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Power off the machine. Inspect the linear scale cable from the read head to the amplifier JD1A/JD1B connector (α/β series) or appropriate scale input connector. Check for physical damage, oil contamination, or pinched cables near cable carriers.

LOTO required - verify zero voltage before handling cables

Disconnect and reconnect scale connectors at both the amplifier and scale head. Inspect pins for corrosion, bent contacts, or debris. Use contact cleaner if contamination is present.

Check cable shield continuity and grounding. Measure resistance between shield and machine ground (should be <1Ω). Verify shield is grounded only at amplifier end, not at both ends which causes ground loops.

If cable damage is suspected, temporarily swap the scale cable with a known-good spare or another axis cable to isolate the fault. Monitor if alarm follows the cable.

Inspect the linear scale tape and read head for chips, coolant residue, or contamination. Clean scale tape with isopropyl alcohol and lint-free cloth. Do not touch grating surface with bare hands.

Verify read head gap using manufacturer specifications (typically 0.5-1.0mm for optical, verify with feeler gauge). Check mounting screws for looseness that could cause intermittent misalignment during motion.

Access DIAGNOSIS screen > SERVO > page through to scale diagnostic data. Check signal amplitude indicators (specific screen varies by amplifier: αi series shows at DIAGNOSIS #300-#399 range). Amplitude should be consistent and within range.

If read head failure suspected, temporarily install spare read head if available. Monitor alarm status. Check scale manufacturer diagnostics if equipped with LED indicators on read head housing.

Navigate to SYSTEM > PARAM > search for Parameter 1815 (serial scale/linear scale setting) for the affected axis. Verify correct setting: 0=no scale, 1=serial pulse coder, 2-7=various scale types. Confirm matches actual installed hardware.

Parameter changes require PWE=1 and machine restart - document original values

Check Parameter 1825 (linear scale pitch) matches physical scale specification. Incorrect pitch causes position calculation errors that may manifest as phase abnormality. Verify against scale manufacturer datasheet.

Inspect amplifier LED diagnostic codes. For αi/βi series, check 7-segment display and red ALM LED status during alarm condition. Refer to amplifier manual for specific LED patterns indicating scale input circuit faults.

If other causes eliminated, suspect amplifier scale input PCB failure. Swap amplifier with identical spare or another axis amplifier (after documenting parameters). If alarm moves with amplifier, replace amplifier and restore axis parameters from backup.

Ensure replacement amplifier is identical model and firmware compatible with CNC control