It's the two metal prongs at the bottom of the shooter lane on Avatar LE. Steel ball completes the circuit, ceramic ball doesn't. Not sure how they connect under the apron. 'White ball detector' is dedicated switch D7 in TRS manual. Part numbers listed for the metal 'switch' parts. Lightly sanding the metal pieces and cleaning the steel balls with alcohol (not polish, which can leave a film) may help your problem.