First Post- A1502 820-3476-A no power

[sangolossyfon]

Hi everyone, I am a high school student and i am running a MacBook repair business in my free time. I have only recently gotten started on some board level repairs and have a few boards that I haven't been able to figure out.

I have the boardview for this A1502 laptop and a schematic for another A1502. Bought it from a guy who said it had just been sitting for a few years but no liquid damage. Had orange light but no power.
It would initially continually try to spin up the fan when just the charger and fan connected, like little spins, but after cleaning the board with alcohol, it now just shows a yellow light and no fan spin.
NOTE- there was a tiny amount of blue corrosion on three components only- R7520, R7522, and the J6601 connector. I didn't see any major signs of liquid damage or damage to these components though. I cleaned it off when I cleaned the whole board with alcohol.

I measured the following residences and voltages (voltages with the laptop plugged in to MagSafe, resistance when unplugged) (grounds measured at magsafe charger ground)

PP3V42_G3H	3.4V	31K
PPBUS_G3H	12.56V	97K
PP5V_S5	Alternating 4.7 and 2.1V	47K
PP3V3_S5	Alternating 3 and 0.2V	413 ohms
PP5V_S4	Alternating 0 and 0.5V	0 ohms
PP3V3_S4	Alternating 0.3 and 0.6V	1.1K
PP5V_S3_LTUSB_A_F (Only PP5VS3 there was on boardview)	Slowly alternating 0.2 to 0.7V	0 ohms
PP3V3_S3	Alternaitng 0 to 0.04V	4.4K
PP5V_S0	0V	14 ohms
PP3V3_S0	0.14 to 0.3V	627 ohms

I don't have any professional equipment and at the moment i'm using a laser thermometer only. I tried injecting PP5V_S4 with 5V 0.6A but a lot of areas slowly got warm but nothing in particular over 100*F. I might have done the voltage injection wrong though.

One of my biggest questions is this- if both the S0 and S4 rails are shorted, could this be due to a short on only the S0 rail, for example, due to it coming after the S4 rail? If I'm injecting into S4, would a short on S0 make S4 seem shorted, or vice versa? I guess, in theory, could a laptop have a shorted S4 but a not-shorted S0? Because I have both.

thank you are any help. looking forward to being a part of this forum.

 

[2informaticos]

First of all, welcome to the forum!

If you have a pure short (0 ohm) never inject 5V; even the power rail supports such voltage.
1-2V should generate 5-10A anyway; depends of the exact ohm value of the short.
Enough to find hot component with the fingers; 5-10W cause lot of heat.

Limiting PSU current to 0.6A is an error.
It will get in protection mode and doesn't inject anything.
Always set amp limit to max, before injecting.
At least 10A capable PSU is needed; recommended 20A.

The power sequence looks like G3H -> S5 -> S4 -> S3 -> S0.
Short on different power states can imply short between corresponding power rails too, but not always.
A component powered buy different rails can cause such problem.

As a recommendation, always wait 10-15s once power is removed, before checking in diode mode, or ohm scale.
This way avoid any error caused by remanent voltages.

Now post the exact ohm value of mentioned shorts found on the board; if still exist.

Do NOT inject voltage at this moment!
No need to apply power, until eliminate any short from the board.

 

[sangolossyfon]

i actually measured all the resistance values before injecting voltage so they are accurate.

correct me if I am wrong, but from the chart I made, 14 ohms of resistance on PP5vS0 is basically the same as 0 ohms on Pp5vs4... i would say anything under 20-30 ohms is a "pure short."

what is the lowest value of resistance that would not be worrisome on any power rail? is the 627 ohms on pp3v3S0 a cause for concern? I am hoping you can see the chart I posted above. thanks.

 

[2informaticos]

Power rails must be checked in diode mode.
Ohm scale is used for low readings; to confirm a real short.

HOW TO collection

I hope this "How to" collection will help our members. I will possibly modify this post in time, if updates available. Trigger SMC_ONOFF_L (PMU_ONOFF_L): It reffers to simulate power button function, when the keyboard is not connected. We need to do this when we work with the board...

boards.rossmanngroup.com

Same link mentions voltage injection method.
You can try injecting into 5V_S4 first.
Start with 1V and go up if needed.
Pure short can be discovered with less than 2V injected there.
Do not go over 6V, under any reason.
Don't forget about amp limit!

Once you've discovered the culprit on 5V_S4, do the same on 5V_S0.

"what is the lowest value of resistance that would not be worrisome on any power rail?"
That is difficult to say; CPU coils have low readings as normal.