Abstract: In a memory processing unit, a magnetic-head positioning device having a longitudinally actuated carriage with a guide plate carrying the head, or heads, which plate has longitudinal edges and an upper and lower face, one of these faces being in engagement with a spool wheel driving the plate via a belt and the longitudinal edges of the guide plate being guided between a pair of anti-friction bearings. Linear, three-point contact of the guide plate with the bearings and with the spool wheel is insured, regardless of lateral displacement of the guide plate, by mounting the bearings so that they are tiltable, in a plane transverse to the longitudinal movement of the guide plate, by a limited amount.