A light circuit is a continuous metal-to-metal path from AC plug through fuses, wires, sockets and lamp filaments. A light circuit will operate if it has a power source, good fuses, good lights and good wiring. Since light strings start out working, we typically only have to find one or two problems to get the string working again.
A typical light string has 100 bulbs in 2 circuits of 50 bulbs each. Each circuit is connected in series, and any bad connection or bad light shunt can darken an entire circuit. A bad fuse in the plug will darken the whole string, as well as any strings plugged into the AC socket at the end. So if some lights are on, or if lights plugged into the AC socket are on, we know we have power and that the fuses are good.
Some strings may have just 1 circuit, or they may have 3 circuits instead of 2. Some may have 35-bulb circuits with 3.4V bulbs instead of 50-bulb circuits with 2.4V bulbs. Some strings may not have an AC socket at the end, and then a 1 circuit system needs only 2 wires instead of 3. Other strings may have 3 circuits and an electronic controller that lights them in sequence for a "running" appearance, and these typically need 5 wires. But all mini-lights operate in series and any bulb and shunt failure will darken an entire circuit. The mission is to trace down the fault and fix it.
The first issue is power. No lights are going to be lighting up without power. No circuit tracing is possible without the hum of AC power.
If any lights at all are lit in the string, we have power and good fuses and can move on.
If no lights are lit, we still might have good power if all the lights are burnt out (with at least one bad shunt). If, as usual, we have an AC socket at the end of the string, we can just plug in a low-power AC light and see if it works. A small neon nightlight would be ideal, but any small light would be fine. If it lights at the end of the string, we have power.
With no power at the end of the string, maybe we have no power at the AC plug. Pull the plug and try the test light there. If not, and now on an extension cord, try the power at the source outlet. If there is power at the source outlet, check the extension cord and also make sure it is plugged in. If no power at the source outlet, check the breakers.
If there is power at the wall or extension cord, with no
power at the end of the string, we are looking at blown fuses,
or an actual wiring failure.
So check the fuses.
Then look at the wires.
The LightKeeper Pro (about $15 to $20 full retail) tool is the best modest-priced tool I know. We can pay much more and get some advantages. Or we can pay much less and get by. But we certainly need something to trace an electrical voltage that we cannot see. I used a cheap hum-tracer (about $5 full retail) for years, and cover that below.
The advantage of the LightKeeper Pro is that it may be able to solve the problem without any tracing at all. The most common light problem seems to be lights which burn out but whose shunt does not operate. The LightKeeper Pro can put a tiny high-voltage pulse into the circuit which usually activates bad shunts. That lights the circuit, then we can replace burnt-out bulbs as usual. Obviously, the LightKeeper Pro will not repair bad sockets, bad wiring, or corroded socket and lamp contacts.
To get at the bad circuit, we remove one of the bulbs from that circuit. Then we plug that bulb socket into the front of the LightKeeper Pro gun. Be sure to orient the socket so that the contacts match the gun. Then we pull the trigger, and, as often as not, the circuit lights up! If not, we pull the trigger up to 20 times.
If the light string sockets will not fit the LightKeeper Pro
gun, we can unplug the string AC plug, and plug that into the gun.
However, because a string can have multiple circuits, we have to
first disable each good circuit so the tiny high-voltage
pulse is not drained away.
We want to attack the open shunt in the bad circuit only.
We disable the good circuits by pulling a lamp from each.
We mark those sockets with tape or otherwise remember where to
put the lamps back.
If the LightKeeper Pro "quick fix" feature does not solve the problem, we can use what they call an "audible continuity detector" but which I call a "hum tracer." Their version is very sensitive, and while that generally is an advantage, it can cause problems as well.
When a circuit is lit, the voltage starts out at 120VAC and drops by 2.4V per bulb until we get to ground at the far end. The various voltage levels will confuse the hum tracer, which is thus not particularly helpful when the lights are on.
When a circuit is dark and we have power, there will be a full 120VAC hum all the way to the first "open." By tracing from the start of the hum, eventually we find the hum is gone and then we know something is open between there and the previous test.
By reversing the plug, usually we can put the hum at either end of the circuit.
Because the LightKeeper Pro hum tracer is sensitive, we can
touch a bulb and get a signal.
So we can move from bulb to bulb until things change.
However, because the detector is so sensitive, it may detect
voltage from only one lamp filament support wire, when there is
no voltage on the other.
So the actual position of the problem could be one lamp prior to
the indication of no more hum.
When a circuit is dark and other means have not worked, we may need to trace the AC voltage from where it is good to where it stops. When some part of the circuit is open, we should get the same strong AC hum up to the point of the open itself.
To trace a circuit, we first have to make sure we know what hum looks like on our tracer. We also may need to reverse the AC plug to put the hum at the end we want, or perhaps find that only one end will work.
Many hum tracers are not good at detecting hum from inside the lamps themselves. To use insensitive tracers, we need instead to place the wire being tested right on the pickup of the unit, while moving other wires away so they do not complicate the reading. Then we move along the circuit, from lamp to lamp, pulling each socket up to expose its wires, then we check those wires.
When we have hum before a socket, but no hum after that same socket, we have found an open. While the open is normally a bulb with a bad shunt, it could be a lamp not making contact with a socket, or a bad socket or wire. Typically, we can test the lamp, and move it around enough to be sure it is making contact. If the string lights up, we can replace bad bulbs as usual. If not, we can continue hum tracing until we have found every open.
If we have a sequencer-controlled 5-wire system, that means
we have 3 light circuits, plus the hot and cold AC lines to the
AC plug at the end.
If we get the controller to turn all 3 circuits on, then pull
a lamp from two of the circuits.
Now, every third lamp should be on (but is not because the circuit
is dark), and we can try to trace the hum through every third