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00840 Pulsing rails

2informaticos

2informaticos

Administrator
Staff member
You can compare ohm readings with good board.
Seems to be good values for CPU power rails.

Check diode mode on all other big coils and behind all Power FETs.
Just in case, try to start in SMC bypass.
 
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grmacs

Member
Well. damnit I must've messed something up. All those coils and areas looked good. SMC Bypass didn't work.

I checked solders around U7650 and reflowed it. Something smelled like it burned up while putting heat to board. Board showed same symptoms initially and was pulling 20V, with rails pulsing after this but when I went back for second check no longer boosts to 20V intake. NOw stuck at 5v. Makes no sense to me that something burned up. Is there something that would be temp sensitive near U7650? Or U7650 burned maybe? Thinking maybe this was the case and not seeing any shorts on PP3V3 G3H or around CD3215's, I replaced U7650. NO change. I hate the newer boards...
 
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grmacs

Member
Those are all 0V.
Maybe what I explained prior was not done well: the board is now only drawing 5V from charger. PPBUS G3H is no longer present, only PP3v3_G3H.

Diode check of PPBUS_g3H is .441V
 
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grmacs

Member
diode values of caps appear to be good at both U3100/3200 however i do not have 5V at C3201 but C3101 =5V.

Fuses are ok there.

I swapped out U3200 to see if any difference but no change and diode values of caps near U3200 still match U3100.
 
2informaticos

2informaticos

Administrator
Staff member
You cannot get 5V at C3101/201 at same time; depends where the charger is connected.

"Something smelled like it burned up while putting heat to board"
Check what happened then.
Measure diode mode to ground on all caps from that area and closer coils.
 
2informaticos

2informaticos

Administrator
Staff member
Something happened when you apllied heat on the board for sure; if the problem appeared just after that.
Some BGA chip could be affected by possible accumulated crap.
 
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grmacs

Member
I think this is first time i;ve had something random fry just from heat and not be visibly noticeable like a nearby component not meant for heat. Possible maybe the board could hold enough charge if there was an issue somewhere that would then short something out? It definitely smelled like something shorted, yet no power was being supplied to the board while I was reflowing U7650.

May have to put a fork in this one as I'm just not finding anything out of sorts. So strange that there are no discoverable shorts on the rails and PPBUS G3H seems good, CD3215 measure legit, yet power draw to 20V is non-existent.
 
G

grmacs

Member
Ended up pulling a board from A1708 with bad LCD and everything works and passes AHT but external display does not work. No corrosion on board, tried resetting smc and PRAM, Clean ME with Medusa. Nothing detected when plugging in external display to it while booting or after boot. Looking in settings nothing shows for external options when connected to external display.

Seems very unusual for otherwise fully working mac to have no external display with a board only has IG.

Suggestions?
 
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grmacs

Member
U2800 looks like a fun job to replace. haha...
Yes I was able to load OS from USB 3.0 adapter/USB installer.
I'm sorry, what chip on this board is the USB-C redriver? I tried searching a MPN you mentioned in another thread but it isn't showing in my schematic
 
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