Open the POT and spray the wiper blades/opposite contacting surface they ride on with non-residue electrical contact cleaner. SLIGHTLY bend the feelers (wiper blades) out to insure they are making good contact with the other surface they ride on when it's put back together. Not too much, as that will cause premature wear on the surfaces. Just enough to insure contact. Repair any damage that caused the POT to turn past it's stopping point (was designed to prevent it).
I would re-try the pot to see if it works (measure it with a multimeter as you slowly turn the pot...is it smooth is it's transition?). If it's not, then time for more drastic measures.
Use some computer printer paper or some other very mildly abrasive form of paper to sand the contact surface the feelers ride on. Then clean all surfaces again with non-residue contact cleaner. I have fixed many pots in servos on things like RC planes by cleaning them with just contact cleaner, still others by slightly bending out the feelers, and still others by taking the more drastic action of sanding the contact surface, with all of these actions of course followed by spraying with contact cleaner to finish up.
Some pots come apart rather easily, by just bending some clips. Others are a bit harder to crack open, such as having a c-clip on the end of the pot shaft or such.
I'd say out of about 12 or 14 servos I've cleaned over the years on RC electrics, only about 4 or 5 I couldn't recover, and even then I probably could have but just didn't feel like cracking them open again to sand with some paper and going thru the whole cleaning routine. When you are talking some servos costing as little as $4 there is only so much point to taking the time to fixing it being worth the trouble. Only ones I ever got intense about fixing were the $20+ servos I used in more demanding applications.
Incidentally, a bad servo pot will cause the servo to flutter instead of holding still, or to wander on you all the way to one side when it shouldn't. That should tell you something about POTs on detectors, in how they can make a machine, it's volume, it's threshold, etc....do some wacky things. If you've damaged the feelers due to turning the POT beyond it's proper movement area then probably no hope to fix it. If the feelers look intact then hope lives.
