Two final cases from Fanuc's spindle amplifier troubleshooting chapter that don't fit the normal alarm-code pattern — and both regularly mislead people into condemning the wrong unit.
Case 1 — SP status display shows 4, 11, 30, 33, 51, 57, 58, b1, b2 or b3
These codes on the SPINDLE amplifier's display mean an alarm condition occurred in the POWER SUPPLY (PSM) — the spindle amplifier is relaying its power source's distress, not its own. The instruction: check the Power Supply module's own status display and troubleshoot the PSM per its codes — our PSM alarm 04, alarm 2 and alarm A guides cover the common ones. Same principle as alarm 80: read every display in the rack before condemning anything — the unit showing the code is often just the messenger.
Case 2 — CNC alarms 756 / 766 (abnormal axis number) on Dual Check Safety machines
These CNC-side alarms occur only with Dual Check Safety, and Fanuc's fix is wonderfully specific: check that the K76 connector is attached to the JA7A connector of the SECOND spindle.
- On two-spindle serial chains (CNC JA41 → SP1 JA7B, SP1 JA7A → SP2 JA7B), the chain must be terminated: K76 is a small terminating plug on the second spindle's JA7A — a 20-pin half-pitch connector bridging X2NDSP (pin 15) to 0V (pin 13).
- K76 is unnecessary if only one spindle is used — but on two-spindle machines a missing/lost K76 (it walks away during amplifier swaps) produces 756/766.
- Wiring normal, K76 present and correct → then the spindle amplifier control PCB is the suspect — board repair or exchange.
Related guides
- Fanuc spindle alarm 80 (the messenger alarm)
- Fanuc αi PSM alarm 04 (single phase failure)
- Fanuc αi drive alarms — full index