Servo tracks on SMD disk

Hi, I have some trouble with a 14" removable SMD disk drive (Ampex DFR 996): When it comes to the outer tracks it starts making noises like a cat and recalibrates. Endlessly. With some disks I have it manages to crouch up to the end, track by track with many retries. The problem seems to exist with most of the packs I have. I played around with the head cables while the heads are further outside. Seems to be no contact/strain problem here. The drive seems to be working fine for the rest of the surface. But having the end unusable makes the disks unusable for RT11 with its replacement tables. And if the drive doesn't even manage to arrive at the end, RT11 won't even write a new directory to the disk. The drive does not recalibrate on bad blocks. So the problem should be in the servo system and nowhere else. Does my problem sound familiar in any way? Playing with the servo head amplifier gain adjustment did not help. In fact I found out that there's a working range, and the original adjustment was quite in the middle of that. So I kept it the way it was adjusted. For real readjustment of the drive I'd need a disk exerciser board, CE cartridge and some other special purpose bits. None of them are available to me. Another idea is that the servo data is decaying more quickly in the center of the disk: The disk hub is held by a magnet in on the spindle. The magnetic field and the mechanical shock while inserting the cartridge could affect the disk. Or/and the problem could be caused by thinner magnetic coating in the center (too close to the inner edge of the coating?). Vague theses.... The disk drive also has a fixed disk with three disks and 5 data plus 1 servo surface. That works flawlessly. Any comments? If somebody can provide one or more known good and recently tested cartridges, I'd be glad to give them a try. Packs are named CDC 1204. Best wishes, Philipp