Why not call or E-mail VPI....it sounds like something is wrong.
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What strobe disc and light are you using? The VPI supplied strobe disc is pretty much useless, and I didn't have much more luck with a decent quality Hansen strobe disc and a "neon" light until I got the KAB Speed Strobe and used the quartz illuminator to determine the platter speed. You really need to be able to see the strobe disc markers clearly, which could be difficult without proper light.
If the bars move counter clockwise the platter is slow, increase the frequency of the sds to correct. If the movement is clockwise then the platter is fast so reduce the frequency to correct.
If the rate of rotation of the bars does not change when you change the frequency of the sds then that would suggest the the frequency is not changing and something is wrong with the sds.
If you are changing by .01 hz it will take quite a bit to make a visible change on the strobe disk. Try changing the frequency by 5 or 10 hz and see what happens.
This may be an irrelevant or stupid (or both) response, but make sure you are not using any type of battery-powered light source; it needs to be powered by AC to result in the necessary "strobing". How do I know? I tried a battery-powered fluorescent light for hours until I called VPI and they told me to use an AC-powered (plugged into the the wall outlet) fluorescent...DOH!