What am I looking for with strobe disc?

You've got to have a strobe light to use with the strobe disc. If you've got that, you want to adjust the speed so that the line of "dots" sits as still as possible for the speed and frequency selected. Since some countries use 50 Hz and some use 60 Hz as the AC frequency, many discs have four rows of "dots". There are two rows for 33 rpm and two for 45 rpm, one row each for 50 Hz and 60 Hz accordingly. If playing a 33 ( standard LP ) in the USA, you would want the 33 rpm / 60 Hz row of dots to "stand still". If you can't get it to stand perfectly still, you want it to hold as steady as possible. If it seems like it is gradually slipping forward, the music will be slightly faster / more up-beat. If it seems like it is gradually slipping backwards, the music will be slightly slower / more drawn-out. If you can't achieve "perfection", it is up to your personal preference to set it to what you think sounds "better" to your ears. Sean
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A hit of acid?

The $95 KAB stobe kit works for me {strobe and plate with 33rpm,45rpm and 78??}.

Forgive me,I thought you were looking for a unit.
When on 33 rpm,the 33 marked portion should be at rest and stable.If it drifts,then an adjustmentment is in order.Provided the table has such a device.
Seans' description is right on target.My observations are referenced to the KAB unit.

Thanks much for the info! I think I'm getting the hang of it. The PLC on my VPI is MUCH better than without, go figure.

Related questions: Clearaudio has a strobe disk with a drag groove which allows one to measure the speed with the drag created by the stylus. Is the drag created by the stylus significant? Will it vary from record to record (or even track to track (or even moment to moment as the pitch of the music changes))? Will the drag vary depending on the weight of the platter? Is the industry standard 33 1/3 assuming no stylus drag? Are we all nuts to worry about this?

The "strobe light" can just be an inexpensive neon circuit tester. Any light source that flickers with the power line frequency. Fluorescent lights used to work, but I think they have improved them so they don't flicker these days.

The effect, if any, of stylus drag will depend on your particular turntable: platter mass, motor drive power, etc.