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My Macintosh II Restoration


Well-known member
Not entirely. I attached the “most circulated” diagram for PM8500. The thread itself no longer exists.

The 47k pull-down resistor introduces too high of a voltage drop, resulting in low input signal and therefore the PSU is always on. Lower values like 10k resulted in momentary high signal, then low again, so the PSU would turn on, then off again after 1 second or so. Using 1k pull-down resistor would and up having the PSU always off. So I ended up using a trim pot to tune the proper resistance and then measuring it for a proper pull-down resistor value. It was 5.9k ohm.
Thats fair - I was just checking you didn't expect two designs to be identical.


Well-known member

I referenced the Guide to the Macintosh Family Hardware to confirm the specs/pinouts on the power supply connector and then saw that the soft power signal is coming from the two motherboard batteries to pin 15 on the PSU connector. Then just needed to invert that signal for the ATX power supply.

I just used the same inverter circuit that others have used in the past: https://bylenga.ddns.net/index.php?page=Centris_ATX.php

Link to GMFH: https://archive.org/details/Guide_to_the_Macintosh_Family_Hardware_Second_Edition/page/n1/mode/2up

Awesome, thanks! Now I have another fun project.