A long standing customer reported recurring fault messages on his older machine equipped with a MELDAS 500 control system. The machine stopped sporadically during the power up sequence or shortly after servo ON. The codes 65 rush relay error and 6C main circuit error appeared frequently, sometimes accompanied by 68 watch dog. The customer reported increasing startup problems when the cabinet was warm, which is a clear indication of thermally unstable components in the rush or main circuit.

After arrival of the unit we performed a visual inspection. The fans were dusty, the heatsink fins partially clogged and several capacitors showed signs of aging. Especially around the rush relay we found discoloration which typically indicates long term thermal stress and contact wear. The customer mentioned that the machine sometimes required several startup attempts before running stable. Such behaviour is typical when the precharge circuit is unreliable.

During initial electrical measurements we observed fluctuations in inrush current and a delay in the transition from rush to main operation. The relay chattered during several test cycles which confirmed the plausibility of alarm 65. At the same time certain areas of the control board heated up faster than expected. Measurements on the main charging circuit showed unstable values trending toward a 6C main circuit error.

After disassembly all power components were tested individually. The rush relay showed clear contact erosion and reduced spring force. The precharge resistor had drifted from its nominal value due to thermal fatigue. The DC bus capacitors showed elevated ESR values which impaired stable voltage regulation. On the control board we found several cold solder joints and traces of thermal cycling near the watchdog circuit.

During the repair we replaced the complete rush section including the relay and resistor. All relevant high voltage capacitors were renewed to ensure long term stability of the DC bus. The control board was reworked, critical solder joints were reflowed and the watchdog circuit stabilised. Additionally the fan was replaced with a high quality industrial model to prevent future overheating and to avoid errors such as 77 PCB overheat or 74 regeneration resistor overheat.

The final test run on our in house test bench was successful. We performed more than fifty rush test cycles including extended thermal load and varying load profiles. All transitions between precharge and main operation were stable and reproducible. The DC bus voltage remained constant and no faults occurred. Even under long term thermal stress the unit remained fully stable. The customer was able to reinstall the unit and confirmed flawless operation after reassembly.

Preventive Measures

• Annual fan inspection
• Check DC bus capacitors every 7 to 10 years
• Reduce cabinet temperature
• Monitor regeneration loads
• Inspect rush circuit during maintenance

Conclusion

The MDS-A-CR-55 is durable but after decades benefits greatly from a preventive overhaul. Rush relays, capacitors and thermally stressed areas of the control board show the clearest improvements after refurbishment.

To mentioned Mitsubishi Drive: Mitsubishi MDS-A-CR-55 Power Supply Unit

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Technical Specifications

Operating Environment and Compatible Devices

The Mitsubishi MDS-A-CR-55 is used in older machine tools equipped with MELDAS 500 control systems. It supplies servo and spindle amplifiers of the MDS-A series and provides the required DC bus voltage. Typical applications include lathes, machining centers and transfer machines from the early to mid 1990s. In many installations several amplifiers operate on one shared CR unit, making its stability essential for machine availability.

Functional Description

The MDS-A-CR-55 is an AC to DC converter that rectifies the mains voltage and initially precharges the DC bus through the rush circuit. After precharge, the main circuit maintains a stable DC bus voltage. The regeneration circuit dissipates braking energy. The unit monitors voltage, temperature, CPU status, memory integrity, overcurrent, ground faults and parameter validity. Errors are displayed via LED codes. Severe alarms shut down the supply to prevent secondary damage.

Alarm Messages and Troubleshooting

Components