820-00292 iMac 27-inch 2015 no light on diagnostics leds

Hi,

Tried good working power supply - no light on any diagnostics LEDs. Inspected board for corrosion or explosions - nothing. Seems like there are no schematics for this board either. Any known issues with this board? Any idea what it can be?

Thanks!

Leakage on a Power FET; usually 12V_S0.
Use 3478 and 00291 schemtics...

Checked voltage on pins 4,5,6, 10,11,12 - 0V
Checked for short - shorted
Replaced Q8450 (12V_S0) FET - no short
voltage on pins 4,5,6, 10,11,12 is 0.45V now.
Please advise.

You should have 12V at pins 1/5; there is 12V_G3H...
If not, post diode mode to ground.

497 in diode mode

Missing 12V_G3H from power supply?
Check at power connector.

It is 12V on the output cable of power supply. When I plug it in to the board, it becomes 0.41V on J6900.

"497 in diode mode"
Is that with power supply connected to the board?
No 220V at power supply, of course...

497 with power supply connected
624 without

No short, you must get 12V there, when power supply connected.
Check the power cable, between power supply and MLB.

You can test the board alone on the desk, with 12V from lab power supply...

I injected 12V and the results are almost the same.
I get 1.5V on the connector and 1amp.

Did you inject directly on the 12V_G3H power rail?

No. First time I injected it at J6900. Now I tried to inject it directly at 12V_G3H and I got green light on the first diagnostics led.
I went poking around for more short and found another one on PP12V_S0 line. Tried to inject 12V there but it only takes 1V since its shorted to ground. Nothing heats up. Whats my next move? Thanks!

Discard first any shorted high-side MOSFET from a secondary power supply connected to 12V_S0.
Compare short value with ohm value to ground from correspondinf big coils.
If nothing strange, then start injecting with 1V and slowly go up.
Let ampers adjust at max on the lab power supply.