Pioneer DJM 900 NXS color knobs not working properly

Hi,

I've had my mixer (1st gen NXS) for almost 3 years and recently my color knobs on channel one and two have not been working correctly.  

When the color effect is engaged, even if the knob is at 12 o'clock (off), it will still affect the signal.  

I was hoping someone knew the cost of repairing these knobs as I was told by one retailer that if I send it in it's going to cost me a $75 shipping fee that is non-refundable. Just trying to figure out if it's worth my while or to keep playing with this problem.      

Thanks, 

PD