Ressurecting FDDs (was Original IBM PC (was Re: Prices to

For a floppy driver, there are essentially 4 adjustments that you have to make to do a full alignment : 1) Spindle speed (not on synchronous-motor drives). This one is obvious 2) Head radial position. This involves adjusting the head position radially with respect to the spindle so that it's positioned over the centre of a data track, and not off to one side. This is the one that most people mean by 'alignment'. You use a special disk recorded with ofset tracks, look at the signal from the head on a 'scope and move the heard (or more correctly the stepper motor) so that the 2 offset tracks are the same amplitude on the 'scope 3) Track 0. You've now got the heads over a track. And you know which track it is, since the alignement disk only has said pattern on selected tracks. Now step the head out the appropriate number of tracks and adjust the track 0 sensor (switch/optoswitch) so that it operates at the right place 4) Index timing. This one is not that important, actually, on most controllers. One track on the alignment disk had a burst of data positioned accurately wrt the index hole. What you do is read at that track, look at the index pulse and the data burst on the 'scope and move the index sensor until the delay between them is right. In general only (2) and (3) are that important with modern controllers. All of them are important on hard-sectored controllers, though. OK... Modern hard disks don't need alignment - they're servo-tracked, and the head uses information on the disk platters - the so-called servo bursts - as a position reference. Older demountable hard disks in general either used one surface of a multi-platter stack for this servo information (when you have to align the heads wrt each other), or had a stepping actuator (not, in general as stepper motor, but an actuator that took its reference from something in the drive and not on the disk), when you have to align the heads wrt the spindle. You do these like a floppy drive, with an alignment pack and a 'scope. Some older winchesters have a stepper motor. In general there's not a lot you can do with these other than to low-level format them and keep on using them. Of course you lose data if you do that. CD-ROMs don't need radial alignment as such - they're servo-positioned in general. The average CD-ROM drive has 3 or 4 servo loops - focus, spindle speed, radial position (or coarse and fine radial postion). There are electronic adjustments for these sometimes, and maybe also a laser power control. Don't fiddle without a service manual!. You set these up like an audio CD player. Tape drives also have alignment adjustments. The old 9-track drives (this is classiccmp, after all!) have motor speed, tape tension, head skew (basically angle of the head), sometimes head tilt, tape sensor, etc adjustments. Even cartridge tape drives have tape speed, head height and maybe head azimuth adjustments. I'll leave paper tape units, card readers, printers, teletypes and monitors until another time (read : until someone asks about them). Suffice it to say they all need alignment sometimes. -tony