MacBook Air Won't start up. Pulsing voltages

[ACS-Repair]

Hello All

I'm just starting to work on a MacBook Air 820-00165 that will not start up, I have the following measurements. Some voltages are steady while others are pulsing. There was a tiny bit of liquid spill on the board but I believe that I have cleared it up. There doesn't seem to be an obvious short on the board. The board is out and not connected to any of the normal hardware.

STEADY VOLTAGES:

PPBUS_G3H 8.59V
PPBUS_S5_HS_COMPUTING_ISNS 8.59V
PP3V3_S5 3.32V
PP3V3_SUS 3.32
PP3V42_G3H 3.41V
PPVRTC_G3H 3.32
PP5V_S5 5.03

PULSING VOLTAGES:

PP5V_S4
PP1V2_S3
PP1V05_S0
PP1V8_S3
PM_SLP_S4_L

COIL RESISTANCES

L7310 / L7320 11.5 (CPUVR)
L7630 173.2 (PP1V05)
L7430 570 (PPDDR_S3)
L7520 > 20K (PP5V_S4)
L7560 > 20K (PP3V3_S5)
L7130 > 20K (PPBUS_G3H)
L7095 > 20K (PP3V42_G3H)

Some of the liquid spill was on the PP3v3_SUS line at the EFI ROM chip. I'm not sure if I would have this issue if the ROM chip was not loading. It was more of a goo that cleaned up with water rather than corrosion and it cleaned up OK.
Compared to another board that I have the resistance measurements are quite similar. I'm curious as to whether anyone had a board with a similar issue?

[petehall347]

Re: MacBook Air Won't start up. Pulsing voltages

i have had boards that needed a good scrub with hot soapy water then isopropyl alcohol then a blow off with compressed air to get them properly cleaned up .

[piernov]

Re: MacBook Air Won't start up. Pulsing voltages

https://logi.wiki/index.php/820-00165_Pulsing

Pulsing every 2 seconds is typically SPI ROM communication issue.

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

Thank you piernov,
I was thinking that as well so I removed the SPI ROM and read it so I had a backup and the serial number (I bought the board without the computer). I have also checked all the SPI terminating resistors and they are all present and testing to spec. I'm assuming that the SPI ROM is still the issue because the symptoms are as the document shows, pulsing about every 2.5 seconds. I also assume that the ROM chip is OK because I was able to read it. That leaves only that the code may be corrupt. This 00165 is a 2017 model but I do have a 2015 with a ROM I could transplant to see if that corrects the problem. I have also removed J6100 because there was corrosion on one side of the connector, and even though it was cleaned, it is better off the board. I'll try this and comment back.

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

I've re-flashed the ROM and returned it to the board and the result has not changed. I accidentally placed the chip on upside down (how dumb was that), but then corrected it. I removed the chip and tested it again and the chip was still readable so I don't know that any damage was done to it or the board. I reprogrammed another ROM and placed that on the board to be sure and again, no change. I have measured the ROM pins 2 and 5 which are at 3.3v, and that matches the test board.
I do see that pin 1 of the ROM, the CE or enable pin, jumps to 1.24v on power up, then drops to zero. On a working board it floats around the same level, but then jumps to 2.3v, then to 3.3, which starts everything up. I'm continuing to explore inputs for U6101 to see if something there is the issue.

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

Unfortunately, constant voltage measurement on pin 1 of U6100 appears to have blown U6101. There is a very close ground plane near pin 1 and I have inadvertantly shorted the output after scratching the trace surface. There is no voltage at all on the output now. Looks like I need to get another one of those. If this doesn't work I may give up on it. It's a shame that is so close yet so far away.

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

I've replaced U6101 and now have the output back. The output at pin 7, CS* shows a DC voltage for a couple of seconds but I'm unable to see any activity on my old 5mHz scope. The input pin 5 of U6101 has brief activity before staying at 3.3v. Pin 3 stays low and has no activity. I've tried an SPI ROM file that piernov provided on another thread, but no luck.

[guruji]

Re: MacBook Air Won't start up. Pulsing voltages

Hey
This is a common problem with air you need to check spi resistors specially going to master you will a open track check 16E and 43E resistors and connection to tracks.check pg45 spi debug conn schematics

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

Thank you guruji. I had tested all of the SPI resistors on that page but decided to do it again. All of the resistors test to spec. All of the traces look good as there is no corrosion in these areas. The only corrosion was on the JTAG and that has been removed. Pin 2 of the JTAG (PP3v42_G3H) appears burned and remains black. I hope this has not contributed to the problem but it is close to the SPI signal on pin 4. When you say that the SPI resistors on these boards are a common fault, what is you experience? Do the resistors come off, change value, etc? What have you seen?

[guruji]

Re: MacBook Air Won't start up. Pulsing voltages

The resistors have dry solder and pls again check every trace as this board has a very horrible design this doesn’t use nails like other board to layers that very important I have found always open trace to cpu from spi also you mentioned the jtag connector that can be a problem as the spi signal terminate there the terminator resistors
Pls look carefully best of luck

[guruji]

Re: MacBook Air Won't start up. Pulsing voltages

Isolate R6127 and check

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

Thanks for that. The resistor tests good out of circuit at 43 ohms. Continuing to look at this board with a fine tooth comb.

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

I did a further close-up inspection of the board and found a couple of corroded test that I hadn't seen before. one of them is the TP[1]SPI_MOSI just to the left of the EFI ROM. I hadn't noticed it before because the copper was completely gone living only the end of a VIA coming through. From what I can tell this is only a test point for traces on the other side and doesn't connect to anything in the area.
The other test point is the one above it, TP[1]SPI_CS0_L. It was just tarnished and I was able to scrape the pad clean. While I am able to see continuity between R6120(1) and the test point, I don't have continuity with R6110(2). Bad traces were suggested by guruji and this will be my next focus.

[ACS-Repair]

Re: MacBook Air Won't start up. Pulsing voltages

After running a jumper between R6120(1) and R6110(2) I now have a working board. This one was tricky to find but it came down to very mild corrosion. I want to thank all that responded to help relocate this problem. I hope this will help others.