To help find and replace these small incandescent candle-shaped bulbs gone bad, here is a description of how they work.
These sockets are permanently attached in series to the main bulb wire.
The bulbs themselves are first seated into their own small plastic bases, which fit into these sockets where contact is made with the their wire.
The inner cores of these sockets are somewhat rectangular in shape.
The final string of lights, which includes this wire of sockets and their contents, are twisted together with the two 120-volt power wires in a rope-like manner.
The bulb set is mentioned above, which usually contains 50 bulbs connected in series on one wire.
Each end of this set is connected in parallel to the two power wires that plug into an electrical outlet or into another string to make the whole string longer.
Often, a single string of 100 mini-lights consists of two 50-bulb sets, each one connected separately in parallel to the two power wires.
Thus, each of two 50-light sets operates separately from the other one in the same string.
To confuse things a bit, the bulbs are held by two form-fitted sockets so to speak: 1) the small plastic rectangular base holding the bulb itself, and 2) the permanent socket on the bulb wire into which it and its base are seated.
Thus, before a bulb can be seated into its main socket, its two bare wires must first be inserted through the two holes in the bottom of the plastic base, and then wrapped vertically around the two grooves on the outer sides of this base.
This bulb and its base are then seated into the socket where its two bare wires make contact with this wire via two metal side-strips inside the socket.
When a new candle-shaped glass bulb stands alone, it has two bare lead wires protruding straight down from its lower end.
These are the ones that wrap around its plastic base.
From there, peering upward into the sealed bulb itself, these wire leads become small metal posts held steady buy a glass bead attached between them.
Just above this glass bead a small wire is wrapped horizontally around both posts several times.
It is called a bypass shunt.
It passes the wires electricity onward if the light's filament burns out.
Above the shunt, the two posts protrude further up into the bulb where they are connected together by the fine-wire filament.
The filament produces the light when the string is plugged in or turned on.
In time, however, the filament will burn itself out.
At that moment, its current or electricity starts crossing through the shunt to keep the rest of the lights in the series set lit.
However, if that shunt goes bad after the filament does, the entire 50-bulb set will go dark because the electricity no longer passes through it.
Thus, the whole light-set goes dead.
In this case, to repair the light set, one needs to find that bad bulb, and then replace it with a new one.
Normally, a new one will be rated near 2.
4-volts for a 50-light set.
Finding the bad bulb.
Unless one buys a special testing device, the fastest way to find a bad bulb with a broken or missing filament is to look at each one with a magnifying glass in front of a mild background light.
The background light enhances this process for the colored ones whose filaments can be hard to see with naked eye.
Also, a darkened or color-changed bulb might indicate a burnt-out filament, which will be easier to spot outright than by using the magnifier.
Otherwise, to locate the bad bulb, the repair person might have to remove and replace each one in the set, one at a time temporarily while the string is plugged in, with a new bulb known to be working okay.
Normally, one or two extra bulbs come with a new string of mini-lights.
When the bad one has been replaced with a good one, presto, the whole set comes back on.
Much repair time can be saved by replacing a bad bulb soon after it goes out, i.
, before the whole set goes out.
That is, the bad one is much easier to spot and replace while its shunt is still passing electricity to the rest of the set, which is still lit at this time.
For information and diagrams on incandescent mini-lights and the testing of their bulbs, see the following sites.