22.08.2025 by Viktor Siebert
Mitsubishi MDS-B-CV-150 Power Supply Unit with Alarm 8
The customer reported that the machine stopped after a few minutes, showing error “8” on the display of the Power Supply Unit. This alarm indicates that the internal software process of the PSU did not complete within the required time frame. As a result, the entire machine was down and production came to a halt.
When the unit arrived in our workshop, a visual inspection revealed heavy wear on the power stage. The insulation of several components showed discoloration caused by heat, and dust deposits had restricted cooling. The more serious issue, however, was a fault on the control board: several ICs and traces showed malfunction.
The repair was challenging and required multiple steps:
- Cleaning
All boards were carefully cleaned with industrial-grade cleaning solutions to remove dust, oil, and conductive debris. Afterwards, the modules were dried in a special oven to prevent moisture damage.
- Diagnosis & Fault Isolation
Using an oscilloscope and test routines, the control board was analyzed. Several defective ICs and passive components were identified.
- Control Board Repair
Faulty ICs and capacitors were desoldered and replaced. Damaged traces were reinforced and reworked.
- Power Stage Overhaul
Power transistors and electrolytic capacitors were replaced as a precaution. Insulators and thermal interface materials were renewed to restore proper heat transfer.
- Testing Procedures
- Functional test with simulated machine environment
- Load tests under various conditions
- Thermal stress testing at elevated ambient temperatures
- Complete documentation following standardized checklists
The repair process required many hours of detailed work and expertise. In the end, the unit was successfully restored to reliable working condition. For the customer, this meant significant cost savings compared to purchasing a replacement unit and much shorter downtime.
Preventive Measures for Customers
- Regular cleaning of the unit and airflow channels to prevent overheating
- Fan replacement every 3–5 years, depending on environmental conditions
- Insulation measurements of motors and supply cables to detect early-stage faults
- Seal inspection to prevent moisture ingress
- Thermal management by monitoring machine room temperature and ensuring proper airflow
Conclusion
The MDS-B-CV-150 Power Supply Unit is a robust and essential part of Mitsubishi CNC drive systems. However, after many years of continuous service, typical wear issues such as control board faults and power stage degradation may occur.
With professional repair and preventive maintenance, the service life of the unit can be significantly extended. For end users, the key to reliability lies in consistent care combined with timely expert repair.
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Technical Summary of the Unit
| Parameter | Value |
|---|
| Type | MDS-B-CV-150 Power Supply Unit |
| Rated Input | 3AC 200–230 V, 50/60 Hz, 49 A |
| Auxiliary Input | 1AC 200–230 V, 50/60 Hz, 0.2 A |
| Rated Output | DC 270–311 V |
| Output Current | 58 A |
| Standard | DIN VDE0160 |
| Manual Reference | BNP-B3890 |
| Hardware Version | F |
| Weight / Dimensions | approx. 12–15 kg, 380×270×180 mm (typical) |
| Date of Manufacture | 1998/01 |
Application Environment & Compatible Devices
The MDS-B-CV-150 Power Supply Unit is used in CNC machinery powered by Mitsubishi MDS-B series servo drives. Typical applications include lathes, milling machines, and machining centers.
It is most often combined with servo and spindle drive units of the same MDS-B series.
Functional Description
The Power Supply Unit (PSU) MDS-B-CV-150 provides stabilized DC voltage (270–311 V) to the connected servo amplifiers. Its main functions include:
- Conversion of three-phase AC input into DC voltage
- Supplying stable power to drives and motors
- Protection against overcurrent, overvoltage, and overheating
- Monitoring and diagnostic interface
Alarm Messages and Troubleshooting
| Alarm No. | Display | Description | Cause | Solution |
|---|
| 61 | 1 | Power Module Error | Overcurrent in the IPM | PR |
| 63 | 3 | Compensation Regeneration Error | Regenerative transistor active too long | PR |
| 65 | 5 | Rush Relay Error | Inrush current relay not switching | PR |
| 67 | 7 | Open Phase | Missing input phase (R/S/T) | PR |
| 68 | 8 | Watchdog Error | Software process not completed | AR |
| 69 | 9 | Ground Fault | Ground fault in the motor | PR |
| 6A | A | Contactor Error | External contactor malfunction during “Ready” | PR |
| 6B | B | Rush Relay Error | Inrush relay remains active | PR |
| 6C | C | Main Circuit Error | Capacitor charging error | PR |
| 6E | E | Memory Error | Fault in memory circuit | AR |
| 6F | F | AD Converter Error | Fault in A/D converter | AR |
| 71 | H | Instantaneous Stop | External contactor off during “Ready” | PR |
| 75 | L | Overvoltage | Voltage >410 V | PR |
| 77 | n | Power Module Overheat | Overheating of IPM | PR |
Components
| Type | Marking on PCB | Qty |
|---|
| Control Board | RK412A or BN634A770G51A C | 1 |
| Power Board | RK427A-185 or BN634A781G51A G | 1 |
| Capacitor Board | RK443A-150 | 1 |
| Power Module | Integrated | 1 |