You should be able to replace it, but it may require you to snip the power cable for the fan and splice in the new one. I recently replaced the fan in a digital audio power supply (it was seized) with a newer, quieter model. The fan doesn't have a power connector on the board in the power supply; it is soldered directly through the board. Replacing it was easy, though. Snip the old line, splice in the new line, put the lid back on the power supply, and it's good to go!

Peace,
Drew
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Power to the PowerPC!