14.02.2026 by Viktor Siebert

In this repair case, we were dealing with a 320 Volt DC bus power supply type 93-69-2000C from a HAAS TL-1. The machine intermittently reported the error “A/C Phase Fault.” At first glance, this suggested a classic phase failure.

On site, thorough checks had already been performed. No short circuit was measurable. The cooling fan operated correctly. The DC bus capacitors measured approximately 4000 microfarads and appeared acceptable at first inspection. The power transistors switched properly, and a test lamp confirmed correct operation. Nevertheless, the fault kept appearing under load.

These are the technically demanding cases. When no obvious defect is visible, the real diagnostic work begins.

Workshop Analysis

After arrival at our facility, the power supply was completely disassembled. The first step was thorough cleaning, as HAAS machines often accumulate coolant mist and fine metallic dust. Next, the entire power section was inspected:

• Visual inspection of all solder joints
• Rectifier inspection
• DC bus capacitor measurement
• IGBT unit testing
• Terminal block inspection

Typical signs of aging were found. Some solder joints at the rectifier showed thermal stress. Although the electrolytic capacitors still met nominal capacitance values, their stability under load was no longer optimal. Slight oxidation was also found at terminal contact points.

These components were replaced or reworked as a preventive measure. The cooling fan was also renewed to eliminate thermal risk.

The Fault Persists and Becomes More Complex

After the first load test, the alarm still occurred. It was now clear that the root cause was not in the classic power section.

The real troubleshooting on the control level began.

Under real machine load, input voltages, DC bus stability, and the internal phase monitoring system were analyzed. It became evident that the issue was related to unstable signal evaluation within the phase monitoring circuit. The input voltage was present, but the internal logic reacted sensitively to minimal voltage fluctuations within milliseconds.

This fault could not be reproduced during static bench testing. Only under dynamic load with realistic acceleration and regenerative cycles did the problem become visible.

Through systematic signal tracing, comparative measurements, and component-level analysis, the unstable control behavior was identified and permanently corrected.

Final Test Under Real Conditions

During final testing, the power supply operated for several hours inside a HAAS machine. Various load scenarios were simulated:

• Acceleration cycles
• Braking cycles with regeneration
• Continuous operation under thermal stress

No further phase alarms occurred.

Conclusion

This repair case clearly demonstrates that not every phase fault is caused by an actual missing phase. Often, the root cause lies in unstable components or in internal control logic.

Sustainable repair does not only mean replacing obvious wear parts. It requires deep diagnostic analysis down to the signal level. Only then can long-term operational reliability be ensured and unexpected machine downtime avoided.

Preventive maintenance, regular terminal inspection, and timely replacement of thermally stressed components significantly extend the service life of such power supply units.

Learn more about our HAAS repair services here:
HAAS Drives and Motors

Regarding the mentioned unit:
HAAS 320V Power Supply 93-69-2000C

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Technical Specifications

Feature	Specification
Voltage	3 phase 200–230 VAC
Output Voltage	approx. 320–340 VDC DC bus
Current	depending on load approx. 15–25 A
Power	5 HP class
Weight	approx. 4.5 kg
Dimensions	approx. 280 × 150 × 120 mm
Type	DC bus power supply with integrated brake chopper
Cooling	Active fan
Control	Internal phase monitoring with LED diagnostics
Manufacturer	Haas Automation
Production Year	approx. 2000s
Manual Reference	Haas Service Manual 320V Power Supply

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Application Environment & Compatible Equipment

This power supply is used in HAAS machines with 320V DC bus architecture.

Typical applications:

• HAAS TL series lathes
• HAAS VF series machining centers
• HAAS toolroom machines

Compatible with:

• HAAS axis amplifiers
• HAAS Vector Drive systems
• Servo axes X Z and possibly Y

Its main purpose is supplying stable DC bus power to axis and spindle drives.

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Functional Description

The 93-69-2000C is a central energy supply module.

Main functions:

1. Rectification of three phase input voltage
2. Creation of stable 320 V DC bus
3. Supplying axis amplifiers
4. Absorbing regenerative energy via brake chopper
5. Monitoring phase, voltage and protection logic

In this case the unit reported Alarm A A/C Phase Fault even though no short circuit was measurable and switching transistors operated.

Typical reasons:

• Unbalanced input voltage
• One phase unstable under load
• Internal phase detection malfunction
• Loose terminal connections

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Alarms & Troubleshooting

Code	Description	Cause	Solution
A	A/C Phase Fault	Phase loss or imbalance	Measure input phases
160	Low DC Bus Voltage	DC bus undervoltage	Check mains
119	Over Voltage	DC bus above 390V	Check brake resistor
123	Spindle Drive Fault	Unstable supply	Check DC bus
2040	Short Circuit	Internal short	Disconnect amplifiers
992	Amplifier Overcurrent	Axis overload	Check load
993	Short Circuit	PSU overcurrent	Inspect wiring
01	Main circuit undervoltage Converter Alarme CIMR_MR5-2	Low supply	Check mains
05	Heatsink overheat Converter Alarme CIMR_MR5-2	Fan failure	Replace fan
10	Insufficient voltage Alarme _ Troubleshooting Mitsub…	Bus undervoltage	Check capacitors

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Components

Assembly	Description	Function	Inspection
Rectifier	3 phase bridge	AC to DC	Diode test
DC capacitors	approx. 4000 µF	DC smoothing	ESR test
IGBT transistor	Chopper module	Regeneration	Gate test
Cooling fan	24V axial	Cooling	Speed test
Phase monitor	internal logic	Fault detection	Voltage measurement
DC bus terminals	P and N	Power distribution	Tightening