SportDOG FieldTrainer 425x/SportHunter 825x Add-A-Dog Electronic Collar - Orange - The Collar Receiver for FieldTrainer 425X, SportHunter 825X and SportTrainer 575E/875E Remote Trainers. Add another dog to your current SportDOG Brand electronic training collar system or replace a Collar Receiver with the SDR-AFX Add-A-Dog Receiver Collar.