Start-up procedure for Fanuc Alpha servo amplifiers.

Start-Up Procedure for Fanuc Servo Amplifier (Alpha Series) – Overview

Here is a quick guide overview to start your Fanuc Alpha servo amplifier after it has been replaced, removed, or repaired in your machine.  This is an excerpt from the official Fanuc maintenance manual.  For all Alpha Series Control Motor Amplifiers (servo amplifier units  A06B-6089-H*** and  A06B-6090-H***).

(NOTE: When maintaining or inspecting the servo amplifiers, keep the power supply switched off. Also, make sure that the “CHARGING” LED (red) beside the circuit breaker on the front panel of the servo amplifier is off as well.)

Fanuc amplifier unit

An overview of the Fanuc amplifier unit Start-up procedure.

  1. Confirm the CNC model, and the specifications of the control motors, detectors, and servo amplifier units.


Fanuc Servo Amplifier Module

Fanuc A06B-6089-H104 label

2.  Check for damage to the appearance.

3. Connecting the Power Supply – Confirm the line voltage and power requirements

Fanuc Amplifier Start-up Procedure

Line voltage
Before connecting the power source to the machine, check the voltage
of the power source.


4.  Connect grounding line, power cords, and motor power wires.

Fanuc servo amplifier

Connecting grounding lines, power cords, and motor power wires before start-up.

Connecting to the Protection Ground – When it is necessary to make a motor operating with the SVU qualify for European CE marking, ground the motor using the accessory plate terminal.  Note that installing more than one protection ground line with single screw makes it impossible for the motor to qualify for European CE marking.

Leakage Current and Selecting a Ground Fault Interrupter – Because the drive circuit for the servo amplifier unit operates by pulse width modulation control system using IGBT’s, high-frequency leakage current flows from the motor windings and power lines to the ground through stray capacitance. This leakage current may cause the ground fault interrupter or leakage protection relay installed in the power line on the power supply side to malfunction.  So they must have a measure against influence from inverters, so that they do not malfunction.  To determine the leakage current from each motor, see the table on page 11 from original Fanuc document.

5.  Check the Settings

There are four channel switches above the 7-segment LED behind the terminal board cover on the front of the servo amplifier.  These switches should be set as described below before use of the servo amplifier

(1) Positions

The switches are sequentially numbered 1, 2, 3 and 4 with the one at the bottom as switch 1.  The OFF position is on the left, and the ON position on the right.

Fanuc amplifier repair

Fanuc switch position on servo amplifier

(2) Switch 1 Setting

The setting of switch 1 varies with the interface type used between the NC and servo amplifier.
→ If the setting is incorrect, an alarm occurs.
→ If the load is light, the motor may keep running.

OFFType A Interface

ON Type B Interface


* The following NC unit has the type A interface: FS-0C,  FS-15A,  FS-15B,  FS-16A,  FS-16B,  FS-18A,  FS-21TA,  PM-D,  PM-F
*  The following NC has the type B interface: FS-20, FS-21TB , FS-21GA, FS-16B, PM-H

(3)  Switch 2 setting

For the SVU, set switch 2 to off.
For the SVUC, set switch 2 to on.
→ If the setting is incorrect, the VRDY OFF alarm may occur.

(4)  Switch 3 and 4 setting

The setting varies depending on the regenerative discharge resistance used.
→ If the setting is incorrect, the regenerative discharge control circuit failure alarm (DCSW) cannot be detected correctly.

Diagram of discharge unit.

Settings for correctly detecting the regenerative discharge control.

*Footnote – For further information on the start-up procedure, please see Section 3 in the OEM manual titled, ‘Alpha Series Control Motor Amplifier Servo Amplifier Unit’ where this excerpt is borrowed from. 

If you have a Fanuc Alpha amplifier that is throwing an error code or has just stopper running, first check the servo motor that it is connected to it, then check the cable.  If you find that your amplifier is at fault, send your unit to an experienced repair center like  We can give it a free evaluation with no obligation on your part to have it repaired.

For more information on our Fanuc repair service, watch this Fanuc repair video.

Other information: Repair Zone can repair all Alpha, Alpha i, Beta, and C series amplifiers from Fanuc. When we receive your Fanuc servo or spindle amplifier unit:

  • We do a thorough inspection including run-testing to verify any faults before we start any repairs.
  • We then clean & bake the unit to assure all contaminates are removed
  • We perform comprehensive component testing then replace any diodes, resistors, IGBT’s, or other damaged components that we have identified as an issue, along with preventative replacement of components that we have seen as a common failure on amplifiers like electrolytic capacitors and fans
  • We perform as many static tests as we can on each of the individual circuits as part of our repair process.
  • Finally, we test-run your unit again for a minimum of 2 hours before shipping it back to you.

Since they are used in CNC machines, robotics, and other machine applications,  we have the ability to help no matter what the application. So, give us the opportunity to repair your Fanuc amplifier today!  All repairs carry a 1 year warranty.

We repair Fanuc servo motors, drives, amplifiers, power supplies, monitors, and PLC units. We can also repair spindle motors, drives, amplifiers and power supplies.

Repair Zone, your servo motor and Fanuc amplifier repair experts.


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