Hello, i just found this thread. I have been doing Christmas displays for decades and LEDs since they came out. Since Christmas is right around the corner I figured I'd chime in.
It is true these LED strings aren't as perfect as we'd been led to believe, but many of the issues can be resolved. The problems spoken of on this forum are many because the failure modes are many. Some problems are similar or identical to those with incandescent light strings, and new ones have been added! (of course.)
1). As mentioned by some, the only truly reliable way to fix a string that has a bad section is to take each lamp (LED or incandescent) out of the nonworking part and swap it into a socket of a known working string of the same type. You may have to do this with every last lamp in the bad section! Do so with great caution, as you're working with a live string. Work on a non conductive surface and make sure you are neither touching anything grounded nor any moist surface. If you doubt your abilities or the situation, it would be far better to just buy a new string than possibly risk your life!
2) Some of the posts from several years ago mention plugging in lights one way and half the string lights, flip the plug and the other half lights. This issue came about because around 2007~maybe 2008 one brand of string available at The Home Depot, ACE Hardware and possibly others contained a bridge rectifier (four extra, non- light emitting diodes that direct both half cycles of the AC waveform into the same direction to make the LEDs brighter and with less flicker, as mentioned previously.) This is not unusual in itself- many types of Christmas light strings have rectifier diodes to reduce flicker, and they're used in some LED rope lights as well. The problem here is the manufacturer used a GLOBAL rectifier that affects, not only the LEDs in that particular section (25, 30, or 35 LEDs typically), but the add-on socket at the end as well, which unfortunately for most people is a standard Edison type connector! This was only done for a year or two, then they realized the error of their ways (or the safety agencies had their say) and stopped doing it. This is found exclusively in certain strings that have non-replaceable LEDs with each base covered in a thin green or clear shrink wrap where you can see light leaking through the base itself, and where there is a single, bulbous plastic molding at the male end only, which comes before any of the lamps. This contains the global rectifier. DO NOT plug anything requiring actual AC into such a string (for example a motorized yard decoration, as it could overheat and burn up!) Some LEDs and probably all incandescent can be plugged in, up to probably about an ampere of current, with some caveats.
Some LED strings are wired with both halves (or more) in polarity, and some out of polarity. Why? I have no idea! The out-of-polarity ones will not work properly with the global-rectified strings, exhibiting the behavior already mentioned. The in-polarity ones will work in their entirety (if the string itself is OK) and will be brighter as a result and have less flicker (unless they have their own rectifier too in which case you wouldn't notice a difference). HOWEVER-- the current limiting resistors in the unrectified LED strings plugged into the global-rectifier string may overheat and possibly melt, so you shouldn't use this as a feature unless you really know what you're doing! Check the enlarged knob or bulbous plastic molding on the plugged-in strings to see how hot they're getting. It might be best to get rid of those offending string(s), or give them to someone who knows what they are and how to properly use them. Yes, yet more waste for a supposedly environmentally friendly product:( Unfortunately even some very rare Jewel cover LED strings were made this way.
3). LED strings sold in brick-and-mortars (at least here in Tucson, Arizona) have not used mild-steel-leadframe LEDs for several years now (yay! The manufacturers are to be commended for doing something right!!!), so corroded-off wire legs should be a thing of the past, even if the sockets are not sealed from rain. This change can be verified with a magnet, and is probably at least partly responsible for LED string prices not having dropped as fast as we expected over the years. Interestingly, the packs of bare replacement LEDs available on eBay, etc. still use steel:(...
4). Where there are enough LEDs in a section (about 25 or 30 seems to be plenty) they are usually not provided with any protective electronics for each LED other than the current limiting resistors in my experience. All the LEDs are run in series, not parallel, except for the really low voltage strings that run off a small solar panel. The nature of the LED semiconductor itself is such that they share the total voltage (170 volts peak for a 120VAC line) as long as the reverse voltage rating of each LED is not exceeded by too much. (The manufacturers fudge it a bit, you see, as we've come to expect from this quality of product- sometimes 6 or 8 volts is close enough to 5 for them). So you can have 5 x 35 lamps = 175V which is higher than 170 and we're OK. I would hope when they get down into those 25-light C9 strings, and especially the 15-LED icicle sets, that they're including reverse blocking diodes to protect the LEDs but... who knows?!?
5). Yes, just like with an incandescent series string, leaving bad LEDs in the string or cutting them out entirely will tend to reduce the life of the others in that same series section. This is because the same total line voltage now is shared aamongst fewer LEDs, so the total forward voltage of all the LEDs in that series section is reduced, causing the line voltage to be able to push more current through the remaining lamps. If you want your series light strings to last longer, try splicing in a few more lamps than were originally in the string, not taking some out. This will make the lights unnoticeably dimmer.
With LEDs, bear in mind that if you splice any in by wire splicing (using waterproof connection methods of course!) that the polarity of the new lamps needs to be correct or the new lamps will probably fry or at least be damaged due to excessive reverse voltage (the voltage that's applied to the string of LEDs when the AC cycle has reversed and the LEDs are not conducting, unless they're running from a rectifier.) An LED probably designed for 5 reverse volts could easily see 50 or even more this way!
6). Almost universally these LED strings are pushed by the manufacturer to their limits to make them as bright as possible, rather than running them conservatively to help ensure rated lifetime. Current limiting resistors are usually too small, letting too much current flow, particularly when used in a warm climate. This is especially true of the 25-light C7 and C9 types in my experience. (Some LED types handle this service better than others.) This sometimes makes it hard or impossible to realize the supposed 25,000 hour lifetimes, let alone the 100,000 hours they were touting when these first started coming out about 10 years ago!
6). As for the comment about standardized strings and LEDs, they are already available... well, kinda. Check out the GE brand strings. These are characterized by a string that has alternating 4-wire, 3-wire, 4-wire etc. numbers of wires between each adjacent socket. They always have 50 lamps per section, though the string might be 100 lamps. Both the male and female power connectors are largeish and rectangular, with a screw showing in one side that looks like it has been sealed with Loctite or similar. 3 wires protrude from each male and female plug, rather than the usual two. There is no stacking male plugs with these ones.
The four wire/three wire thing is because every 4-wired-together pair of sockets is in parallel with itself and in series with every other pair of 4-wire-connected sockets (called a 2-parallel, 25-series arrangement), so that if one lamp falls out, the string continues to light, but with the remaining lamp of the affected pair being a bit brighter now. You could in fact lose up to 25 lamps from these strings (every other lamp) and they'd continue lighting! (hopefully...)
To achieve standardization, the LEDs for these, both red/orange/yellow and green/blue/white alike all have a forward voltage of about 3 volts and are thus interchangeable!!! (Well, mostly anyway...). They accomplish this seemingly impossible feat by adding an actual extra non-LED diode junction inside the package of each lower-forward-voltage (red, orange, non-filtered warm white yellow) LED! Pretty cool!
The only-kinda-interchangeability I alluded to consists in the fact that the manufacturer has not been consistent, over the years, in what polarity they assemble these lamps in! Go figure. :(...
And I must mention, unfortunately, that these strings have 2 major problems, unless they've fixed them recently:
1: they include a small circuit board in each male and female plug. Due to water ingress into the plugs the circuit board is subject to corrosion which will take down the string itself and any other string plugged into it :(
2: on those circuit boards rests a rectifier (good- less flicker) and a capacitor ( further reduces flicker) which unfortunately causes major problems when trying to dim these strings. Some dimmers will not do it at all, becoming latched on while the GE string remains connected, and the ones that do dim the string cause a great deal of audible noise and noise on the AC line due to this capacitance. I haven't had anything get damaged though after a couple seasons of running a dozen or so of these strings on dimmer, but you can tell it's not a happy setup and I'm not going to be dimming any of them at all this year. So don't go thinking these strings would be good with your light show controller if it does any dimming at all.
I hope this all is not too confusing and offers some help before this holiday season!