cctech Digest, Vol 12, Issue 33, Re: Drive Alignment

RE: Drive Alignment 8" drives are usually pretty stable once aligned, but perhaps not over a period of decades. There are two ways to check/set alignment. The "classical" (mid-70's) way is to use an analog alignment diskette and a dual-channel oscilloscope. You connect the scope to two channels of the read head amplifier and seek to the alignment track of the disk (my recollection is track 38 decimal, the center of the 77-track diskette). You should see a kind of "filled in dual sine wave". If it's symmetrical or close, the alignment is correct. If one lobe (every other one) is of significantly greater amplitude than the lobes in between, you are out of alignment and you adjust the position of the head to get it right (this part varies from drive to drive). Then you have to check some other things, like the track 0 sensor and track 0 phase (on some drives). You need the analogy alignment disk, a scope and an "exerciser", or a computer with software that could let you manually seek and load the head as desired. The "modern" (about 1981) way was with a digital alignment diskette. This was a normal data disk, almost, but various tracks on the diskette were either correctly aligned, or intentionally mis-aligned by varying known amounts. By seeing which tracks could be read, and which could not, you could determine the alignment status of the drive. This was useful for checking alignment, but not nearly as good for setting it. SOME of these disks ALSO had the analog patter on track 38. You needed the digital alignment disk and software that knew what to do with it. The problem today is that the alignment disks (of either type) are not readily available, and most PC hobbyists don't have a scope. Documentation, knowledge and experience are also issues.