For one who has the time but is low on funds ... how does one fix those
strings of Christmas tree / colored lights that have the bulbs in series -
presumably, if one bulb blows they all get knocked out? I have a multimeter
and several non-working strings (so hopefully there are plenty of spare
I have an electrical tester that I picked at the dollar store. If you hold
it next to a plugged in extension cord, it will light up when held next to
the hot / live wire, but won't light up when held next to the neutral /
ground (even if there is nothing plugged into the extension cord). If you
could find this tool, you could use it to trace the line, and find where it
breaks, then replace that bulb, and continue till you get a working set.
I think the tester is called DEET or something.
If you just have a multimeter: set it to continuity test or resistance test.
Remove a lightbulb and test the two contacts. If it beeps on continuity, or
registers a resistance (near zero), then you know that bulb is good. Plug
that bulb in and test the next one.
If all the bulbs work, then you might have screwed up sockets or wiring,
which is a little harder to test. You remove a bulb, test continuity from
both the prongs on the plug to the contacts in the socket, replace the bulb,
and continue to the next socket till you find a problem.
Make your own.
Hook up a single socket to a battery ......
In series, the voltage is divided amonst all the lights on the string
so just calculate what you need, ands screw each one in one at a time.
Alternatively, find one STRING that works, unscrew one light and then
test all your other lights on the working string.
It's actually pretty rare to have a "one bulb blows, the whole strand goes out"
anymore. It _is_ still true in most sets though that if "one bulb is not fully
seated the whole strand goes out". That being the case you usually only have to
go through the set looking for the loose bulb(s) to get the strand to light up.
Once the strand lights, the bulbs that are actually burned out are easy to spot.
Except for the very deluxe sets I only give them about 20 minutes worth of
dicking with them. After that, they hit the trash and I buy another set. There
just not worth more effort than that.
Cheap grain-of-wheat xmas lamps are designed for no-more-than 3-volts..
and will (dimly) light with a regular C-cell battery.
50-bulb strings are thus in series. 100-bulb-strands are usually 2
separate 50-bulb series twisted together so that the bulbs alternate.
If I were testing the bulbs, I'd use a battery and a chunk of wire.
I've seen fault-tolerant light strings, but I don't have any idea how
they work. If OP had one, he wouldn't be asking how to test
the lights in a string, though.
They are cheaper to run and no repairs. OTOH, the OP is low on funds so
my preferred method was changing one light bulb at a time. Most mini
light sets come with a couple replacement bulbs. With any luck, he can
troubleshoot enough to get the strand going.
Now-a-days, the strings are so cheap. However, I still fix them, to a
point. The big problem with using a multimeter is that the voltage
(ohmmeter) is too low. Years ago, I built a box which we refer to as
the Muppet Box, because it looks like something my kids saw on the
Muppet Show (more on the box later). The lamps each have a shunt inside
the bulb. The shunt doesn't make contact as there is some type of
coating and the voltage across one lamp, in a working string, is only
1.5 to 3 volts. However, when one filament opens the entire 110 volts
is now across the open circuit. The higher voltage breaks down the
shunt coating and starts to conduct. If there are many open filaments
in the string with all the shunts shorted, it will put too high voltage
on the working lamps. I have seen a run away condition where the lamps
start going one at a time and eventually they all die.
The problem usually comes from 2 or more open filaments in the string.
This probably happens from rough handling. There is not enough voltage
to "share" between several shunts, so the string will not light. In the
test box I have a 110 to 220 volt autotransformer. The output feed the
light string through a series diode. There is a switch to switch
between regular 110 volts and 220 volts through the diode. The string
is plugged in at 110 volts and I momentarily throw the switch to the 220
volt side (half wave rectified). The higher peaks will, in many cases,
break down the shunts and the string will light. I then switch back to
the 110 volt side and change out the bulbs that don't work. The higher
voltage will also arc through corrosion in the sockets in some cases.
BTW, I also have other things (series resistors, etc.) in the box to
allow manually probing the lights. There's also a GCFI to keep me alive
in case I get careless.
All this said, there is (was?) a company that builds a high voltage
pulser to break down the shunts. It was called Lightkeeper, however, I
suspect they are out of business as their web page doesn't work. I
suspect people just didn't bother with this product and just throw out
I found www.lightkeeper.biz. Neat device. eBay also has 'em, but they
look like "older" versions.
This discussion has me thinking. Perhaps one could wire a BBQ ignitor,
also a piezo device, to the string of lights and that too would send a
pulse out to the lights. However, what I don't understand and perhaps
somebody can explain, is why (following the Lightkeeper's
instructions) they make you remove one bulb from each set of lights
prior to plugging the light string into the device. I mean that way
there's no continuity to the shunt. Maybe I'm missing something.
On 11/29/2004 6:54 AM US(ET), Paul Giroux took fingers to keys, and
typed the following:
device. They don't want you to test the string from a good bulb socket,
so if you remove a bad bulb before unplugging the whole string from the
outlet, you won't have to try to remember which bulb was bad (it's the
I don't have one of these devices, but I'm going to look for one.
Yes, but I'm talking about the test procedure if the socket is too big
to fit into the device; they make you remove one good bulb from each
string until the entire run no longer lights up, then you insert the
plug (the 2-prong 110v thing) into the Lightkeeper. That piece is what
is confusing me: there's no continuity anywhere, how does the circuit
completes so that the shunts "activate"?
On 11/29/2004 9:28 AM US(ET), Paul Giroux took fingers to keys, and
typed the following:
"Plug the light string *bulb socket* fully into the LightKeeper Pro
The metal contacts of the bulb socket should line up with the metal
contact of the LightKeeper Pro Socket Connector.
When properly inserted, the flat indentations of the Socket Connector
will match the parallel the indentation of the bulb socket".
I saw that, but I'm referring to the "Partially Lit Sections -
Alternate Method Step 1D: (...) The Lightkeeper Pro's Socket Connector
should be your first choice to repair a partially lit set. If the
Socket Connector can not be used, the Alternate Plug Connector is a
versatile option. (...) It is important to note, if the light set is
partially lit, one bulb must be removed from each lighted section
before using the Quick Fix Trigger. Removing the bulb from each
lighted section isolates the problem. (...) A click will be heard each
time as the charge flows to teh defective shunt."
But you know, as I read this again, I think they want you to remove a
bulb so that only the loop with the bad bulb and shunt is actually in
Doh! Doh! Doh!
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