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
the string.

Hybyd2 wrote:

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
light bulbs).

Suggestions?

Thanks,

Henry