purchased used, at first the control sort of slipped and it was hard to get it to engage. Then it worked fine for several months. Now it works fine but it only runs at the fast speed.
Look at the carbon brushes in the motor. Theyre graphite blocks which transfer mains power to the copper segments of the motor. v. cheap parts and easy to replace. Look at the copper segments to check for burning or pitting. other than that its the motor control circuit board which is faulty.
check out www.2ndwave.co.uk/manual.html for more info