the wires but over time somehow had corroded and were no longer making a good solid connection. To fix this I started by pushing the 2 wires up into the socket enough that the spade crimped contacts came out the top (Pic 8) and then I pulled both wires out of the socket (Pic 9). Using soldering iron, flux and solder, I was able to heat and flow solder into the crimps thus making a much better connection between the spade contacts and wires. Try to keep solder away from the spade contact's back side so that it doesn't interfere with getting the spade contacts back into the socket. Then, when putting the contacts back into the socket, they must slide down and snap into place inside the socket.

** Your sockets may be different **

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    2 Discussions


    3 years ago

    I need to do this, a section of our lights went down but it's hard to tell where it starts when it's all built into the tree!


    Reply 3 years ago

    Sounds like one string, probably one bulb has gone out so this tester should work. I have no idea how difficult it will be to trace that wiring and bulbs. I just don't have any experience with these, sorry.