FD400 drive troubleshooting

Of course it is. I believe you can do it for any speed and light frequency, sctually. 50Hz presumably is 100 flashes/second (I am assuming you are using a mains-energized neon lamp or something). First try making one for 300 flashes per second. That one is trivial, 360rpm is 6 revs/second, so 1/50th of a revolution per flash. So 50 spots works. Now realise that said disk will also work at 100 flashes per second. The fact that a given spot has moved 3 positions, not one, between flashes is not noticeable since all dots look the same This is actually a problem with strobe disks. A disk that will indicate, say, 360rpm will also appear to be stationary at 720rpm (or any integer multiple of the speed). I susepct you'd notice if your disk drive was spinning twice as fast as normal, but I would alwaus check the index pulse timing 'looks sensible' There should be (here) 6 index pulses per second. You can check that quite easliy with a 'scope or counter. The result will not normally be accurate enough to set the speed, but it will make sure you're near enough the right speed that the stroboscope will let you lock to that speed, not twice it or whatever. -tony