ATX PSU conversion Powermac G3

Discussion in 'PowerPC Macs' started by Laterne, May 29, 2014.

  1. Laterne macrumors newbie

    Feb 1, 2014
    Finally my original G3 beige's PSU died. So I took the opportunity to convert to an 80 Plus Gold certified one.

    There were three reasons for upgrading:

    1. active PFC => high efficiency ; low consumption
    2. ultra silent operation of the 120 mm PSU fan being temperature-controlled
    3. almost doubled nominal power output

    To fit the G3 Mini-Tower case without modifications the PSU has to be ATX form factor. The correct location of the power unit's On/Off switch is another aspect to be considered.

    Some general info:
    Both beige G3 cases - the Desktop and the Mini-Tower - feature the Gossamer mainboard, but only the latter was shipped with an ATX-based power supply. Before converting to an actual ATX PSU make sure the small PSU-specific jumper (found on the mainboard in the vicinity of the PCI expansion slots) is set to PS/2 position.

    In addition, two modifications must be carried out in order to make a 24-pin PSU connector work with these old-world Powermacs:

    1) pin 18
    Since year 2005 most 24pin ATX power units don't carry pin 20 (-5V) any more. So we can take the (useless white) wire of pin 20 to obtain "ground" for G3 pin #18 e.g. by connecting it to black wire of pin24.

    2) pin 8
    The standard ATX pin #8 called "Power Good" (often grey-colored) is not required because the Powermac's mainboard provides a built-in boot delay. Instead the beige G3 asks for +3.3V ("orange") at pin #8.

    I purchased a short ATX wire adapter (24-pin female to 20-pin male) that is well suited for these two mods. You don't have to alter anything at the PSU wiring loom itself!

    This modified 24-to-20-pin adapter will allow the following Power Macs to be upgraded with a state-of-the-art ATX PSU:

    G3 Desktop
    G3 Minitower
    G3 All-in-one
    G3 Blue & White
    G4 PCI Graphics

    Attached Files:

  2. harrymatic macrumors 6502

    Dec 30, 2013
    United Kingdom
    Great post, using a modern power supply sure beats hunting down a used or new-old stock one on eBay! I've been considering shoehorning a new PSU into my G4 - I'm just waiting patiently for the original one to die! :rolleyes:
  3. Laterne thread starter macrumors newbie

    Feb 1, 2014
    Well, for G4 towers such a PSU upgrade will be even more beneficial :)

    Owners of the low height beige G3 Desktop Power Mac also will be able to upgrade to a modern silent and low power consumption ATX PSU.

    There are differences though:

    The G3 Minitower, G3 B&W and G4 PCI Graphics will accept a standard ATX PSU featuring physical dimensions of 150 x 86 x ... mm using the already present back-plate screw mounting (3 or 4-point).

    In contrast to that the stock G3 Desktop PSU (non-ATX jumpered on the mainboard) comes with a custom design of 210 x 86 x 120 mm. And its fan is located at the bottom side.

    Such form factor is not available within the actual ATX ranges. Fortunately there's still the regular 30 mm distance between the PSU side panel and the center axis of the AC power connector.

    So we can mount most 150 x 86 x ... mm ATX PSUs horizontally in the G3 Desktop (max PSU case depth should not exceed 150 mm).

    But, for free access to the AC power connector and to the PSU On/Off switch four short cuts have to be milled into the Desktop's ABS plastic applying a Dremel preferably (see red dashed lines in below picture). This case mod isn't required for the G3 Minitower beige.

    Mounting a 150 x 86 x 140 to 150 mm ATX PSU using the regular backplate screws is not realizable in the G3 Desktop sadly. So we should apply strong self-adhesive Velcro strips at the PSU bottom side.

    Due to positive long-term experience with these PSU makers and because of the correct position of the ON/Off switch I'd like to recommend the following reasonably priced modern power supply units featuring a temperature-controlled variable speed fan:

    They will fit the beige G3 Minitower and G3 Desktop.

    In the context of upgrading to an ATX PSU in the Desktop please don't forget to set the jumper to PS/2. Cooling within the G3 Desktop case will change because the internal quiet 120 mm fan of the new PSU now sucks in the air from the top side.

    Attached Files:

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