Sensitivity correcting circuit of servo signal detection on data surface and offset measuring circuit in magnetic disk unit

A magnetic disk unit using a sensitivity correcting circuit for a servo signal detection on a data surface having

• disk media each having first servo information (A) recorded thereon with an outward offset of predetermined quantity from an ontrack position and second servo information (B) with an inward offset of predetermined quantity from the ontrack position;
• a plurality of data heads provided in correspondence with the disk media;
• offset detection means for reading signals (V.sub.A, V.sub.B) on the first and second servo information (A, B) on one disk medium with the data head, detecting an offset of the data head by multiplying a differential signal (V.sub.A -V.sub.B) of the read signals by a proportional coefficient K representing an output sensitivity of the data head;
• offset storage means for storing the offset of each data head;
• and
• offset correction means for correcting the position of a data head selected for reading or writing one disk medium in a way to remove the offset of the selected data head stored in the offset storage means, the sensitivity correcting circuit comprising
• sensitivity detection means disposed in the offset detection means, for outwardly offsetting one data head from an ontrack position by a given quantity (XI), said sensitivity detection means including means for reading signals (V.sub.A, V.sub.B) on the first and second servo information (A, B), means for detecting a differential signal (V1) of the read signals, means for inwardly offsetting the data head from the ontrack position by a given quantity (X2), means for reading signals (V.sub.A, B.sub.A) on the first and second servo information, detecting a differential signal (V2) of the read signals, and means for computing a proportional coefficient K for the data head according to the differential signals (V1, V2).