Trimmer Pot Autopsy and Repair

[Cyberdragon]

I found out why these upright trimmers fail. It appears to be that the plastic knob is rubbing off some sort of conductive coating causing a high resistance or break where the knob touches the track. (shown by the two light colored bands on the track). You can repair them by applying a very thin solder bridge across the break, as long as it doesn't get in the way of the knob.