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WilliamRader

macrumors newbie
Original poster
Mar 25, 2023
2
1
Hello all and thanks for having me! I’ve been lurking for a few weeks while I’ve been working on an iMac g4 M1 conversion mod. Big thanks to everyone who has contributed to this pool of knowledge - it is extraordinary!
I hope this has not been asked before, I gave a good look but could not find anything directly related.
I have completed the monitor conversion, but have a question concerning the power supply.
The base iMac is a 1.25 ghz 17 inch G4. I am using the original power supply and display.
As such, I have 5v and 3.3v downconversion running from the PSU, and it works great when powered on.
However, booting is an issue - It seems I have to have one of the 12v and ground lines directly bridged when plugging it in, or the PSU will not start. If the lines are connected, the PSU will begin to click until the lines are disconnected. Then it starts, and the inverter kicks on.
I have wired a simple button for testing purposes, which works, but if held too long, the PSU starts to whine. This has me somewhat concerned, and I would prefer if the PSU would simply come on when the power cable is plugged in. Then, I could wire a separate bat switch to function as an on/off for the monitor and possibly run it out one of the extra 3.5mm jack ports.
I am under the assumption that the power supply is peaking when the 12v and ground are bridged.
Ultimately my question - Given the situation, is there a way I can wire the PSU to power on automatically? I’m hoping it's something simple I missed or am unaware of.
If you need any additional information, I am happy to provide.

Thank you everyone for your time and consideration ~
 

DeltaMac

macrumors G5
Jul 30, 2003
13,484
4,413
Delaware
That's pretty normal for a computer power supply. It needs to be connected to the motherboard (logic board), and there has to be a live connection before the power supply will come on, as you have discovered. (I think that is normally called "trickle voltage") Now, your task will be to discover which pin on the M1 logic board needs to be connected with the power supply. When you make that connection, the power supply should turn on when you want to boot, as it should.
You could connect the power supply to the old G4 logic board, plug in the power cord, and (no need to try booting) check where you have 5 volts (maybe it's 12 volts, I don't know) when only the power is plugged in, but not turned on.
AND, supply that voltage at the same pin when using the M1 board. Should be fun!
 
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