The Granville‑Phillips 270059 is a Series 270 ionization gauge controller designed for dual thermocouple gauges with auto‑ranging and process‑control output. Common failures include loss of high‑voltage isolation leading to leakage currents, cracked or corroded thermocouple connections causing drift or open‑circuit readings, and power‑supply board faults (e.g., capacitor degradation) that produce erratic voltage regulation. For this class of vacuum instrumentation, also watch for improper grounding of the gauge housing and controller chassis, which can create dangerous high‑voltage potentials. First‑check diagnostics: verify the 24 V or 48 V supply rail with a multimeter, inspect all thermocouple leads for continuity and proper solder joints, and confirm that the controller’s output is within spec while monitoring the gauge input voltage for spikes; if available, review any stored error codes in the controller’s diagnostic log.[oss-gpt-20b]