SDS speed control

Shakey- Belt slippage doesn't have to be, "severe" to show with a strobe/disc and not everyone can discern minor speed/pitch variations aurally. The OP did say, "a slow spin to the right."

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!