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IIfx Momentary Power Switch? (Power Circuit Bypass)

I have offically given up on repairing my power circuit on a Mac IIfx. Brand new batteries and all I just cant figure out what is preventing the power circuit from working. Everything works fine otherwise but I am left having to pop the case lid and jump start with a coin battery. Is there a good solution for mounting an external switch that can be used to turn off and on the power supply? For that matter, is there anyway to actually turn off the power supply with a switch without having to unplug it?


Well-known member
Start with the Nand Gates UJ18 (startup) and UI18 (shutdown) and search for broken traces due to the leaky capacitors near by. Here's image of the Startup circuit.

Also a link to Doug Brown's write up on the Mac IIx start up circuitry. Even thou there is a difference with the IIx and IIfx, he has a great explainer on how the circuit works with the Mac II.



  • macIIfx startup circuit copy.jpeg
    macIIfx startup circuit copy.jpeg
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