I have quite a number of Xmtrs that are 10 or more years old and have never had a potentiometer give me problems. In fact I have some old assemblies that I have cannabalized for the stick assemblys to make servo testors out of and all of them have shown no tendency to malfunction. Now if youv'e dunked them in a lake or something like that, you could get a problem or two. The same goes for synthesized modules, they either work or do not work, they don't gradually deteriorate. The airborne components are different; they suffer a lot from vibration if you are running piston engines and the pots can definately wear in the servos. The gears also wear, especially the metal ones more so than the plastic ones. This type of failure usually shows up gradually in plenty of time for you to catch it and replace the faulty unit. In short, it would be prohibitively expensive (in manhours) to expect a radio repair technician to catch such items at pot wear in a Xmtr. You would get by less expensively by just buying a new one.