Teledyne Gurley Model 400W is a vintage mountain‑transit telescope used for precision angular measurements in surveying. Common failure modes for this class of instruments include misalignment or damage to the optical tube, wear or loosening of the vernier scales and horizontal circle bearings, corrosion or loss of lubrication on the telescope’s pivot, and degradation of the internal electrical components (if equipped with a battery‑powered readout). First‑check diagnostics: verify that the power supply or battery is functioning and correctly connected; inspect the optical tube for scratches, misalignment, or debris; examine the vernier scales and circle bearings for looseness or wear; review any error codes displayed on an electronic readout.[oss-gpt-20b]