I have to change the "Cut Of Freq"-pot (Low pass filter) on my Yamaha CS15D. Does anyone know if the potentiometer should be linear or logarithmic?

When i was playing on my Yamaha CS15D turning around the Cut Of Freq-knob it just stuck all of a sudden. I can´t find what´s wrong with it, anyone who knows how to fix it? I can´t remove it a millimeter.