I have 2 Makita 3612C's. I wouldn't recommend using an external speed control. It won't necessarily "fry" the router but, if it is soft start. You won't have that ability.
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"Whether active duty, retired, national guard or reserve - is someone who, at one point in their life, wrote a blank check made payable to "The United States of America", for an amount of "up to and including my life". That is HONOR, and there are way too many people in this country who no longer understand it." -Author Unknown