Along with rustyJ comment all springs are made to certain tolerance. Meaning the height, diameter, and stiffness produced certain resistance. So measure your sping height. then stretch little at time to get the right resistance or spring pressure you desire to let the advance spool release one click at time when you lightly bump it. If you have to smack it on the ground usually the string will tie itself in knot and not release. If the spring is colasped enough it will release with the slightest bumping. So to make this short depending on the model and design. the spring has to be certain height to alow the right tension on the bump advance. Too weak (or Short) line comes out to easy. Too strong (spring height longer that it's suspose to be makes string hard to advance (providing the string is not knotted up on the spool which happens from time to time). All of us at one time or another has bumped it extremely harder thinking I didn't bump it hard enough when all it was the string became knotted and locked up on the spool reventing string advancement.

Years ago (I'm talking like at least 12) we bought Target's Room Essentials brand string lights - they were on sale for like $7 for a strand. I needed 4 strands to go around the perimeter of my screen porch and I bought a few extra strands figuring that they would eventually go bad and I wouldn't be able to find matching ones to replace them. Here is it 12 years later and all of the extra light sets are still sitting in the original bag in my basement. Every single bulb on the original strands still works. Granted, I only turn them on a few times a year when we have parties, but I would have thought that weather and dampness and corrosion would have done at least one or two of them in by now, but no. They are on a covered porch but up against the screens so they do get rained on, and I leave them up all year round....I certainly haven't babied them at all! And Target still carries the exact same lights too -- figures! (You know, if I hadn't stocked up on matching extras, they would be discontinued for sure!!!) Every year I see them in the flyer on sale several times a year, so if $12.99 is too steep for you, wait a few weeks -- they'll go on sale. http://www.target.com/p/25ct-clear-globe-lights-room-essentials-153/-/A-17186971 Also, they are not LED but I actually prefer that -- they have a very vintage old-fashioned carnival-y kind of look that I love! And since I only turn them on a few times a year, I don't feel too guilty. (I've converted most of my home lighting to LED but there's something about that incandescent glow for string lights -- I'll never convert my Christmas lights either!)

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!

The prongs are typically made of brass or nickel-plated brass. Brass is used in bathroom plumbing and in marine applications because it doesn't rust...it has little or no iron in it. Over time brass can tarnish a bit, but that shouldn't be an issue. I suggest you just let it be, and see if anything happens that looks concerning. Personally, I wouldn't worry about it. My main concern would be if it was plugged in to power in an exposed location and a rainstorm happened: water could short out the connection where it was plugged in to the extension cord.