Either the main head switch or trip switch in spine, they are both to the left of the power switch.
I have attached a couple of pics, the trip switch with the red shaft does fail, as does the normal switch with it's latching function.
You may find if you press and hold the switch firmly, the head motor may stay on.
there is another switch, on the axis point under the side housing, its a micro switch. My bet is either of the 2 switches pictured.