820-3481-A Late 2013 A1419 Imac 1st LED on 2nd LED flickers.

[sparkcoordination]

Hi there,
I have an Imac LB here that refuses to power on. I changed out the blade SSD for an M2 and encountered this problem. Swapped back and the problem persisted. The Imac was working up to this point. Have since successfully installed that drive in another Imac. Tried reseating the processor in its socket but no improvement. S5 rails are good but all power rails after are pulsing on power button press.
I am using the 820-3478 schematic and boardview as 820-3481-A is not available.

Voltages and drop to ground (positive multimeter probe on ground):

Rail Voltage Drop

Always on:
PP3V_G3 3.3V *
PP12V_ACDC 12.04V 540
PP12V_G3H 12.04V 540
PP3V42_G3H 3.43V 319

S5:
PP12V_S5 12.04V 513
PP5V_S5 4.97V 463
PP3V3_S5 3.31V 141

S4:
PP5V_S4 5V (pulse) 154
PP3V2_S4 1.1-3.3V (pulse) 322
PP3V3_S4_AP 1.1-3.3V (pulse) 322

S3:
PP3V3_ENET 1.1-3.3V (pulse) 375
PPVDDQ_S3 1.5V (pulse) 296
PPVDDQ_S3_DDR 1.5V (pulse) 181

S0:
PP12V_S0 12V (pulse) 443
PP12V_S0_GPUCORE 12V (pulse) 446
PPHDD12_S0 12V (pulse) 612
PP12V_S0_HDD 12V (pulse) 611
PP12V_S0_BLC 12V (pulse) 501
PP12V_S0_FBVDDQ 12V (pulse) 447
PP5V_S0 5V (pulse) 379
PPHDD_S0 not measured 379 - Only available on back of LB.
PP5V_S0_HDD 0V 379 - Hard drive not plugged in.
PP3V3_S0 3.3V (pulse) 247
PPSSD_S0 3.3V (pulse) 246
PP1V5_S0 1.48V (pulse) 142
PP1V5_S0_PCH 1.5V (pulse) 142
PPDDRVTT_S0 700mV (pulse) 112
PPCPUVCC_S0_CPU 1.76V (pulse) 020

GPU:
PPVCORE_S0_GPU 880mV (pulse) 010
PPFBVDDQ_S0_GPU 1.54V (pulse) 046
PP1V05_S0 1V (pulse) 061

Thunderbolt:
PP3V3_TBTLC 3.3V 397
PP1V05_TBTLC other side 001
PP1V05_TBTCIO other side 130


I see there is what looks like a short on a thunderbolt rail PP1V05_TBTLC. I am not sure whether this would stop the logic board from powering on though. I put 1V into it and it takes 1 amp however i cannot find any components heating up. Should i just start pulling capacitors on this rail? Look forward to your insights as to why this board might not be turning on. Please let me know if i have missed anything obvious...
thanks from Scotland,
Will

 

[2informaticos]

First of all, welcome to the forum!

"PP1V05_TBTLC other side 001"
Remove U3015 and check again; also test if board starts.

 

[sparkcoordination]

Thanks glad to be here!

U3015 removed but short still present. Still not starting...

 

[2informaticos]

Probably bad U2800, removed it.

BTW, I'm looking at 3482 schematic.
I hope the mentioned chips have the same position into 3481 board.

 

[sparkcoordination]

U2800 removed and the short is gone which is good.
I am looking at 3479 schematic.... same apart from gpu area i believe and same board outline.

Still same symptoms. Do i need U2800 to get power good? Am scanning the board for anything visible...

 

[2informaticos]

"I am looking at 3479 schematic"
Do you mean 3478?

So the machine was working untill changed the SSD?

 

[sparkcoordination]

Sorry yes i meant the 3478 schematic. Yes it booted with the original blade ssd in it and then i swapped it for the m2 (now in another imac), i got the led2 flickering and swapped back to the original blade and still the same... Have been scanning the board for missing or damaged components but nothing found yet...

 

[2informaticos]

If U2800 was burnt, possible PCH is damaged too.