The Positron 7501‑16C is a telephony isolator card that operates from a –48 V DC source and is mounted on Three, Five, or Eight‑card Teleline shelves or Central Office side loops. Common failure points for this class of isolation cards include degraded isolation resistors, failed MOSFETs in the bi‑directional modulator/demodulators, and power‑rail decoupling capacitors that have lost capacitance under high‑current transients. First‑check diagnostics should verify the –48 V supply voltage and ground reference continuity, inspect for burn marks or corrosion on the isolation pads, and review any error logs from the Teleline shelf for abnormal loop‑back or line‑loss indications.[oss-gpt-20b]