Calibration of 8" floppy drive?

I have two intermittently functional 8" floppy drives that I debugged to the best of my ability, and as far as I've been able to tell their problems must be limited to the circuitry that deals with the actual magnetic interface with the disk. The intercommunication seems normal and the head load solenoids fire, but after that the drive often aborts the process, presumably because it wasn't able to read data properly. I think the signal for reading the disk-rotation hole even fires correctly. The drives are Siemens FDD 100-8 drives used in an S-100 bus system, and controlled by a Jade "Double D" disk controller. The machine was in a functional state when stored, and I have known-good copies of disks that I've been able to boot from at least a few times. (Unless the drives somehow damaged them.) Can someone knowledgeable about 8" floppy drives share information about how these things were serviced and maintained, and what sort of procedures were required to test and calibrate them? Information I've picked up through osmosis leads me to believe that there's a floppy disk with a special pattern on it, and you use specialized test equipment to check the flux off the head and dial it into proper settings. I suspect there the drive has trim pots or similar that allow for this. So, how do you deal with your 8" drives, and what do you do when they don't work? Thanks, Dan