If you've had a Telecaster for any length of time, there's a reasonable chance you've experienced Wobbly Jack Syndrome. It's an annoying condition that afflicts Teles from time to time. You know it… That little recessed metal dish or cup that holds the output jack starts to get a little loose. After a while, it's very loose and—sometimes—even causes nasty signal cracks and output loss.
Inside the hole in the side of the guitar, the output jack is mounted through a metal 'retainer clip' and the metal cup. The retainer clip (photo on the right below) should hold it all tightly in the wall of the hole but sometimes works its way loose.
The clip works on a really simple principle: it goes into the hole with its sides bent (as shown above) and is forced to straighten. This causes the sides to dig into the hole-wall and holds it all in place.
Easy. And it's an easy fix if you have the right tool. If you don't—despite the easy principle—it's almost impossible to do properly.
And that ugly looking hunk of metal in the photo on the left is the right tool. Leo Fender may have given it a proper name but, for me, it's just the Tele Jack Clip Installation Thing. It makes it easy to remove an existing clip or to properly install a new one. Without it, you end up hacking aimlessly and hoping for the best.
If you're fed up with wobbly Tele jacks, a device called an Electrosocket can be screwed in to replace the, rather fiddly, clip and cup arrangement. It's not something you'd really do with a nice vintage Tele, though. In that case, occasional wobbly jacks are just part of the magic.