Introduction
The Macintosh IIsi has a case fan mouted on inside back of the computer case.
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In all of the years that I have worked on this machine, I have found no good way to remove the fan.
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The best way to start, is to push in here on the side of the fan.
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Being careful not to exert too much force, try to push in, and wiggle the fan outward as such. (2nd Photo)
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You can now lift the fan all the way out, and remove it.
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The Fan Power Contacts on the Logic Board:
 
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                            Conclusion
                          
                          To reassemble your device, follow these instructions in reverse order.