Preset is a small variable resistor generally used to provide a variable resistance in a circuit in order to tune the circuit. They are directly mounted on the PCBs. A screwdriver or similar tool is required to adjust them.Working of presets is quite simple. It works by having a resistive track and a small plate having dents to short the track. This small plate can be rotated using a screw driver or some similar tool. Position of the dents on the track determines the amount of resistance that is to be used in the circuit.