Service Home Servo Amplifier - Troubleshooting Guide - CHC - TG0124
Last updated: 10/14/2025

Servo Amplifier - Troubleshooting Guide - CHC

TG0124

Introduction

There are four versions of Haas amplifiers. 30A [1], 45A [2], 60A [3], and 90A [4]. All four operate the same way.

Smart Amplifier assemblies are powered from the 320 VDC Bus from the Vector Drive assembly or Minimill Power Supply. They each have a red LED FAULT light, and a green LED POWER ON light to show the status of the amplifier.

Electrical Safety

 DANGER: Working with the electrical services required for CNC machines is extremely hazardous and can result in serious injury or death.

Before connecting line wires to the CNC:

  • Turn off all power to the machine at the source.
  • Perform a Lockout-Tagout (LOTO) procedure to ensure the power remains off during service.
  • Verify that power has been disconnected by using an AC voltage detector on all incoming lines.

If you are uncertain about how to safely disconnect power or perform LOTO procedures:

  • Do not proceed.
  • Contact qualified personnel or obtain appropriate assistance before continuing.

Failure to follow these precautions may result in electrical shock, equipment damage, or fatal injury.

 Danger: Before beginning any work inside the control cabinet, verify that the High Voltage indicator light on the 320V Power Supply / Vector Drive has been off for a minimum of five (5) minutes. This waiting period ensures that residual voltage has dissipated and reduces the risk of electric shock.

Certain service procedures involve high-risk electrical components and may pose serious injury or fatality hazards. Technicians must not attempt any procedure unless they have a complete understanding of the steps involved and the associated risks.

If there is any uncertainty regarding a procedure, contact your Haas Factory Outlet (HFO) to arrange for a qualified service technician.

Symptom Table

Symptom Possible Cause Corrective Action

Alarms 161-164 AXIS DRIVE FAULT

Alarm 993 AMPLIFIER SHORT CIRCUIT

Alarm 2040 VECTOR DRIVE OR SPINDLE AMPLIFIER-SHORT CIRCUIT

 Amplifier detected a short. Check cables for contamination or damage.
The amplifier has a internal short circuit. Do the Amplifier Short Circuit Inspection section.

Note: If a short-circuit is found, inspect the cable and motor for a short circuit before installing a new amplifier.
Alarms 161-164 AXIS DRIVE FAULT and Alarm 986 CALIBRATION FAULT The machine booted up before the amplifier calibration cycle finished Press [Reset] to clear the alarm and continue to use the machine. Refer to HBC-N-02/27/2025 for more details on this issue.
Amplifier internal calibration. Check the command cable from the Maincon / Mocon PCB to amplifier. Check the 320 VDC voltage to amplifiers.
Alarms 161-164 AXIS DRIVE FAULT and Alarm 992 AMPLIFIER OVER CURRENT Axis mechanical blockage. Axis mechanical blockage.
Alarms 161-164 AXIS DRIVE FAULT and Alarm 994 AMPLIFIER OVER LOAD Incorrect application. Change speed and check for tool damage.
Axis mechanical blockage. Remove Blockage.
Incorrect Low Voltage Input supply to Maincon or Amplifier. See the Low Voltage Inspection section.
Alarms 161-164 AXIS DRIVE FAULT and Alarm 991 AMPLIFIER OVER TEMPERATURE Problem with the cooling fan. Check cooling fan.
Alarm 991 AMPLIFIER OVER TEMPERATURE. The Maincon/Mocon axis channel is defective, or there is a broken pin on the connector. Remove the encoder/drive cables from the Maincon/Mocon PCB axis channel. Inspect the connectors pins. Test the axis on a different Maincon/Mocon channel, if the problem goes away there is a problem with the axis channel.

Mechanical Blockage

Corrective Action:

Make sure the axis has a clear path of travel. Tools, part fixtures, or workpieces that are too large can crash into the machine enclosure.

Look at the machine's alarm history. Overload alarms followed by over current alarms are a sign of a crash.

Look at the machine enclosure, spindle and tooling for damage. Damage to the tooling, fixtures and enclosure are signs of a crash.

If there are no signs of a crash, inspect the ballscrew and linear guides. They must move or turn freely by hand.

Amplifier Short Circuit Inspection

Check the amplifier for a short. Power off the machine. Disconnect the cables for the axis servo motor at the amplifier that generates the alarm.

Place MultiMeter Leads Here Instructions

Set the meter to ohms, and take the following readings:

Ground screw to RED, WHT, and BLK terminals

If the meter does not show open (o.l) the amplifier is damaged.

Set the meter to Ohms, and take the following readings:

Black lead on HV (+) and Red lead to HV (-), RED, WHT, and BLK terminals.

The meter should show high resistance readings typically in the Kilo or Mega ohms.  If the meter reads very low resistance the amplifier is damaged.

Set the meter to diode test mode, and take the following readings:

Red lead on HV (-) and black lead to HV (+), RED, WHT, and BLK terminals

If the meter does not show a voltage drop ranging from 0.2 to 0.8 volts the amplifier is damaged.

Set the meter to diode test mode, and take the following readings:

Black lead on HV (-) and red lead to HV (+), RED, WHT, and BLK terminals

If the meter does not an open circuit (O.L) the amplifier is damaged.

Servo Motor Short Circuit Inspection

Corrective Action:

Check the cables for a short: Make sure the axis servo motor cables are not contaminated or pinched. Contaminated or pinched cables can cause a short. Replace the cables if necessary.

Inspect the servo motor. Disconnect and inspect the power cable connector at the motor. Make sure that there is no coolant contamination. Coolant contamination can cause this alarm and damage the amplifier. Measure the resistance from the pins labeled A, B and C at the motor connector to chassis ground.

  • The reading should show an open circuit.
  • If there is not open resistance, the servo motor is at fault.

Review the Part Program

Corrective Action:

The machine load is too high. Decrease the feeds to decrease the machine load. Examine the cutting tool for damage.

Test the axis that generates the alarm. Write a program in MDI to move the axis. If any movement at any feed rate causes the alarm, then the servo amplifier is at fault.

Low Voltage Inspection

Non Smart Amplifiers Low Voltage Input.

Non Smart Amplifiers - Use a low voltage power supply module to supply +12/-12VDC to the amplifier.  This voltage is critical to the correct operation of this amplifier.

With the power on, using a volt meter set to DC carefully measure acrross Yellow/Black and White/Black.

  • Yellow = +12VDC
  • White = -12VD
  • Black = Ground

Use the table below for the correct voltage range.

Mocon/Maincon Low Voltage Input

Corrective Action:

Measure the low voltage power supply voltage levels at the Maincon or Mocon PCB. Make sure you do not short or cross your multimeter pins.

Cycle power to the machine. If the fault stays, then the amplifier is faulty.

If every axis amplifier in the machine generates the alarm at the same time, and the low voltage power supply is correct, then the Maincon or Mocon PCB may be at fault.

Pins Correct Voltage
+5 and GND +4.90 to +5.20 V dc
+12 and GND +11.85 to +12.50 V dc
-12 and GND -11.85 to -12.50 V dc

Cooling Fan

Corrective Action:

 Note: The maximum temperature for safe operation of Haas CNC machines is 122 °F (50 °C). The amplifier over temperature sensor will trigger at 195 °F (90 °C).

Make sure the cooling fan for the servo amplifiers operates correctly.

If the fan does not operate correctly, measure the voltage it receives. Disconnect the cable from the cooling fan. Measure the voltage between the leads of the cable. The correct voltage 120 VAC.

  • If the cable has the correct voltage, the cooling fan is defective.

Electrical Diagram