In this repair case, we received three Mitsubishi Servo Drive Units MR-J4-40B-RJ from the same customer, all showing the identical fault condition. All units reported the Main Circuit Error Alarm 24 immediately after power-up. What was striking was that these were relatively young devices, externally in good condition, with no visible mechanical damage. Despite the series still being comparatively recent, it was already in the phase-out stage, which made spare part availability and replacement with new units more difficult.

The customer consciously decided against scrapping the drives. In addition to a strong environmental awareness, the individual parameterization of the servo drives played a decisive role. The drives were precisely matched to the machines, and replacing them would have required extensive adjustments and caused additional downtime. Furthermore, the customer had proactively stocked spare units and collected defective devices, allowing them to be repaired without time pressure.

After receipt, all three units first underwent a detailed visual inspection. This was followed by electrical input measurements, which confirmed that the fault was reproducible in the main circuit area. Analysis of the DC bus revealed irregularities during the charging phase, clearly indicating a defect in the charging circuit and adjacent components.

In the next step, the units were completely disassembled, cleaned, and freed from dust and deposits. Critical components in the main circuit were replaced preventively to avoid future failures. Special attention was paid to thermally stressed components, which experience has shown can age prematurely even with moderate operating hours. After completing the repair, a multi-stage functional test was carried out on our in-house test bench. The servo drives were operated under realistic load conditions, thermally monitored, and repeatedly powered on and off to verify the behavior of the DC bus.

All three units passed the final tests without any abnormalities and demonstrated stable control performance. Through preventive overhaul, the service life was significantly extended and the risk of renewed downtime was minimized.

Preventive Measures for the Customer

Regular visual inspection of fans and airflow paths
Early replacement of thermally stressed components
Clean and dry control cabinet environment
Avoidance of frequent power interruptions
Planned maintenance instead of reactive repair

Conclusion

This repair case clearly demonstrates that refurbishing modern servo drives is worthwhile even when repair costs initially appear high. In addition to sustainability and resource conservation, customers benefit from stable processes, reduced downtime, and the preservation of proven machine configurations. A professional repair with a preventive approach is often the most economical and technically sound solution.

More Info & Contact

To mentioned Mitsubishi Drive: Mitsubishi Servo Drive Unit MR-J4-40B-RJ

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

We regularly repair similar models, including:
MR-J4-40B
MR-J4-40A

Operating Environment and Compatible Devices

The Mitsubishi MR-J4-40B-RJ is typically used in modern CNC machine tools, special-purpose machines, and high-dynamic automation solutions. It operates in combination with suitable Mitsubishi J4 series servo motors and is often installed in machines where precise control, high dynamics, and repeatable accuracy are required. In this specific case, the units were parameterized specifically for the customer’s machines, which made replacement with new units economically and technically impractical.

Functional Description

The MR-J4-40B-RJ is a digital AC servo amplifier with high-resolution control for torque, speed, and position. It continuously monitors the condition of the power stage, DC bus voltage, current control, and internal protective functions. Faults in the main circuit, especially within the DC bus charging circuit, are reliably detected and lead to a safe shutdown to prevent consequential damage to the motor and machine.

Alarm Messages and Troubleshooting

Components