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Macintosh IIsi – Purple screen with ATX PSU: Problem solved!


Well-known member
Some months ago (before the great crash), I asked for help with a freshly recapped Macintosh IIsi which was showing a purple screen for 30 seconds until it booted. It appears that the ADB microcontroller (or something else) holds down the reset signal for this amount of time.

View attachment IMG_2966.MOV

CPU reset signal (with original audio from the headphone jack), taken from pin 15 of the ADB Controller (UB1)
View attachment Boot-RST-Signal.mov

Can you see the wooden peg in the upper video? I used this to pull the PS_ON signal to ground to automatically switch on the PSU. It now appears that the ADB controller gets confused if the PSU switches on while the power failure warning signal (pin 26 of the ADB controller) is low. This signal is pulled high for about one second if one of the power buttons is pressed. So when I press the power button and instantly connect PS_ON to ground, the system boots up immediately!

So I now just need to restore the original PSU and my IIsi reborn!