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So! Every year you take the box of holiday lights off the shelf, plug them in, and only half of them work. Don't pitch 'em, you can FIX them, and it's pretty simple, actually! Just follow this recipe - it's not frustrating - in fact it's very gratifying.

The -very- first thing to do is to take your string of lights, and stretch it out on the floor or handy table. Here I am using a table in the Hub area of TechShop Pittsburgh. The string of lights is plugged in at the top left corner of the photo, and loops around counterclockwise. The end is at top right in this photo.

In addition to the string of lights, you will probably need some replacement bulbs. We don't entirely know how many yet. Maybe you saved the extras that came in the box last year, maybe you will need to go buy some new ones. Alternatively, if you have several strings, you can scavenge the lights from one string and use them to fix several other strings. (That's what I do.) [Any scavenged lamps should go in the "Untested" bowl described later.]

Step 1: SECRET TRICKERY #1 - Bulbs Are Wired in Series, and in Groups

In the olden days of LARGE holiday lights, one bulb would burn out, and it would not affect its neighbors. That's because all of those bulbs were 110 volts, and wired in parallel. (Just like the individual light fixtures in your house - each one is independent of all the others.)

These little guys are different. They are arranged (electrically) in "chunks" - some number of bulbs connected in series. (See the photo.) Each bulb is rated for a low voltage, 2.5 volts (AC) in this case. When you string a bunch of 2.5v bulbs in "series", they share that voltage between them, like holding hands. A string of ten bulbs would operate properly at 25 volts, 40 bulbs would take 100v, and so on. If you were to plug one of these little guys into 110 volts, or even 10 volts, it would burn out in an instant. *pfft*!

So! If you have a LARGE string of lights, it probably has several of these chunks. When you take the set out of storage, some chunks may have ALL of the lights working fine. (Happy happy!) Some chunks may have just a few lights burned out. (Meh.) And some chunks may have ALL of the lights off. What's going on here?

Step 2: Secret Trickery #2 - the SHUNT!

Unlike regular house lights, there's a little extra gizmo inside each of these bulbs. It's called a "shunt". It's sort of the inverse of a fuse. When the bulb is new, all of the electricity goes through the main filament, emitting that nice warm light. The shunt does not pass any electricity. When the main filament burns out, the shunt is supposed to take over, allowing electricity continue to flow through the bulb. (The shunt does not get hot enough to glow.) When the shunt works properly, every other bulb in that Chunk should stay ON.

Ah, but there is mischief afoot. Sometimes the shunt-magic does not "turn on". This breaks the circuit in that chunk, and all of the lights in that chunk go dark.

Step 3: Think, and Chunkify!

OK, so let's step back and review the big picture.

(A) If -any- of the lights in a chunk turn on, that means that all of the bulbs either are working happily, or else they burned out and their Magic Shunt worked properly. All of those bulbs are still "holding hands", and electrical current passes all the way from the left end of the chunk to the right end.

(B) If -all- of the lights in a chunk are OFF, its very likely that -one- of the bulbs burned out, but its shunt did not work properly. There is a gap in the line - like someone "dropped hands" - and now the power does not flow. :( The other bulbs in that chunk may be OK, or they could perhaps be shunted out. We can't tell. Yet.

We can take this new knowledge and create a scheme to find and replace all of the bad bulbs. We'll get to that in the next step.

The first thing, though, is to mark the boundaries between "chunks". I use colored masking tape for this - you'll see yellow tape in the photo. You could use yarn, or a piece of ribbon, or anything at all. We simply want to break the big project down into smaller sections. Divide and conquer! My light string had four chunks.

You'll notice in the photo above that SOME of the lamps to the left of the yellow tape are OK, and some are dark. ALL of the bulbs to the right of the tape are dark.

Step 4: The Sorting of the Bulbs

So here's the plan, Stan. This is the straightest line between here and DONE. You can do this.

  • Draw three circles on a piece of paper, or grab three bowls, and label them "Untested", "Working", and "Failed". Seems silly, but it helps keep track of things in case you get interrupted or distracted. ("What, cookies for me?!?! How nice!")
  • If your string of lights has a "dark chunk", pull out -ALL- of its bulbs and put them in the Untested bowl. We don't know whether they are dead or alive, but we are about to find out.
  • Go to a chunk that has at least one lamp turned on. Pick a socket that has a working bulb, and put another mark on that socket. It's going to serve as our Special Testing Socket (STS). You can put some different colored tape tape on the STS socket, or a piece of ribbon, or nothing at all. I just took care to keep the STS centered on the yellow envelope that you see in the photo.
  • Pull the working lamp out of the STS, and set it into the "Working" bowl. All of the remaining lights in that chunk will go dark.
  • (1) Take a bulb from the "Untested" bowl and plug it into the STS. If the bulb lights up, remove it and place it in the "Working" bowl. Yay!
  • (2) If the bulb does not light up, but all of the other lights in the Testing Chunk -do- come back on, this indicates that the bulb has failed, but the shunt worked. Pull the bulb out of the STS. Pull the dead bulb out of its plastic base. Put the base in the "Failed" bowl, and the lamp in the trash can. (Avoiding future confusion about that lamp.)
  • (3) If the bulb does not light up, and none of the other lights in the Testing Chunk come on either, this indicates that the bulb burned out AND had a failed shunt. This is the critter that turned the whole chunk dark. As before, pull the bulb out of the socket, and remove the dead bulb from its base. Drop the dead bulb in the trash NOW, and put the base into the Failed bowl.

Repeat the previous three steps until the Untested bowl is COMPLETELY empty.

Once the Untested bowl is empty, put new (or scavenged) bulbs into the empty bases found in the Failed bowl. Repeat the above testing process for these bulbs, too. If they are new, they should work. If they were scavenged, we don't know, we have to test them to see.

Step 5: Put Good Bulbs to Work!

OK, now you have some bulbs that you KNOW are working properly. I use the first of these to go and replace the dead singletons in the other Chunks, because I like the quick gratification.

When you take out the dead singletons, be sure to put them immediately into the "Untested" bowl. At some point, you will need to go back and test them as described in a previous step.

-- We are almost done! --

The last step is to refill all of the sockets in the "dark chunk". Nothing will happen until you fill ALL of the sockets in that chunk. When you do, you should be greeted with many happy little lamps all coming on at once. Hurray!

...And what if the dark chunk does NOT come on? In my experience, it means that I either skipped a socket, or else a bad bulb snuck into the "working" bowl while I was having a cookie. Pull all of the unlit bulbs out of that chunk, run them through the test/repair process one more time. I bet you will find that the same thing has happened to you. :)

THAT'S IT! You are done. Have two more cookies as your reward.

Wow, I have a &quot;chunk&quot; of lights out but they are already installed on the house lol. I didn't bother to test them before I installed them(dumb huh?)<br>Good instructable. When I go to take the lights down I will use this procedure to fix em. I was planning of tossing them and buying new ones next year. Hopefully I won't have to do that now.<br>Thanks!
thank you so much! This will save quite a few strands of my lights!

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