Sometimes a little dirt or dust on the Hall sensor can screw up the operation of the feedback network. Been there. Done that. Fixed that by just a careful cleaning. Of course, if you did anything to alter the spatial relation between the sensor and the magnet, that too would or could cause the problem you describe. It's rather critical that the two pass within less than a very tiny distance of each other (max distance between the two is mentioned in the instructions but I forget exactly).
What the heck did I do
So I pulled my platter to look at the bearing on my VPI prime. I have the eagle PSU, and the road runner tachometer that control my speed. After I re-greased the bearing I dusted the platter with one of those swiffer duster‘s and the duster went near the little PCB board that the Road Runner tach picks up at signal with.
I put everything back together and switched on the turntable, it spins at 33.3 RPM but now the tachometer isn’t picking up any signal to control that speed. Did I statically electrically charged the PCB board and damage it? If so what do I do now?
I put everything back together and switched on the turntable, it spins at 33.3 RPM but now the tachometer isn’t picking up any signal to control that speed. Did I statically electrically charged the PCB board and damage it? If so what do I do now?