[SOLVED]820-4668 - PP3V3_S4 low - If you power cycle the device ±50 times it will post

[dukefawks]

I would first suspect one of the USB over current protection chips like U4600 or the one associated with the dead port.

 

[smiba]

Thanks, I guess replacing it would be part of a quality repair anyways!

Any tips on testing it on a bench? Because reassembling these boards in the iMac itself is a pain in the ass and I don't feel comfortable having the PSU with exposed 230V joints on my desk

 

[dukefawks]

I just use the guts of a scrapped machine for testing. Yes there will be wires hanging everywhere.....

For the love of God try and source REAL adhesive strips to put this thing back together. Most of the shit sold everywhere are Chinese rip offs and you will never get the LCD off again.

 

[smiba]

No worries I did, thats why it hurts so much taking it apart again because they're not your regular $1 tapes. I still have nightmares about iPad screens so I'll never cheap out on tape haha
Got apart with exactly the same amount of force then it came with, the tape is again easily removable and leaves no or almost no residue

Alright I guess thats the only solution for the bench testing then, thanks! I'll replace U4600 + any other parts of that circuit if I find them to be bad and will report back

 

[dukefawks]

Could also be U4700 of course, I don't know which port is bad in your case.

 

[smiba]

Port A damaged and Port B broken, should be U4600

 

[smiba]

Took a bit longer then planned due unexpected sickness.

Anyways U4600 actually has a small hole inside of it and PP5V_S4_EXTA_ILIM was semi-shorted with only 16Ohms to ground. And PP5V_S4 went from 10.4Ohms to ground to >15kOhm
Not surprised this can cause some issues

Its currently running and i'll be monitoring the device for a little while before I return it to the customer