How to debug iMac 27" power circuit?

tl;dr; This A1312 can not start up. PS_ON voltage appears to be too low, despite testing with two different PSU’s.

The story so far…
Several months back, my son’s A1312 started suddenly turning off with no specific trigger cause. The iMac could always be turned back on again, and there was no evidence to be found in the logs. Then one day it turned off and stayed off. (Luckily I had another spare machine for my son).

Searching for how to debug it, I found Can anyone tell me what these PSU pin abbreviations mean? PSU 614-0446 and iMac won't turn on, which together with the diagnostic LED’s helped me determine that probably the PSU was broken, so I ordered a “new” used, but tested, PSU. The only source I could find was in the US, and shipping to Denmark was expected to take about a week. However, it took longer, much longer… the PSU finally arrived just before Christmas!

I’ve spent today trying to get the iMac to work again, but it looks like it suffers from exactly the same problem with both PSUs.

The two PSU’s are not exactly the same design: One is LITEON, the other DELTA. Connectors and major components are more or less identical, but it’s two very different PSUs.

Unfortunately, I haven’t been able to find a schematic for either of the PSUs or for the motherboard.

When testing, the diagnostic LED#1 is always on, and no other diagnostic LED’s ever turn on. There are no other audio or visual indications from the iMac ever.

Diagnostic LED #2 is expected to light up when PS_ON is 12V, but measuring the PS_ON voltage on the power button, I get 3.98V. Checking on pin 12 on the DC Power Connector on the motherboard, and also on pin 3 on the PSU DC Power Connector I find the same voltage. Disconnecting the motherboard, the PS_ON voltage drops to 3.64V. The strange thing, however, is that I measure exactly the same voltages for both of the PSUs I’m testing with.

Measuring some other voltages on the DC Power connector, I realized that the power button is able to power up the PSU, but not allow the iMac to start. I’m guessing the PS_ON voltage somehow gate this. Once the PSU is supplying 12V, it’s also not possible to use the power button to turn off the PSU.

My measurements on the motherboard DC Power Connector before and after pressing the power-button:

#1 : 0V (GND)
#2: 0V -> 12V (PP12V_S0_PS)
#3: 0V (GND)
#4: 0V -> 12V (PP12V_S0_PS)
#5 : 0V (GND)
#6 : 0V -> 12V (PP12V_S0_PS)
#7 : 0V (GND)
#8 : 12V (PP12V_C3H_ACDC) Standby supply.
#9 : 0V (GND)
#10 : 0V (GND)
#11 : 0V -> 3.3V (Either SDA or SCL)
#12 : 3.98V (PS_ON) - Short to GND to start PSU - can not turn off.
#13 : 0V (GND)
#14 : 0V -> 3.3V (Either SCL or SDA)
Note, pins are counted 1-2, 3-4, 5-6, 7-8, 9-10, 11-12, 13-14, starting from the squared pad.

Performing the same measurement on the PSU Power Connector, I get the following:

#1 : 0V (INVERTER -)
#2 : 0V (GND)
#3 : 3.98V (PS_ON) - Short to GND to start PSU - can not turn off.
#4 : 0V (GND)
#5 : 12V (PP12V_C3H_ACDC) Standby supply.
#6 : 0V (GND)
#7 : 0V -> 3.3V (SMB_ACDC_SCL_RC)
#8 : 0V (GND)
#9 : 0V (INVERTER -)
#10 : 0V -> 12V (INVERTER+)
#11 : 0V -> 12V (INVERTER+)
#12 : 0V -> 12V (PP12V_S0_PS)
#13 : 0V -> 12V (PP12V_S0_PS)
#14 : 0V-> 12V (PP12V_S0_PS)
#15 : 0V -> 3.3V (SMB_ACDC_SDA_RC)
#16 : 0V (GND)
Note: Pins are counted 1-8, 9-16, with 1 and 9 next to each other. (As marked on the PCB, but not as marked on most drawings I found online).

But I’m wondering.

1. Is there something wrong with the motherboard, that somehow has damaged the new PSU, so that it now exhibits exactly the same error as the old PSU?
2. Is the PS_ON really expected to be 12V (as I’ve read a few places across the web) or is 3.98V the expected voltage, and the error lies somewhere completely different?
3. I have another working 27” iMac (Mid-2010, EMC 2390), which I could take apart and test with… But for now, I don’t dare. Would you?

Suggestions on how to debug and repair this issue are highly appreciated.

Thanks in advance, and Happy New Year to you all.