Fanuc spindle alarm 10 (AL-10) means the amplifier's control power supply voltage has dropped below tolerance — not the big DC link that drives the motor, but the low-voltage supply that runs the amplifier's own control electronics. Fanuc's manual is unusually terse here: ‘the control power supply, power cable, Power Supply, or Spindle Amplifier is defective — replace one of them.’ Four suspects, one line. Here is how to work through them in order without replacing three good parts to find one bad one — because the chain is testable with a multimeter in twenty minutes.
How the control power reaches the spindle amplifier
On αi systems, the ~200 V AC control feed (and on some configurations a 24 V DC feed) arrives at the amplifier row and daisy-chains from unit to unit through the CX1A (in) / CX1B (out) connectors: control transformer → PSM → SPM → SVMs. Each amplifier then makes its own internal control rails from that feed. AL-10 means the SPM's control input — or what it makes from it — sagged. So the fault is somewhere along: source → cable/connectors → the amplifier's internal supply.
Step 1 — Is it only the spindle amplifier, or the whole row?
- Several units complain together (servo amplifiers dropping, blinking dashes on SVMs, PSM unhappy) → the shared source or the daisy-chain upstream of them all: control transformer output, its fuses, or the first CX1 cable. Measure the control voltage at the transformer and at the first amplifier's CX1A — under operating conditions, not just at idle.
- Only the SPM shows AL-10 → the fault is its own CX1 feed (the link between the previous unit's CX1B and its CX1A) or its internal supply. Continue below.
Step 2 — The cable and connectors (the usual culprit)
- Power OFF. Inspect the CX1A/CX1B jumper cables along the row — these small connectors run warm for decades and suffer: loose crimps, heat-darkened housings, and half-seated plugs after any cabinet work. Reseat every one from the transformer to the SPM.
- Look for melted/discoloured pins — a high-resistance CX1 contact drops volts under load and gets hot doing it; replace that jumper cable, do not just reseat it.
- Measure the control voltage AT the SPM's CX1A while the machine runs a spindle accel — a feed that reads fine at idle and sags during acceleration is a resistive connection or an overloaded source.
Step 3 — The source
- Control transformer: verify its output under load and check its primary taps match your incoming mains — a plant that moved from 415 V to a sagging 380 V feeder leaves every downstream rail marginal.
- Fuses/breakers on the control feed: a fuse that has fatigued (not blown) can go resistive — measure across it under load; more than a fraction of a volt drop means replace it.
- Incoming mains dips: if AL-10 clusters at fixed times or coincides with big equipment starting, it's the feeder, not the machine — same story as the DC-link undervoltage alarms (βiSVSP alarm 51).
Step 4 — The amplifier's internal control supply
If the CX1A input measures healthy through a full spindle cycle and AL-10 persists, the SPM's internal DC-DC/control supply stage has failed — dried filter capacitors, a tired switching stage, or the voltage-monitoring circuit itself reading low. Fanuc's remedy is ‘replace the Spindle Amplifier’; the economical path is board-level repair of the control supply — one of the most routine repairs on our bench, because these supplies run 24/7 for 15–20 years before the capacitors quit.
Diagnosis checklist (in order)
- One unit or several? Several → source/upstream chain. Only the SPM → its feed or itself.
- Reseat and inspect every CX1A/CX1B jumper; replace any with heat-marked pins.
- Measure control voltage at the SPM's CX1A during spindle acceleration — idle readings prove nothing.
- Check transformer output, taps and control-feed fuses under load.
- Input healthy, alarm persists → internal control supply → bench repair or exchange unit.
Related guides
- Fanuc SVM blinking dash (control power supply) — the servo-side twin
- Fanuc αi PSM alarm 04 (single phase failure)
- Fanuc spindle alarm 09 (heat sink overheat)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained