Thanks MarkT and jackrae!
It worked! I can control it by pwm - low pass filter! I had to adjust the filter though...
I tried the filter with 1k ohm and 1 uF, and although it was stable with no load, over the multimeter, when connected to the control box, the motor oscillated, as the voltage was increasing and decreasing slowly, trying to keep the signal steady..
I raised the capacitor value till 47 uF (electrolytic), and although the response time is slower now, the signal is steady and the motor runs smoothly, exactly the way the pot used to run it.
Thanks for all your help!
