The J300X‑40 is a Simrad Robertson autopilot junction unit that interfaces with AP20/21/22/25/26/27/35 and AP11 controllers. It provides two NMEA 0183 input/output ports, a 10 Hz heading output from Tx port 2, a radar clock/data interface, an external alarm, and drives reversible drive units or solenoid‑activated steering up to 20 A (continuous). Common failure points include power supply degradation (12/24/32 V DC), corrosion or loose contacts on the NMEA ports, firmware corruption leading to loss of heading output, and overheating of the internal regulator. First‑check diagnostics: verify input voltage and polarity at the VDC terminals; inspect all connector pins for bent or corroded contacts; check the 10 Hz heading output signal with an oscilloscope or logic analyzer; review any stored error codes via the AP controller interface.[oss-gpt-20b]