If individual bulbs are dead, they just burned out. If complete strands are dead, and there is no obvious break in the wire, then probably just a cheaply made strand which should be discarded. If the indivitdua bulbs are removable, save them.

That many consecutive bulbs going unlit could be because that section of the strand has individual lights connected in series. Each bulb in the dark section would have to be replaced by a working bulb, in turn, until that strand section works.

Older incandescent strings need to be handled with care, have spare bulbs handy, and try wiggling the bulb and socket on problem areas.

I can understand hanging on to old Christmas lights as a tradition; we are limping along with a circa 1996 string of pearl lights although 2 or 3 are dark, and no spares.

Do you have a cat batting at the lights or other disturbances?

The bulbs are in series in the string, meaning that if one bulb would go out, all the bulbs in the string would go out. To prevent that from happening, the bulbs have a shunt at the base of the bulb (what looks like a second filament at the bottom of the bulb).

When all the bulbs are working, since they are in series, the mains voltage (120 volts) is divided by the number of bulbs, around 2.5 volts in a typical 100-light string (which contains two 50-light strings in parallel). If a bulb burns out, the whole 120 volts is applied across the shunt, which has a coating that makes it conduct electricity only at higher voltage, and it completes the circuit. However, as more bulbs burn out, the 120V is divided by a smaller amount, so a higher voltage is present across the remaining bulbs. That higher voltage puts more stress on the filaments, which makes them fail sooner. If enough burnt-out bulbs are left on the string, at some point the voltage becomes higher that the bulbs' filaments can handle and they all fail catastrophically, leaving that blackening inside the bulbs. Changing the burnt bulbs promptly will prevent problem #2 . On my Christmas trees, I check once a week for burnt bulbs, and replace them.

When you take the strings off the tree, you move them around, they get shaken, vibrate, and all that moving around and shaking breaks the filaments of bulbs that were just on the verge of failing. Since this happens while the strings are not plugged in, their internal shunt never gets "activated" by the 120 volts and these bulbs cannot complete the circuit. Unfortunately, there is not much that can be done to prevent problem #1. However, in many cases (not always), I would say 40% of times, you can use a non-contact voltage tester, plug the string in and run the non-contact voltage tester along the string. Where it stops glowing is the burnt out bulb. If it glows along the entire length of the string, rotate the plug 180 degrees and repeat. More than likely you will have more than one burnt bulb in the string.

If the non-contact tester method does not work for you, and IF you have lots of patience, you can take the bulbs one by one and try them in a known good string. Discard any bulb that does not light up, even if it makes the string light up.

Happy new year, and good luck with your light strings next year!

W0lley, no, I don’t have that kind of patience. Logician, no, no cat or any other disturbance I can think of. The strand on the tress was about 3 years old.

No matter how careful you are, you are still manipulating the strings, turning the bulbs in different ways, even just the vibration from walking with the strings in your hands is enough to break an incandescent bulb's filament that is on the verge of failing. That's just the nature of the thing.

If you have a non-contact voltage tester, try it! I am sure it will help you salvage at least a couple of strings. But don't try to put a single new bulb in a string with many blackened bulbs while it is plugged in, for if you happen to reach the critical voltage, the shunts will start to conduct and you are likely to lose all the remaining good bulbs. But you could try, with the string unplugged, replacing all the blackened bulbs with known good bulbs, THEN plug it it, and use the non-contact tester method if it does not work.

And I, too, hold on to my good old long lasting incandescent strings. I purchased some LED strings to put in some exterior trees, and they just don't last more than a few years. But I still have the strings my aunt gave me for Christmas back in 1983, and most of the bulbs are original!