Нет плавного пуска, проблема кнопки шуруповерта DeWALT DCD734 14.4V

In this video, there is a problem in which there is no adjustable low speed in the screwdriver when the button is turned on, only high with the maximum clamped key. What is the problem of how to establish the cause of the breakdown and the order of replacement - we look and comment ...