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Power down the machine and lockout/tagout. Locate the regenerative discharge resistor assembly (usually mounted on or near the spindle drive unit).

Ensure DC bus capacitors are fully discharged before touching any components. Wait minimum 5 minutes after power-off and verify with voltmeter.

Visually inspect resistors for burn marks, discoloration, cracks, or physical damage. Check all wire connections and terminals for looseness or corrosion.

Using a multimeter, measure resistance across the discharge resistor bank. Compare reading to specifications in the spindle drive manual (typically 20-100 ohms depending on model). An infinite reading indicates open circuit.

If resistor is faulty, replace with exact OEM part number. Verify regenerative discharge transistor is triggering properly by monitoring DC bus voltage during controlled deceleration test after repair.

Access the spindle drive alarm history and parameter diagnostics. Check DC bus voltage parameter (typically #3027 or similar) to confirm overvoltage condition during alarm occurrence.

Power down and access the spindle amplifier control board. Locate the regenerative discharge control circuit and transistor module (refer to drive-specific service manual for location).

High voltage present. Only qualified personnel should open spindle drive enclosures.

Inspect the discharge transistor module for physical damage, blown components, or burnt traces on the control PCB. Check gate drive signal connections and opto-isolators if accessible.

If damage is found, replace the regenerative discharge module or control card. If no visible damage, contact Fanuc support for board-level diagnostics or replacement drive module may be required.

Review recent program changes or tooling modifications. Check if heavy chucks, rotary tables, or large face mills were recently installed that increase spindle inertia.

Examine spindle deceleration parameters. Check parameter #4021 (spindle deceleration time) and increase value by 20-30% to reduce regenerative energy rate. Test with gradual increases.

Monitor DC bus voltage during spindle stop operations using maintenance diagnostics screen. Normal range is 280-320VDC for 200V class, 530-680VDC for 400V class. Alarm triggers typically above 380V or 800V respectively.

If deceleration adjustment resolves the issue but cycle time is affected, consult with Fanuc about upgrading to higher capacity regenerative resistors or adding a braking resistor bank to handle increased load.