Manufacturing of magnetometer units employs a test socket having a substantially rigid body with a cavity therein holding an untested unit in a predetermined position proximate electrical connection thereto, wherein one or more magnetic field sources fixed in the body provide known magnetic fields at the position so that the response of each unit is measured and compared to stored expected values. Based thereon, each unit can be calibrated or trimmed by feeding corrective electrical signals back to the unit through the test socket until the actual and expected responses match or the unit is discarded as uncorrectable. In a preferred embodiment, the magnetic field sources are substantially orthogonal coil pairs arranged so that their centerlines coincide at a common point within the predetermined position. Because the test-socket is especially rugged and compact, other functions (e.g., accelerometers) included in the unit can also be easily tested and trimmed.