Most of the arms I see, if not obviously broken (shattered/missing balls), will typically present with at east one pitted ball in a bearing. These can seem to work fine until that pit works its way around to the running surface of the race or pivot, at which point the bearing is ’loose’ with play. It’s also typical for there to be fine detritus in the bearing increasing friction.
Half the time on seemingly good arms I find indentations from the balls in the pivot surface. These surfaces need to be re-ground and polished out for best performance, or the arm will tend to have resting positions when the balls rotate around to fit in the indentations. Usually because of the detritus in the bearing the friction is high enough that this condition isn’t obvious from obersvation.
The silicon nitride balls I use have better roundness and surface finish and a lower friction coefficient than the original rubies. This makes the outstanding bearings in these arms just a bit better.
With these arms, even in poor condition relative to their potential, the performance is beyond most contemporaries. This is to say that people don’t realize they’re leaving performance on the table. If your EPA-100 is nearly impossible to get to hold balance and moves around on air currents in a seemingly completely still room, then there’s an excellent chance it’s still in great shape. If not, it could be a lot better.
EDIT: Nearly forgot - on all of them with original wiring the insulation has reacted with the copper causing a good deal of corrosion and fragility.
