Here is some general information on repair of miniature series string
Christmas lights.

If it's an "end to end" string with an outlet on the end you can check if
the fuses (plural because both sides are fused on these non polarized
units) by plugging another known to work in the end or some other VERY
LOW draw item or a voltmeter or neon test light. Likewise if it's a
string made of several substrings (like 100 or 150 light strings are made
of sets of 50; 70's are made of two 35 light strings) and some group(s)
light and some don't then it's not the fuses. If you suspect fuses and
replace them and still no lights anywhere then you have a connection
problem somewhere along the line.

The most common problem when a string (or substring) doesn't light is
going to be a bulb that is missing, is not fully inserted, or has burned
out and the built-in shunt has failed to keep the connection going.
These are the things that take the longest to find and often fail the is-
it-worth-it test.

First look it over to find any obvious problem.

If not then here is where it gets difficult. You can replace bulbs,
sometimes replacing one then using that one to subsitute for the next and
so on but that won't help if there is more than one problem. (Usually
it's cheaper to sacrifice a string as a source of spare bulbs rather than
buying separately.)

Best bet is to get one of those pen-like devices with an LED that glows
when brought near a "hot" wire. To use effectively you will have to
figure out which way you have plugged in the string (to assure that
"hotness" is being fed from the plug end while the series string of bulbs
is connected to neutral at the far end. The diagnostic work can be done
the other way but then you have to work from the far end back to the
beginning. Easier just to reverse the plug.

And for purposes of this discussion we are talking about the end of a
series string of bulbs which will not the end of the overall string on
100 and 150 light sets. You can tell where one substring ends and the
next starts even without counting by looking for a place where there are
only two wires between bulbs.

Remember these detector pens need only be brought close to an INSULATED
wire that is "hot" for the LED to glow. You don't strip anything or have
contact with dangerous voltage.

You will have to do some untwisting of the wires to ensure that the
detector pen is brought close to a potentially hot single wire which
doesn't have any other conductors near it.

Basically, after you play around a bit with the wires from the plug to
the first bulb to gain knowledge of how the detector works you should try
it on the first few bulbs, untwisting some wires and proving that "hot"
goes in and comes out on the wire that holds the bulbs.

Once you get the hang of this then go out about half way through the
bulbs and see if you still have "hotness" there. If you do then divide
the distance to the end of the string (meaning that group of 50 if it's a
multi string set). If you don't detect anything at that point then go
back halfway to the beginning and see if you find it there. You should
be able to narrow it down to a bulb where "hot" goes in but doesn't come
out. Replace that bulb. If the string or sub string still doesn't light
keep checking as you have more than one problem.

Remember that one of the other wires, of the two that go direct from plug
to end outlet (or to the next substring), is carrying "hot" so you must
separate the bulb-string wire from it to avoid a false positive.
Likewise that other wire is carrying neutral and will cancel the reading
if too close to the wire under test.

If you are dealing with a string that has no outlet on the end you can
use this method but beware that this type of string has alternate bulbs
on opposite wires. In other words, imagine a circle of wire with bulbs
on them at, say, 8" intervals and then is twisted together into a linear
string so the bulbs end up about 4" apart. The work outlined above can
be done but as you work along you skip every other bulb as you go along
detecting.

I have another method of repairing strings but it's too complex to
explain here but it involves using a Variac and lighting a few bulbs on
appropriate voltage, then moving farther along and increasing the voltage
and eventually finding and repair the bad bulbs. I call it the brute
force method as I've never failed to repair a string unless some wiring
defect is found where it would be unsafe to put it back in service.
Those become the bulb donors for good strings and wire salvaged for other
use.