Aloha Tony
My parma controllers are 25 ohm and to keep em "tuned" I diassemble it and place a piece of 220 grit sandpaper between the wiper arm and the resistor,
pull the trigger a few times to smooth out the wiper and making it flat like we do our tires. When finished add a couple of drops of voodoo comm drops or similar comm drops to the resistor. Oil the pivot pin on the trigger, and to my liking I polish the trigger, where my finger touches it. Also you can remove the wire that goes to the metal frame and solder it to the wiper arm,cut it loose from the frame snake the wire under the trigger and up through the hole in the wiper arm and solder it, this way the power doesnt have to travel through the frame.
The power directly goes to the wiper. If the resistor heats up (though it shouldnt) use an old school heat sink, ebayers sometime sell em.
Aloha
Marty