Macbook Air A1466 Fluctuating 3v supply

[ronelex]

Hello
I got this unit with orange light but wont boot up
I notice that its L7560 3.3v supply is fluctuating
this is the reading I got on its PWM U7501 TPS51890A
pin 4- EN1 - 0v
pin 12 - EN - 3.3v Fluctuate
pin 21 - EN2 - 3.3v Fluctuate
pin 22 - vreg3 - 3.3v ok
pin 23 - vin - 8.62V
pin 29 - vreg5- 5v fluctuate

I already check the power rails line in page 62 but I didn't find any short on those lines. I already tried replacing the pwm but same issue.
I hope you can guide me where I can look for the fault of this unit

[mon2]

What is the ppbus_g3h voltage to ground? (Check at the fuse)

Is the 3v42 rail stable?

[ronelex]

ppbus_g3h- 8.62v
3v42 is stable
current stuck on 0.02A

[mon2]

What is the 820-**** board number for your unit ?

[mon2]

Remove all power. Meter in resistance mode.

What is the resistance to ground of PP3V3_S5_REG_R ? (L7560)

[ronelex]

820-0165 sir

What is the resistance to ground of PP3V3_S5_REG_R ? (L7560) - 400 ohms

[mon2]

400 ohms

That is too low of a resistance for this rail. This is likely causing the fault with the current sense loop observing a heavy load and power pulsing this rail.

With flux and patience, lift up one leg of L7560.

Then measure the resistance to ground again on the PCB pads of L7560.

Which side has this 400 ohm resistance to ground ?

Is it producer side (U7501) or the consumer side (L7560, pin # 2) ?

Do you have an adjustable power supply that can support 1 volt (NOT HIGHER)?

[ronelex]

consumer side pin 2
Yes i got a PSU Bench here and also thermal cam but it is in the shop im at home right now, only psu is available

[mon2]

Do wait for other comments but there is someone who is partially shorted on this consumer side of the rail.

Suggestion is to inject 1 volt (not higher) but ok to use higher current. The idea is to heat up the defective part and use your thermal camera to locate that part.

You want to keep the voltage as low as the lowest power rail in case a high side mosfet is leaking (ie. that may power the CPU rail). The CPU cannot handle a high voltage.

Hoping that the short is a shorted capacitor (most likely - the tantalums are the worst for this but have seen ceramic non-polarized caps as well with the same fault).

[ronelex]

ok sir thanks for your guide, I try to check all the power rails and I dont see any shorted circuit, I just try to intect this 3v line tomorror,

[mon2]

Tomorrow, also check the resistance to ground of vreg5 (pin 29) which you have reported is also fluctuating ?

[reformatt]

Take off the heatsink from the CPU and check the PCH portion of the die for damage. Also check resistance to ground of L7630. Seen these shattered and you can just be wasting your time.

[ronelex]

i tried to hang up the L7630 and the fluctuate stop, I will check now the CPU now

[ronelex]

Originally Posted by reformatt

Take off the heatsink from the CPU and check the PCH portion of the die for damage. Also check resistance to ground of L7630. Seen these shattered and you can just be wasting your time.

I open the heatsink and the cpu is looks good no crack on the top of it

[reformatt]

You need to get to stable PP3V42_G3H, PPBUS_G3H (8.6V no battery), and PP3V3_S5. Then troubleshoot from there with PM_SLP_S4.

[ronelex]

PM_SLP_S4 not generated,
i check the 11 signal
1. PPVRTC_G3H - 3.42v
2. RTC_RESET_L - 3.42v
3. PCH_SRTCRST_L - 3.42v
4. PCH_INTVRMEN - 3.3v
5. PCH_DSWVRMEN_L - 3.24v
6. SYSCLK_CLK32K_RTCX1
7. PP3V3_S5 - fluctuate
8. PM_DSW_PWRGD - 0
9. PP3V3_SUS - 0
10. PM_BATLOW_L - Fluctuate
11. PM_RSMRST_L - 0
+1. PM_PWRBTN_L - Fluctuate
I thinks my main issue is this PP3V3_S5

[reformatt]

PP3V3_S5 resistance to ground on a working board should be over 3K. You said earlier this was 400 ohms so something is not right there which is why I said you look at the PCH first. The supply should still be stable even with that low resistance.

Look for corrosion on U7501, especially near pins 9 through 16 as I often see liquid damage here.