I have a real old one (probably ca. 1985-1990, possibly older than that) that still works just fine. It came with a neon light thingie to test it with. You pop that in the barrel opening and if it lights up, the Zerostat is working. The instructions with mine told you to pull and release the trigger with it pointing at 3 different spots around the LP, and then pull the trigger while pointing at the spindle, and then point it at the floor and release the trigger. Mine came in one of the big Discwasher sets with the wood base BTW.