A1208 2006 Intel iMac Power Supply "Power Good" voltage?
Solved this on my own. For reference this is the information needed. The Power Supply is a 12V 15.5A Power Supply, with an Inverted Power Good Signal.
Logic Low (Or Floating) = Good, Logic High = Bad (I don't know what voltage level), tells the computer to shut itself down.

This forum post from 2013 had a similar issue (weak power supply).

Left to Right, when plugged into the DC/DC board (Part 820-1934).



12V (Grey)
12V (Grey)
Gnd (Black)
Gnd (Black)
PG [Power Good] (Gold or Brown, Varies)


The 6x2 connector on the DC/DC board goes out to the Logic Board and SATA Harddrive. All outputs look to be on when the connector is unplugged.



PG (Power Good, Direct from Power Supply Connector)
GND (To LB and HD)
12V (To LB and HD) Switchable with P_ON
GND (To LB and HD)
5V (To LB and HD) Switchable with P_ON
3.3V Switchable with P_ON
?P_ON/Power_ON?
12V Always On
12V Always On
5V Always On
GND
3.3V Always On


Pin 7 on the 6x2 connector that I labeled Power_On, Connects to transistor pair most likely controlling the IRF7424 P-Channel 10A mosfet. Logic Low (Or Floating) turns ON the DC/DC converter's Lower Outputs, Logic High (3.3V) disables it. The Motherboard pulls this High when the computer is OFF. The Harddrive Power is connected to the Lower Outputs.

The setup is fairly common. Some newer iMacs skip the DC/DC board and have that all built into the logic board, but still take 12V input.

I connected an old ATX power supply, with a 12V 7.5A rail, half the current of than the original. It's also out of spec (10V instead of 12V ±5%), so the hard drive wouldn't spin up. I left the Power_Good FLOATING. But the computer booted. Now that I know the majority of the iMac is working, I can order a replacement power supply without worrying about wasting my money.