MacPro 5.1 330-2337-A mainboard No Powerup, PSU OK,

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  • wrighthm
    Mr Wright
    • Jun 2025
    • 2
    • Australia
    #1

    MacPro 5.1 330-2337-A mainboard No Powerup, PSU OK,

    Good Morning All. I know this has some exposure in the forum but hoped somebody could help.

    I have a MacPro 5.1 that wont power on. Power Button LED never illuminates. I followed the Service Guide and removed all parts When plugging in power i get the two CPU TEMP RED led's flash once and I can press the Diag LED and get the Standby LED. If I remove Power I get the same two LED flash. I have tried shorting SYS PWR pads to force start the board and nothing happens. I have also tried shorting the SMC reset with no result. I changed the BIOS Battery too. Finally I pulled the PSU, Shorted the switch pins and the PSU starts proving the required 12V, 5V and 3.3V.

    I then removed the board and checked screw holes to electrolytic capacitors looking for a short, all values here are well above short.

    It appears that whilst the mainboard has the startup 5V the SYS PWR trick will not tell the PSU to start. It also seems that schematics do not exist for this.

    If anybody has any ideas that don't involve replacing the board as its not economically viable for that, please reach out and thanks for reading.
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