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Okay, I've gone back through checking the rails... PP3v3_S4 boosts twice the same as every other S4 onwards rail, but after the others boost twice, they drop to 0v. PP3v3_S4 drops to 0.45v and sits there. The damage was in the U3210 area, so I swapped out U3210 to see if it made a difference...

So with the U6100 attached it boosts twice. Without it attached it boosts constantly. Is that it then? Bin it and replace the board?

We use this stuff in 0.038mm or 0.51mm dia (depending on stock) its 60/40 solder with 1.1% no clean flux core.... not that you wouldn't clean after anyway :) Comes as a 500G reel so should last a fair while. https://www.somersetsolders.com/tin-lead-solder-wire-with-rosin-free-flux/p180

Without the bios, its boosting PM_SLP_S4_L and VCORE in stages by the looks of it. So it boosts after every 2 second pause and changes the voltage up or down each time in a cycle. VCORE = 0.08v (pause) 0.12v (pause) 0.24v (pause) 0.53v (pause) 1.1v (pause) 1.3v (pause) then does the same in...

Okay, I’ll give that a go and see what it does when I get back to work on Monday.

Nothing obvious near the cpu at all. To be honest, apart from the backlight chip, there wasn’t much anywhere. Do you mean if remove the bios chip entirely and then connect the MagSafe, it should power cycle constantly?

From checking a bunch of S0 rails, they all seem to do the same boost twice and then give up

The U6100 I tried was from a 2015 00165 board and this is a 2017.. would that make any difference? I could always try ordering in a new U6100 with a clean ME just to be sure I suppose.

Maybe the bios was no good off the scrap board. What do you think?

I'll swap it back again. I don't understand why I get VCORE boosting twice but no fan spin. What should I look for?

Swapped the bios but no change :(

Okay, I’ll give that a bash tomorrow, home time in the UK now :)

I've never swapped a bios before, I know theres stuff to do to get it to work correctly, but do you mean just straight swap for now just to test for boot?

At C1031 PPVCC_S0_CPU is either 1.7v or 1.8v, I think my multimeter isn't quick enough. No idea where the 3.3 came from.. My bad

VCORE is 1.8v not 3.3v! sorry

I'm checking at L7320 pin 2.. Is that correct?

Yeah just double checked and they both jump to 3.3v twice and then quit

PPVCC_S0_CPU is jumping to 3.3v also, it does it twice. The other may do it twice also but I might have missed the first one.

PPBUS is exactly 8.6v - PM_SLP_S4_L jumps to 3.3v about 3 seconds after connecting the charger but doesnt do it again until you unplug and plug back in

I reballed the SMC. I now get 3.33v on PP3v3_SUS and PM_DSW_PWRGD is 3.42v. Still no fan spin though :(