FANUC Spindle Drive Alarm 2 and 12: Encoder Adjustment Procedure

Overview

The FANUC A20B-0009-053 and A20B-0009-069 series spindle drive boards are legacy analog AC spindle amplifiers used widely on Okuma, Mazak, and other CNC machine tools built in the 1980s and 1990s. These boards typically operate in conjunction with a power base cage containing the IGBT output stage, a main contactor, and a separately mounted pulse encoder (M-revolutions) on the spindle motor. Two recurring alarm states observed on this platform are:

• Alarm 2 (AL-2) - the drive failed to reach commanded speed within the acceleration window.
• Alarm 12 (AL-12) - a regenerative / over-current fault that can be destructive to the power section if the drive is left enabled.

This article documents the field-proven encoder duty-cycle adjustment (RV18 / RV19 at test points CH7 / CH8), the IGBT and fuse checks that distinguish a recoverable AL-2 from a destructive AL-12, and the parameter-storage caveat (the 6055 EEPROM on the top board) that often locks fault memory to a single physical card.

Affected Hardware and Alarm Definitions

Alarm 2 semantics: The velocity command from the CNC is greater than the actual speed feedback by more than the tracking tolerance for longer than the acceleration window permits. Triggers include encoder duty cycle mis-adjusted, encoder failure, blown green fuse on the top board, parameters wiped from the 6055 EEPROM, or a weak IGBT that cannot deliver rated current.

Alarm 12 semantics: The control board has detected a regenerative over-current condition originating in the power stage. Continued operation with the suspect board installed risks secondary damage to the IGBT modules in the lower cage. Remove or quarantine the AL-12 board before swapping in a known-good unit.

Root Cause Decision Matrix

Prerequisites

1. Obtain the machine-specific FANUC spindle drive maintenance manual matching the A20B-0009-053 or -069 board. Parameter listings, schematic, and test-point map are mandatory.
2. Prepare a digital multimeter (DMM) with DC millivolt resolution, or a two-channel oscilloscope for duty-cycle verification.
3. Lock out and tag out the machine main disconnect. Verify zero DC bus voltage before opening the cabinet.
4. Have a non-metallic potentiometer adjustment tool. Metal screwdrivers can slip and short the encoder test point to the 0V reference, destroying the input.
5. Confirm the spindle can be rotated by hand in high gear without drive enable. The motor must turn freely for the encoder to generate a signal.

Encoder Duty-Cycle Adjustment Procedure (CH7 / CH8 / RV18 / RV19)

The encoder interface on these drives expects a 50% duty-cycle quadrature signal on both channels A and B. Mis-adjustment outside the comparator window causes the velocity loop to misread speed and triggers AL-2 even when the motor itself is healthy.

1. With the drive powered and the spindle commanded to a low speed (approximately 100 RPM), locate test points CH7 (channel A) and CH8 (channel B) on the A20B-0009-053 / -069 board. Identify the 0V reference test point adjacent to CH7.
2. Set the DMM to DC volts. Connect the negative lead to 0V.
3. Measure DC voltage on CH7. Target: 2.5 VDC ± 0.1 V.
4. Adjust potentiometer RV19 (channel A) until CH7 reads 2.5 VDC. Turn only in small increments and allow the loop to settle.
5. Move the positive lead to CH8. Measure DC voltage. Target: 2.5 VDC ± 0.1 V.
6. Adjust potentiometer RV18 (channel B) until CH8 reads 2.5 VDC.
7. Re-verify CH7. Adjustments to one channel can shift the other slightly through the comparator reference; iterate until both read 2.5 VDC within tolerance.
8. Command the spindle to half speed for the active range. Re-measure CH7 and CH8; both must remain at 2.5 VDC.
9. Command the spindle to top speed for the range. Re-measure; both must remain at 2.5 VDC across the full speed envelope.

If the voltages cannot be pulled into 2.5 VDC on either channel even at the extreme of the pot travel, the encoder itself (motor-mounted M-revolutions unit) or the interconnecting cable is the fault, not the board.

IGBT and Power Section Verification

AL-2 that survives a correct encoder adjustment typically originates in the power stage. A non-energized resistance check is the safest first test.

1. With the drive locked out and DC bus verified at 0V, isolate the IGBT modules from the bus by removing the gate-driver ribbon if accessible.
2. Set the DMM to diode test. Measure each IGBT module phase-to-phase and phase-to-DC-bus. A healthy module shows a single diode drop (~0.4 to 0.7 V) in one direction and open in the other.
3. A shorted module reads near 0 V in both polarities. Replace before re-energizing.
4. Inspect the main contactor tips for pitting, welding, or coil failure. A contactor that does not pull in cleanly creates a single-phasing condition that the drive interprets as a velocity-tracking fault.
5. Inspect the green input fuse on the top board. A blown fuse removes supply to the encoder interface and produces the same AL-2 signature as a missing encoder signal. Replace only with the exact amperage and blow-characteristic specified in the maintenance manual.
6. Check the 6055 parameter storage EEPROM seating. If the top board has been removed or the battery has expired, parameters may default. Re-load from the machine backup or the manual's factory defaults before further troubleshooting.

Parameter Storage and Board Swapping Caveat

On A20B-0009-053 / -069 drives, the spindle-loop tuning parameters (proportional gain, integral gain, acceleration timer, current limit, gear-ratio constants) live in the 6055 EEPROM socketed on the top control board. Swapping a known-good top board from another machine without re-loading the parameters from that machine's backup will produce immediate AL-2 because the new board defaults to parameters that do not match this motor and gear set.

When field-replacing a top board, either:

• Transfer the original 6055 EEPROM to the replacement board (verify orientation, match part number), or
• Re-load parameters from a known-good backup using the FANUC parameter loader, then re-execute the encoder adjustment procedure above.

Verification

1. Power up with a verified power section. Command the spindle to 100 RPM; confirm zero alarms and stable current draw.
2. Verify CH7 and CH8 both read 2.5 VDC at low speed, half range speed, and top range speed.
3. Run a spindle warm-up program that exercises the full RPM envelope for at least 30 minutes. The historical 30-minute drift behavior must not return.
4. Run a full part program with representative load. Confirm no AL-2 or AL-12 under cutting load.
5. Document final parameter set, encoder voltages, and contactor tip condition for the maintenance record.

FANUC Support Channels for Oceania and Australia

For genuine OEM spares, board repair, or escalation beyond field-level troubleshooting, contact FANUC Oceania through the official service channels. The Melbourne Technical Center is the local point of contact for spindle drive repair on legacy Australian-installed equipment.

Official service and support entry points are listed on the FANUC Asia and Oceania service page and the FANUC America Support portal. For regional service requests use the FANUC Contact form. Legacy spindle drive repair is supported through FANUC's authorized repair network rather than direct field replacement.

Troubleshooting Matrix

Field-Notes and Best Practices

• Quarantine any board that has produced AL-12 until bench-repaired. Continued use risks destroying a known-good power section through repetitive over-current events.
• Always mark which top board is paired with which machine. The 6055 EEPROM makes each board "married" to its host until the parameters are re-loaded.
• Document the encoder voltage readings at low, half, and top speed in the maintenance log. Trending catches drift before it produces an alarm.
• If a retrofit is being considered instead of repair, weigh the loss of OEM support against the cost of board repair plus shipping. OEM FANUC, Okuma, Heidenhain, and Mazak field service organizations retain repair capability for these legacy analog drives.