FANUC Robot Repair: SRVO-021 Before Amplifier Replacement

FANUC robot repair for SRVO-021 SRDY OFF should begin with evidence, not immediate amplifier replacement. On FANUC R-30iB Mate and R-30iB Mate Plus controllers, the documented condition is that HRDY is ON while SRDY remains OFF, with no other alarm explaining the state. This means the servo amplifier magnetic contactor did not turn on as commanded. The alarm identifies a servo-ready problem, but it does not independently prove that the amplifier is damaged. Record the complete alarm, Group, Axis, controller model, event time and operating condition before diagnosis.

Symptoms and Scope

SRVO-021 SRDY OFF applies here only to the documented R-30iB Mate and R-30iB Mate Plus controller scope covered by FANUC Robot series R-30iB Mate/R-30iB Mate Plus Controller Maintenance Manual B-83525EN/07. Connector locations, wiring arrangements and inspection procedures depend on the installed controller configuration and matching manual version. The alarm may involve the emergency-stop unit, servo amplifier, configuration-specific connections or an instantaneous power interruption. It should not automatically be treated as a motor, feedback-related component or mechanical fault.

A robot that cannot enable servo power, remains in a servo-off state or reports SRVO-021 after cabinet work requires controlled troubleshooting. The symptom alone cannot distinguish a connection problem, a safety-chain condition, an interruption of power or an amplifier-side issue.

Information to Record First

Complete alarm code and full text: Include SRVO-021 SRDY OFF and any associated message.
Date and exact time: Use the controller record or plant maintenance record.
Faulted axis: Record the Group and Axis shown by the controller.
Robot position/posture: Record the stationary position without moving the robot merely to reproduce the alarm.
Program step or motion: Note the selected program step and whether the alarm appeared during start-up, enable, or a commanded operation.
Actual speed and load condition: Record the operating state shown before the fault, if available.
Reset result: State whether reset cleared the alarm temporarily or had no effect.
Time until recurrence: Record recurrence timing only when it can be observed safely during normal operation.
Related power, communication or feedback alarms: Identify any earlier alarm in the history before SRVO-021.

Safety and Preparation

Use the site lockout, isolation and electrical safety procedure before opening a controller cabinet or inspecting internal connections. Do not perform energized measurements, bypass safety circuits or alter safety-related parameters unless the task is assigned to suitably qualified personnel and is supported by the applicable FANUC documentation. Keep the controller model, software information and cabinet configuration available because a connector or test instruction from another configuration may not apply.

Diagnostic Sequence

1. Confirm the alarm and equipment scope. Input the complete event record, controller model, robot model, Group and Axis. Confirm that the event is SRVO-021 SRDY OFF on an applicable R-30iB Mate or R-30iB Mate Plus system. If the controller or alarm differs, stop using this sequence and identify the correct manual.
2. Check for an earlier alarm. Review the alarm history in time order and identify whether another alarm explains the servo-ready condition. If an earlier alarm exists, diagnose that event first and treat SRVO-021 as a subsequent symptom. If no other alarm explains the state, continue with the HRDY and SRDY status and the controller-specific inspection sequence.
3. Confirm HRDY and SRDY. The relevant documented condition is HRDY ON while SRDY remains OFF. If the available diagnostic screen or maintenance record shows a different state, preserve the evidence and verify the controller configuration and applicable manual before selecting a component. If HRDY and SRDY cannot be obtained, record that missing information rather than assuming a drive failure.
4. Inspect configuration-specific connections. With the equipment made safe, inspect accessible connections, harnesses and visible signs of damage associated with the installed configuration. Look for looseness, contamination, mechanical damage or evidence of previous cabinet service. Do not apply connector names or pin-level conclusions from a different configuration. A connection issue supports further harness or interface investigation; an apparently normal connection does not prove that the servo amplifier is functional.
5. Check whether an instantaneous power interruption occurred. Compare the alarm time with plant power records, controller restart evidence and nearby equipment events. If a power interruption is documented, investigate the interruption and the resulting controller state before condemning a component. If no interruption is recorded, continue with the applicable emergency-stop unit and servo amplifier evaluation described by the matching FANUC manual.
6. Evaluate the emergency-stop unit and servo amplifier in the documented priority order. The manual defines SRVO-021 as a failure of the servo amplifier magnetic contactor to turn on as commanded when HRDY is ON and SRDY is OFF. That definition directs the inspection; it is not confirmation that the amplifier itself has failed. Professional testing or repair may be appropriate when the preceding checks do not explain the condition, but the result should be based on controller evidence, configuration and component identification.

How to Distinguish Similar Causes

A preceding alarm that identifies a safety or power condition changes the diagnostic path. A cabinet connection disturbed during service points toward configuration-specific wiring or interface inspection. A documented instantaneous power interruption points toward the power event and its consequences. When no earlier alarm explains the condition, the HRDY/SRDY state and the applicable emergency-stop unit and amplifier checks become more important. Motor, feedback-related or mechanical investigation should be expanded only when another earlier or associated alarm supports that branch. An alarm reset, by itself, does not establish that the underlying condition has been corrected.

When Repair or Replacement May Be Considered

Consider professional inspection, repair or replacement only after the complete alarm record, controller scope, Group and Axis, HRDY/SRDY evidence where available, connection findings and power-event information have been reviewed. Match any spare or repaired part to the exact model, part number, hardware revision and connector layout. Do not select an amplifier solely because SRVO-021 appeared. For relevant R-30iB servo amplifier sourcing and repair information, see https://autonews.best/products/fanuc-r-30ib-si-fu-qi-ji-xiu. ZHB is an independent industrial robot inspection, repair and maintenance service provider that also supplies parts to overseas customers; manufacturer authorization should not be assumed.

Verification After Repair or Replacement

After authorized work, verify that the installed component and connections match the controller configuration and that all covers, guards and safety devices are restored. Clear the documented condition only under the site procedure, then confirm the servo-ready state and review the alarm history. Any motion observation should be performed only by suitably competent personnel, under controlled conditions, in accordance with the applicable FANUC operating information and site safety procedure. Do not repeatedly cycle or move the robot merely to reproduce an unresolved fault. Record whether the alarm returns, which Group and Axis is affected and whether a new earlier alarm appears.

Information Required for a Repair Inquiry

Provide the robot model, controller model, complete alarm code, alarm history, component label, fault conditions and clear photographs. For SRVO-021, also provide the Group and Axis, full event log, alarm time, connector photographs and servo-amplifier label. HRDY and SRDY information is useful when it can be obtained from the diagnostic screen or maintenance records, but it should remain optional when unavailable.

Sources and Update Information

The technical definition and diagnostic priority in this article are based on FANUC America Corporation, FANUC Robot series R-30iB Mate/R-30iB Mate Plus Controller Maintenance Manual, B-83525EN/07, SRVO-021 SRDY OFF, page 73. An accessible copy is hosted by Haas Automation at https://www.haascnc.com/content/dam/haascnc/en/service/reference/fanuc-manuals/Fanuc%20Robot%20Series%20R-30iB%20Mate%20%2B%20Mate%20Plus%20Maintenance%20Manual.pdf. Connector locations and procedures must be confirmed against the installed configuration and applicable manual version.

Conclusion

SRVO-021 is a servo-ready condition requiring evidence-based diagnosis. On the specified R-30iB Mate controller scope, HRDY ON with SRDY OFF indicates that the servo amplifier magnetic contactor did not turn on as commanded, but the alarm alone does not prove amplifier failure. Recording the event, checking earlier alarms, confirming HRDY and SRDY, reviewing configuration-specific connections, investigating an instantaneous power interruption and following the applicable emergency-stop unit and amplifier procedure reduces the risk of an incorrect part replacement.