820-00165 ALL POWER RAILS, BUT NO CPU ACTIVITY.

Yalnif

Member
Hi guys. I have a 2017 Macbook Air 820-00165 that came in with very minor liquid damage around the backlight driver. I checked the rest of the board very carefully as a matter of routine anyway, Otherwise it worked from what I could tell- image was there, backlight was not. Replaced backlight driver (I have done many and yes, it is on the correct way around and yes, I used a new driver). Now board turns on,, fan spins, no chime, runs at around 350 to 500ma (varies), shuts off once, restarts then runs (still no chime), ends up around 550 to 600ma., no image or backlight. No activity at USB ports.

Checked my work obviously, re-did it anyway just to be sure. No actual damage there anyway, just the backlight chip was dead.

ALL power rails are fine as in I get all the power rails. All-Sys_PWRGD is there also. Machine runs, fan spins, just no actual CPU activity. CPU gets warm/hot as in normal temp.

ANY pointers would be very appreciated.
 

2informaticos

Administrator
Staff member
First of all, welcome to the forum!

I suppose you've did something wrong when changed the U7701 for second time.
Never change a component so fast, whitout checking signals and voltages first.

As you've mentioned corrosion in that area, I can suppose XW7720 was broken; did you check it?
Did you get 0V, 8.5V or 45V on backlight output, when no backlight present?

After heat applied on the board, possibly another component reacted and now the board doesn't even boot (just stays on); sometime happens, after liquid damage.
PLT_RESET_L stays high?
Check all SPI Bus Series Termination resistors and traces.
Clean/remove J6100.
 

Yalnif

Member
Thanks for your reply.

Edit - sorry, didn't check voltage before changing. Quite used to changing them with great results. Voltage currently was 0v. My mistake.

Checking XW7720 is going to need some guidance as it is not visible. I am guessing it is sandwiched in the layers?

I am not at my workbench now. I will check first thing tomorrow. PLT-Reset will be my first check. I know a bit about these boards, but am fairly new to Macs so bear with me if I don't get just how they should power on as in sequence. I will guess that PLT is to dor with the PCH, so makes sense to look into that.

Again, by SPI terminations, I presume you mean the resistors around the U6100? Or am I just guessing wrong there? I did remove, clean and refit the U6100 already as I thought the erorr may be the bios, but again, I need guidance here and appreciate your help very much.
 
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2informaticos

Administrator
Staff member
Yes, around U6100, but not only; some of them are placed under PCH and other on opposite side, near J6100.
And I said clean/remove J6100, not U6100.

With a bit of patience, you should get some pictures from these links.

BTW, I was asking about backlight voltage before second change of U7701.
 

Yalnif

Member
Many thanks again. Ah, sorry. Didn't check voltage after first change. I have done probably 20 of the backlight drivers, so got used to how they usually work. On the rare occasion one has not, it has been a bad new driver (once before). So I just assumed it was one of those occasions. Obviously there is more wrong and needs a good check now. I will start in the morning. Much appreciated your kind helps so far.
 

Yalnif

Member
OK. New day.

I checked all SPI termination resisters and every one is correct.

I checked the J6100 (apologies, I was tired yesterday) and it too is spotless and new looking/fine. I have not removed it as I want to program through it later and have not yet mastered the art of what temperature will remove one without melting the plastic. Maybe you could advise?

You were correct with PLT_RESET_L . It is 3.3v. This is where these things can play with your mind. Correct me if I am wrong, but the fact it is L means it should be 0v (as in grounded). It is currently not and thus 3.3v is incorrect?

The resister R2071 is 65ohm (in circuit) which should be 0ohm. I have replaced that with a new 0ohm resister and get the following at U2071:
Pin 1 = 3.3v
Pin 2 = 3.3v
Pin 3 = 0v
Pin 4 = 3.3v
Pin 5 = 3.3v

You have helped a lot so far. Any ideas from here? I guess PLT_RESET_L should be 0v, so my guess is I need to find out what SHOULD be sending that to ground. But please tell em if that is correct?
 

Yalnif

Member
Just doing some more checks. I maybe in the wrong train of thought, but I checked Q1920 and I am confused. Is this SUPPOSED to short PP5v_S0 to ground under normal running, or is that possoibly the problem? There is wording under ''SPI_DESCRIPTOR_OVERRIDE_L'' in the schematic that frankly is just confusing me:

PCH uses HDA_SDO as a power-up strap. If low, ME functions normally.
If high, ME is disabled. This allows for full re-flashing of SPI ROM.

Is this supposed to short to ground or is that when it is telling the PCH not to run? When it says ''if high'' when what is high exactly?

Voltages:
R1920 pin 1 = 5.16v
R1920 pin 2 = 0.3v

Q1920 pin 1 = 0.2v
Q1920 pin 2 = 3.419v
Q1920 pin 3 = 1.509v
Q1920 pin 4 = 0.5v
Q1920 pin 5 = 0.3v
Q1920 pin 6 = 0.3v
 

2informaticos

Administrator
Staff member
"I guess PLT_RESET_L should be 0v"
Guess why NOT?
I never saw a machine working with active reset signal.

As you get PLT_RESET_L high, power sequence is complete and there must be a data communication problem.
Inspect the board for knocked of components, or moved in wrong place when you've worked on backlight area.

There may be accumulated crap under some BGA chip.
You can try known good working BIOS, just in case,
 

Yalnif

Member
Thanks for clearing that up for me. Makes sense that it is only 0v for the actual rest of course. But sometimes it is better to ask and then know.

The BIOS on this WAS good as in the machine did run - just the backlight that did not work and also the charger light. That light has come on, then not come on with this DC-in board and others, so needs a bit more looking at, but would not stop the machine running I would'nt have thought. It is currently coming on green then amber. If I have another identical BIOS I will swap it, but I think the others I have are from 820-3437 boards.

One thing to ask as I cannot remember. I know that 3437 boards do the 3 starts. Unless I am wrong, 00165 just starts doesn't it? This one starts, runs a bit then shuts down and restarts but only the once. By that I mean it does that every time I apply power, but just the singe restart and not 2 restarts like a 3437.

Is there a way to tell for certain if the PCH is running? The CPU (as in the heat sink) gets hot (normal hot, not hot-hot). It is as if the lights are on but nobody is home with this board.
 

2informaticos

Administrator
Staff member
PCH works at least partially; detects power on sequence final and sends out PLT_RESET_L.

00165 board starts directly, without any power cycle.
Your board got image before, but try new BIOS, just for test.
BIOS corruption has been reported for corroded boards; you must discard that option.
Do a search on the forum, you cannot imagine how many BIOS links we have...
 

Yalnif

Member
Thanks again for your kind help.

I don't have a spare known good SPI chip, but have ordered one and will update in a few days when it arrives.
 

2informaticos

Administrator
Staff member
You've said want to keep J6100 in place for possible SPI programming.
What hapen, don't you want to use it anymore now?
I was supposing you have a proper programmer.
If not, remove the chip and use USB programmer.

Why did you buy new SPI chip, instead of reprogramming original?
Don't forget to make a backup of original BIOS first.
 

Yalnif

Member
J6100 is now removed as I finished reprogramming original SPI rom (yes I have a back up first).

My programmer connects to J tag, so is easier than removing SPI rom. But I can do both no problem as I do indeed have a proper programmer - 3 in fact..

Ordered new SPI chip because old chip does not seem to work properly even when reprogrammed with flash from identical board. Exactly same situation.

Really scratching my head with this. No idea really what could be wrong, so a new chip and if that doesn't work it will be taken back off and kept as another spare.
 

2informaticos

Administrator
Staff member
I mentioned that we have working BIOSes for this model, clean ME.
Just search the forum.

When you want to discard something for sure, you MUST try with known good!!!
A chip taken from other board, even in working condition, doesn't have proper ME region for the actual board.
 

Yalnif

Member
Thanks. I did not know about the BIOIS'es on the forum. That will be very useful.

The Bios'es I buy are brand new and ready programmed for this exact serial number. I have used the supplier many times and it just makes things much simpler for me. They are cheap, will be here tomorrow or Wednesday and I will fit. I have my doubts as to the BIOS being the problem, but it won't hurt to change it as it is only a 30 second job and then at least we will know. Something about this board is just nagging me. I am very grateful for your great help thus far.
 

2informaticos

Administrator
Staff member
I still think something was affected by heat.
You should heat SMC and CPU with quality flux.
No need to melt the balls, just burn eventually accumulated crap.
Check diode mode to ground at BKL_SCL/SDA lines.
 

Yalnif

Member
Just an update. I cleaned the SMC are and the CPU/PCH area with flux (Amtech 559 of course) and heat. Not enough to melt the solder, just enough to cook any crap away.

Exactly the same.

But this is what I find interesting. The start sequence for this board is:
1. Draws about 26ma for about 5 seconds, then
2. Draws about 350ma for about 12 seconds restarts itself, then
3 Draws 350ma for about 15 seconds then
4. Jumps to about 600ma and stays around that. Which I would call normall running current for these with a screen attached. But this is all on the bench.

It will sit at about 600ma all day, fan spinning, no USB activity at all (Chipmunk). Just power and 5v on the USB.

Backlight circuit tests fine. Ironically the original problem seems fixed but I will only know for sure when it powers on correctly of course.

Mmmmm.......
 
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2informaticos

Administrator
Staff member
PCH is directly connected to U7701; 0 ohm resistor is not a real protection.
There is a (very) small chance to damage PCH because of U7701 failure; in case of overvoltage returned.

Near the backlight area is the Thunderbolt circuitry, which can block PCH somehow, becuase of liquid damage.

These are not calssic failures, but we have to think in any possibility, if you are sure nothing else got damaged working on backlight area.
 

Yalnif

Member
Thanks again. I think I am heading quite quickly towards the donor board pile with this one. I have a good screen, good chassis and a decent hard drive. So I have my money there already and some. Lots of useful parts on this board of course too, so good to scrap if needed. But would be a shame.

The original situation was: no backlight AND no charger light. pp3v42 circuit was fine and dc board was changed to double check with known good ones. Same result though.

BUT, and this is probably most significant, I do not know what else did not work. I only know what did work, ie machine chimed, turned on, had no backlight and had no charger light. Let us asume the thunderbolt was also not working, maybe that has triggered as you say a problem when heat has been applied for U7701 replacement and that might have triggered something else.

I feel close, but also a long way from solving this board and sometimes it is better to keep going, others to call it a day and just fix something fixable.
 
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