---------Original Messages: <snip> <snip> Hi I guess my problem was reading your sentence. It just isn't clear to me where you are saying they are equal and where they are not. The part "60Hz bars = 300RPM and the 50Hz bars = 360RPM" seems to be saying that "300RPM @ 60Hz = 360RPM @ 50Hz". I'm sure I'm reading it wrong but it is still confusing. At least we both agree on the basics and that is all that counts. Dwight --------Reply: Well, I did say "the 60 and 50Hz *BARS*" (as in "stripe" or "band") at the same frequency of illumination (60Hz), but perhaps I should have inserted "the speed at which the bars seem to stop" instead of trying to keep it short and simple (with exactly the opposite effect). OK, how about this: It just so happens that the ratio of 360RPM to 300RPM is the same as 60Hz to 50Hz. Therefore, if you have a strobe disk for a 300RPM drive with both 50Hz and 60Hz bars on it and a 60Hz source of illumination, then you can use the same disk to check the speed of a 360RPM drive, since at 360RPM the 50Hz bars will appear to be stationary. I'm talking about using a 300RPM disk at 60Hz (for the OP and others trying to check the speed of a 360RPM FD400 or equivalent with 60Hz); you seem to be talking about using a 360RPM strobe disk (if you found one) at 50Hz, a somewhat different situation albeit one taking advantage of the same "coincidence". But I think by now both our points have been made abundantly and painfully clear, and I'm glad that we can finally put it to rest and not clutter up the archives further. m