Generally speaking, the only really satisfactory way of reducing the resistance that much is to replace the resistor.
Perhaps there's an expert on Ninco controllers out there who would know if there are any alternative Ninco or other make resistors that will fit.
If you go to the trouble of finding the right value resistance wire you could rewind the 55 ohm resistor.
If you are into transistorised circuit design, you design a transistorised controller using the Ninco parts.
If it were me, I'd buy a 25 ohm Parma rather than go to the trouble of those ideas.
The resistor has a coil of resistance wire would on a ceramic core. The total length of wire should have a resistance of 55 ohms,
Ideally more turns of thicker wire is what you want but resistance wire is difficult to come by in DIY quantities. It should work OK if you rewind what's already there, To produce 25 ohms unwind the wire and rewind 25/55 of the length (about 45 % ) evenly along the length of the former between the bands it was originally connected to. The neater and tighter you wind it the better. That'd leave larger gaps between the wire, but the moving arm will probably tolerate that.
The wires should be coated with something to keep them in place except over the part where the wiper rubs. Several coats of the high temperature paint used on car exhaust systems works well.
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