Calibration of 8" floppy drive?

[Dan K]

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