Abstract: An arc welder has a locking crank mechanism for adjusting and securing the adjustment of a transformer having a moveable transformer core shunt. The arc welder has an adjustment base, with a plurality of locking detents in a substantially circular planar ring configuration. The position of the base is fixed in relation to the position of the transformer. The arc welder also has a substantially circular crank plate with a lever opening, with the crank plate mounted for rotation adjacent to and concentric with the locking detents. A mechanical link operably connects the crank plate and the moveable core shunt for adjusting the position of the moveable core shunt through rotation of the crank plate. A lever is mounted to pass through the lever opening in the crank plate.