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Power down the CNC completely and discharge residual power by waiting 5 minutes. Reseat the main CPU board in the backplane, ensuring proper seating and connector integrity.

Follow proper ESD precautions. Disconnect all power sources before opening control cabinet.

Power up and observe if alarm recurs immediately during POST (Power-On Self Test). If alarm returns within seconds of boot, memory hardware is defective.

Check FANUC board part number on main CPU board (locations vary: 0i uses A20B-8200-series, 16i/18i use A20B-8100-series, 30i/31i/32i use A20B-8101-series or later). Document full board number including suffix for replacement.

Replace main CPU board with exact or compatible part number. After replacement, perform battery backup check and restore machine parameters from backup using SYSTEM > PMC > I/O or memory card restore function.

Battery-backed SRAM will lose parameters during board replacement. Ensure complete parameter backup exists before proceeding.

Power down completely. Remove main CPU board and inspect edge connector for corrosion, bent pins, or debris. Clean contacts with electronic contact cleaner and lint-free cloth.

Do not use abrasive materials on gold-plated contacts. ESD precautions required.

Inspect backplane connector slots for damage, burned traces, or foreign material. Check mounting screws and board retaining clips for proper tension.

Reinstall CPU board firmly into backplane. Ensure board seats completely and retention mechanism engages. Power up and monitor for alarm recurrence over 24-hour period during normal machining operations.

If alarm is intermittent, check environmental factors: cabinet temperature (should be below 45°C), cooling fan operation, and cabinet door seals. Access SYSTEM > MAINTENANCE > SELF-DIAGNOSIS to monitor internal temperatures if available on your series.

Measure DC voltages at main power supply unit: +5VDC, +24VDC, and ±15VDC rails. Compare against nameplate specifications (typically +5VDC ±5%, +24VDC ±10%). Use DMM set to DC voltage with control powered on.

Measure at power supply terminals with control energized. High voltage present - qualified personnel only.

Inspect power supply capacitors for bulging, leakage, or discoloration indicating failure. Check AC input voltage stability using SYSTEM > MAINTENANCE > INPUT VOLTAGE if available on 30i/31i/32i series (parameter display varies by model).

Verify proper grounding of control cabinet to machine frame and building ground. Resistance should be <1 ohm. Check for ground loops or shared neutral with high-current devices.

If noise is suspected, install or verify operation of line filters on AC input and check for proper shielding on encoder cables and I/O wiring. Consider oscilloscope measurement of +5VDC rail for excessive ripple (should be <50mV peak-to-peak).