bracket 16 to control rotation of cam 108 cam 108 in turn presses against the end 105 of lead screw 90 and provides a fine adjustment of the position of block 85 and the transducer mechanism to peak the transducer output signal by bringing the transducer to the center of the selected circular path on the recording disc bracket 92 is fastened to panel 18 by bolts 112 enabling the transducer its mounting and its drive all to be replaced as a sub assembly fig9 illustrates the components and their method of assembly making up a single sided cassette which may be used with the mechanism described above and which constitutes a further feature of the invention the cassette comprises an outer case consisting of an upper section 120 having a rim 121 and a lower section 122 having a rim 123 the rims 121 and 123 separate the cassette sections 120 and 122 to provide each of them with an internal shallow central opening or cavity preferably of circular configuration the cavity in the lower section 122 being shown in fig9 on the upwardly facing portion of the lower section 122 when the sections of the case are assembled the cavities in sections 120 and 122 cooperate to define a shallow internal chamber 124 within which a disc 125 of thin flexible material is supported for rotation this disc has a magnetic recording surface on its underside as viewed in fig9 and constitutes the recording media of the cassette.