Power cord buzzes and MACPro does not recognize it

This is a fault in the power supply itself, I have just repaired one of these power supplies successfully.
The fault is caused by the failure of the filter capacitors on the 12V supply rail, and if the power supply is left connected to the mains supply (In our case 240V 50Hz) without the drive connected as a load, the unloaded output voltage will exceed the voltage rating of the capacitors, causing their failure. The failed capacitors had very obvious bulges at the top, and one was beginning to leak onto the PCB.

The filter capacitors on the 12V supply rail are rated at 16V, and the cpacitors on the 5V rail are rated at 10V, and when operated without a load, the 12V rail rises to approximately 19V, and the 5V rail rises to approximately 8V. All output filter capacitors are 1000uF, and 105deg C temperature rated.

I repaired my power supply by replacing all of the filter capacitors on the DC low voltage output rails with 1000uF 25V capacitors. Be careful to only fit the 105 deg C rated caps, as they operate at elevated temperatures in an enclosed environment with no ventillation to cool the power supply.
To ensure that the repair was reliable, I also replaced the filter capacitor on the 5V rail with the same 1000uF 25V, even though that capacitor showed no immediate signs of failure.

When reassembling the power supply, care must be taken to ensure that parts like the ferrite cored transformer are thermally bonded to the aluminium heatsink with a smear of heatsink compound.
When the power supply was first powered up following repair, everything was normal, the green power LED came on as usual, the supply outputs both came up OK, and the unit was not making any buzzing sounds, completely silent. The HDD now operated as normal when once again powered from this power supply.

This was the fix for my particular LaCie power supply model ACU057A-0512, and the supply once again works normally.


Tim,
Brisbane Australia.