Staefa SM II is a compact DDC/analog controller used for HVAC and VAV applications, featuring proprietary 0‑20 Vdc phase‑cut outputs or universal 0‑10 Vdc analog outputs depending on the variant. Common failure points in this class include power supply board degradation (especially the linear regulator), overheating of the microcontroller IC, and corrosion of the TTL communication contacts that can cause intermittent loss of device discovery. First‑check diagnostics: verify input voltage and regulator output with a multimeter; inspect the PCB for burn marks or swollen capacitors; check all analog/TTL connectors for clean, solid contact and review any logged error codes from the controller’s diagnostic display.[oss-gpt-20b]