Mirtone 7602 is an expansion (adder) board that adds two detection zones to a Mirtone 7604 fire‑alarm control panel, increasing total zone count from four to six. Common failure points for this class of add‑on boards include loss of power or voltage drop at the board’s supply pins, burned or tripped relays on the zone outputs, and faulty wiring connections between the board and the main panel. First‑check diagnostics: verify 24 V DC (or specified) supply to the board with a multimeter; inspect all solder joints and connectors for corrosion or breakage; review the panel’s error logs for “zone add‑on failure” codes and confirm that the added zones are correctly mapped in the system configuration.[oss-gpt-20b]