A Fanuc servo amplifier (SVM) showing a blinking ‘-’ (dash) on its 7-segment display means the inverter's control power supply is abnormal — the low-voltage supply that runs the amplifier's own control electronics and feeds the motor encoders is out of tolerance. The amplifier cannot even reach a ready state, so the axis is dead and the CNC typically reports a servo/amplifier communication or ready-signal alarm. The crucial insight — straight from Fanuc's own troubleshooting sequence — is that the fault is very often not the amplifier at all, but something connected to it dragging the supply down. Diagnose by disconnection, not by replacement.
What is happening
Inside the SVM, a small power supply generates the control voltages (+5 V, ±15 V, +24 V rails) from the 24 V control input / DC link. These rails also feed the pulse coder on the motor, through the feedback cable (JF connector). A short or overload anywhere on that distribution — a crushed feedback cable, a coolant-flooded encoder, a failed component on the rail — pulls the supply out of tolerance, and the amplifier flags the blinking dash.
Step 1 — The JF disconnect test (Fanuc's own first move)
- Power OFF, lock out, wait for the charge LED to go out.
- Disconnect the feedback cable (JF*) from the servo amplifier — for a multi-axis module, start with one axis, and if the dash persists, disconnect all JF connectors.
- Switch the power back on and watch the display:
- Blinking continues with feedback disconnected → the amplifier itself is faulty. Its internal control power supply has failed — board-level repair (typically the DC-DC converter stage: switching transistor, rectifier diodes, or dried filter capacitors) or an exchange unit.
- Blinking stops → the amplifier is fine. The feedback side — cable or pulse coder — is shorting the supply. Continue to Step 2.
Step 2 — Feedback side: cable or pulse coder?
- Inspect the feedback cable run end to end: crush points in the drag chain, oil-swollen jackets near the motor, rub-through at cabinet edges, and the connector shells at both ends for coolant and bent pins.
- Meter the cable (disconnected at both ends): check for shorts between the +5 V and 0 V conductors and from each conductor to the shield. Flex the cable at suspect points while measuring — intermittent shorts show only in motion.
- Reconnect the cable at the amplifier but NOT at the motor and power on: dash returns → the cable is shorted; replace it. Dash stays off → the cable is fine — the pulse coder on the motor is pulling the rail down (internal short, coolant ingress). Replace the encoder — see our encoder replacement guide — and re-establish the reference position afterwards (zero return guide).
Step 3 — If the amplifier is the culprit: check why
- 24 V control input: before condemning the internal supply, verify the 24 V control power arriving at the amplifier (CX connectors, daisy-chained along the amplifier row) — a sagging external 24 V supply or a corroded interconnect can mimic an internal failure, and can affect several amplifiers at once.
- Repeat offender: if a repaired/replaced amplifier's supply fails again, something external is overloading a rail — recheck the feedback cables of ALL axes on that module, and any sensors fed from the same supply.
Diagnosis checklist (in order)
- Blinking dash → JF disconnect test: amplifier vs feedback side in ten minutes.
- Feedback side → inspect and meter the cable; then the half-reconnect test to split cable vs pulse coder.
- Amplifier side → verify the incoming 24 V control supply first, then bench-repair the internal DC-DC stage.
- After any encoder/cable replacement → reference the axis and verify soft limits before production.
Related guides
- How to identify and replace a faulty Fanuc encoder
- Fanuc SVM alarms 8. / 9. / A. (axis overcurrent)
- Fanuc SVM alarms b / c / d (DC link current)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained