The top cap of the Fast Find Ranger is disposable (sacrificial says) it protects the antenna and the activation button of the beacon.
Take care when you replace the top cap, do not over tighten the mounting screws (75cNm).
Before replacing the cap make sure the remaining life of the battery by performing a self-test. (As describe on page 8 of the user manual of the Fast Find Ranger).
If the self-test indicates a level of low or no battery, please return your PLB Fast Find Ranger to after-sales service for ; replacement of the battery, a complete verification of the equipment and the beacon will be returned equipped with a new cap.