Old and new power supply -> trickle voltage LED off

My iMac late 2012 27" fails during the night in sleep mode. It doesn't go on at the morning. No Power on USB at all.

After some days of trying and waiting I decided to open the machine and none of the 4 diagnostic LEDs lit up. I could not measure any voltage. I measured mains cable -> OK. I measured inside the iMac between the mains connector socket and the mains connector on the power supply -> OK. I ordered a new power supply. The old was Delta the new is Liteon with same specs.

After replacement -> same problem again. No trickle voltage and no reaction at all.

What's going on here? What does the power supply (on some pins) need to deliver trickle voltage?

I try to find some technical information or shematics without any luck. I am an EE with all the measurement equipment and the soldering tools needed. But without more information I am at an dead end.

Any idea?

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@wattsekunde remember that LED 1 indicates that trickle voltage from power supply has been detected by logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. The LED will remain ON even when computer has been shut down or put to sleep. The LED will remain OFF only if the AC power source is missing or disconnected , if the logic board is disconnected from the power supply, or if the power supply board is faulty. Since you repalced the PSU, point 1 and 3 do not apply, that leaves your logic board. What exact model is your iMac and what 820-XXXX is your logic board.?


@oldturkey03 Thank you for your help.

All LEDs are always off. AC power is connected. I measure AC on the PSU PCB. I measure around 35mA at 240V on the AC power cord. The new and the original PSU showing the same effect. Yes that sadly sounds like a LB problem. But I want to try to find some fault on the LB before replacing it.

My iMac is Model A1419, EMC 2546 with 3,4GHz i7, 8GB (later 16GB), 680MX 2GB, 1TB Fusion.

I couldn't find any number on the backside of the logic board PCB. I haven't removed it yet, but I think this must be the next step. I post the board number if I could find it on the other side.


@wattsekunde, let me know what you find. If it is a 820-3299 board I probably can help you out with some schematics.


@oldturkey03 I managed it to get the logic board out of the imac. The only number I can find is a small sticker nearby the RAM sockets.

CJ3A-31PI (Q or O) MB00H0.

I guess this is the serial number? Or is there a specific place I haven't seen?

Maybe your schematic could help me to understand in general the trickle voltage LED and communication between PSU and LB? I would be very happy about it if you could send me some schematics. The PSU is used in all 27" iMacs between late 2012 and late 2014 (iMac13,2 14,2 15,1).


@wattsekunde send me an email with your contact. Click on my Avatar to get to my email address. I have the 820-3299 logic board schematic and hopefully it'll work for you.


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