09.06.2026 by Viktor Siebert

When this Mitsubishi MDS-D-V1-20 arrived at our workshop, the customer’s fault description initially seemed very straightforward: the unit would not power up at all. No display, no response and no communication with the machine control.

For this type of failure, our diagnostic process always starts with the power supply electronics. During the first visual inspection, signs of a fault within the internal power supply section were already visible. The unit was then completely disassembled and subjected to a detailed investigation.

After extensive measurements, the suspicion was confirmed. Several components within the internal power supply were no longer operating within specification. As a result, the required operating voltages for the processor, memory and communication circuits could no longer be generated reliably.

The defective components were replaced, the power supply section was completely rebuilt and subsequently tested again.

After the first successful startup, the repair initially appeared to be complete. The unit powered up again and showed signs of life for the first time.

However, this was exactly the point where the real challenge began.

During further commissioning, Alarm 12 suddenly appeared.

According to the Mitsubishi manual, this alarm indicates a memory fault or hardware error detected during the internal self-test. This means that the CPU identifies irregularities within the memory management or control electronics during startup.

In practice, Alarm 12 is often interpreted as a total failure. Many units are replaced immediately, even though the actual cause is frequently limited to individual components on the control board.

Our technicians therefore started an advanced analysis of the RM111C-21 control board.

Through systematic signal tracing, voltage analysis and comparative measurements, several defective electronic components responsible for memory management and communication logic were identified.

After replacing these components, the board was tested again.

Alarm 12 disappeared completely.

Following this repair, the entire servo drive underwent a comprehensive overhaul.

This included:

• Verification of all relevant supply voltages
• Rework of critical solder joints
• Inspection of all connectors
• Cleaning of cooling surfaces
• Examination of thermally stressed components
• Testing of communication interfaces

After completion of the repair work, the unit was operated on our Mitsubishi MDS test bench.

The following tests were carried out:

• Multiple power-up cycles
• Communication tests
• Temperature monitoring
• Load testing
• Long-term operation test

No further alarms or warnings occurred throughout the complete test procedure.

The most interesting aspect of this repair case was the fact that two completely independent faults were present.

The first fault prevented the unit from powering up.

Only after that fault had been repaired did the second fault become visible and trigger Alarm 12.

Cases like this demonstrate why a repair must never be considered complete after the first apparent success. Only comprehensive functional testing under realistic operating conditions can confirm that all faults have truly been eliminated.

After successfully completing all tests, the Mitsubishi MDS-D-V1-20 was returned to the customer.

Preventive Measures for the Customer

• Clean the electrical cabinet regularly
• Inspect cooling fans every 2 to 3 years
• Remove dust from heat sinks
• Check connectors annually
• Document supply voltages
• Inspect capacitors after approximately 15 years of operation
• Perform regular insulation testing
• Monitor cabinet temperatures

Conclusion

This repair case clearly demonstrates that what initially appears to be a simple failure can often involve multiple independent faults.

Only after restoring the power supply section was the actual memory fault revealed. By repairing both the power supply and the control board, the Mitsubishi MDS-D-V1-20 was fully restored and returned to reliable operation.

Thorough diagnostics, preventive refurbishment and extensive testing remain the key factors for long-term reliability and maximum machine availability.

To mentioned Mitsubishi Drive: Mitsubishi Servo Drive Unit MDS-D-V1-20

More details about our Mitsubishi repair services can be found here:
Mitsubishi drive Repair by Industrypart

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Technical Specifications

Feature	Value
Model	Mitsubishi MDS-D-V1-20
Device Type	Servo Drive Unit
Power Rating	1.0 kW
DC Input	270 to 311 VDC, 7 A
AC Control Input	Single phase 200 to 230 VAC, 0.2 A, 50/60 Hz
Output	Three phase 155 V, 4.6 A
Output Frequency	0 to 240 Hz
Hardware Version	E
Software Version	1501W001
Manufacturing Date	05/2003
Manufacturer	Mitsubishi Electric
Manual Reference	IB-1500010
Cooling	Heat sink with forced ventilation
Weight	Approx. 4 to 6 kg
Series	Mitsubishi MDS-D Servo System

The nameplate data was taken directly from the submitted unit. Alarm and fault information originates from the MDS-D/DH Instruction Manual.

Operating Environment & Compatible Equipment

The Mitsubishi MDS-D-V1-20 was installed in numerous Mitsubishi CNC machine tools. Typical applications include:

• Feed axes of machining centers
• CNC lathes
• Grinding machines
• Tool changers
• Ball screw drives
• Positioning axes

Compatible with Mitsubishi MDS-D Power Supply Units and Mitsubishi AC servo motors of the same generation.

Functional Description

The MDS-D-V1-20 controls a single servo axis. The controller receives position and speed commands from the CNC and converts them into precise motor currents.

Its main functions include:

• Current control
• Speed control
• Position control
• Encoder evaluation
• Communication with CNC and Power Supply
• Monitoring of temperature, current and voltage
• Protection against overload, overcurrent and memory faults

One particularly important feature is the internal self-diagnostic routine during power up. Memory areas, processor functions and communication paths are checked immediately after startup. If a fault is detected during this procedure, Alarm 12 “Memory Error” is generated.

Alarms & Troubleshooting

Alarm	Description	Cause	Solution
10	Low Voltage	DC bus voltage too low	Check power supply
12	Memory Error	Hardware fault during self test	Repair or replace control board
17	A/D Converter Error	Current measurement fault	Check electronics
18	Encoder Communication Error	No communication with encoder	Check cable and encoder
24	Ground Fault	Motor cable connected to ground	Check insulation
30	Regeneration Error	Regenerative resistor overloaded	Check braking circuit
32	Power Module Overcurrent	IGBT or motor fault	Check power stage
33	Overvoltage	DC bus voltage too high	Check braking circuit
3B	Power Module Overheat	Insufficient cooling	Check fan and heat sink
45	Fan Stop	Cooling fan failure	Replace fan
50	Overload 1	Motor or drive overloaded	Reduce load
51	Overload 2	Continuous high current	Check axis mechanics
71	Instantaneous Power Interruption	Voltage drop	Check power supply
72	Power Supply Fan Fault	Power supply cooling fan failure	Replace fan

Alarm 12 is defined in the Mitsubishi manual as a hardware fault detected during the power-on self-test. Typical causes are internal memory or processor faults.

Components

Assembly	Designation	Function	Notes
Control Board	RM111C-21 / BN638A503G51	Regulation and communication	Common source of Alarm 12
Power Board	RM162A-V1 / BN638A394G51C	Power stage and current control	Main power section
Power Module	MP04-DH-V1-20	Motor drive stage	Overcurrent protection
Power Supply Section	Integrated	Supplies all electronics	Frequent failure source
Heat Sink	Integrated	Heat dissipation	Clean regularly