MacBook Air 820-3208 Not chiming, fan running on high.

gcain

New member
Hi Everyone,

Signed up because I have this 3208-B board I cannot work out...



One-wire circuit operates as expected (green then orange light all good).

[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Fan spins on high, never chimes or posts and of course, no backlight. IMHO, the CPU gets too hot quite quickly...[/FONT]
[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]C.S. not liquid damaged, but the board was really dirty and needed a really good clean. IMHO it was liquid damage from humidity.[/FONT]


On R7402; Pin 2 == 8.4v and R7402 Pin 1 == 340mV.

The resistance of R7402 == 91.3k and should be 90.9k, so it's right which makes me reluctant to replace it...
I changed R7402 just to test and get the same measurements.


[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Another interesting issue is that when I measure[/FONT] Probe 152, Pin 1 on U6990, it causes the fan to slow down to about half speed. Then it builds its way back up again.


Does anyone have any suggestions?
 

gcain

New member
Thanks, I will check it closer in the morning.
The board is very very clean though. There was no corrosion evident anywhere.
 

gcain

New member
Definitely can't see any corrosion or any missing components.

Can someone suggest some measurements to take so I can start tracing it please?


Just to reiterate;
R7402 is giving me 340mV on pin1 and 8.4v on pin 2

The fan slows to almost a stop then builds back up again if I measure Probe 152 near U6990.

C6994 gives 7.2v on pin 1, and 3.4v on pin 2.
 
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dukefawks

Administrator
No need to measure anything around U6990, you have a green light so 3V42 must be ok.
Do you actually have CPU Vcore on L7510? If yes continue with the next:
I have had a few of these boards with no apparent damage and they usually have PCH problems. Heat up the PCH area and see if it works again after, not reflow just 150C or so.
 

gcain

New member
Do you actually have CPU Vcore on L7510? If yes continue with the next:
I get steady 1.125v

I have had a few of these boards with no apparent damage and they usually have PCH problems. Heat up the PCH area and see if it works again after, not reflow just 150C or so.

Heating it didn't seem to make any difference.

When I plug in power the fan doesn't do the little start-stop-start-stop sequence the Airs usually do. It just spins for a while, then starts to speed up.

Thanks for your suggestions.
 

gcain

New member
Just getting back onto this board today;

Does this look right? pp1v5_s3rs0 1.v35
pp1v5_s0 1v06
I was expecting 1v5 on each of those lines.

Board was working, we changed the SSD, and bang, no boot.


pp1v5_s0 is giving me 1v5 now, but pp1v5_s3rs0 is still giving me 1.3
 
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gcain

New member
Q7801::pin1 == 1.35v
Q7801::pin3 == 5.89v (Dafaq? Meant to be 1.5 right?)

Q7801::pin6 == 1.35v

R7850::pin1 & 2 == 1.35v
 

gcain

New member
So as I understand it;

5v is going to U7801, when it get's EN, it sends the 5v through R7801 TO Q7801 where it gets turn into 1.3v (should be 1.5 though?) and then off down PP1V5_S3RS0_FET7.
It seems like U7901 is getting the current back through Pin6, this would be a feedback loop to make sure its the correct voltage maybe?
 

dukefawks

Administrator
I checked a working board and the output is really 1.5V. Gate voltage on Q7801 is 5.4V to turn it on. So you either have a dead Q7801 or a very heavy load on 1.5V_S3 pulling it down.
Check for low resistance to GND on 1.5V_S3RS0, if that is ok I suspect a dead Q7801.
 

gcain

New member
Out of interest, how would I find a heavy load?
(A document or YouTube clip if you know of one would do.)

I need to order a Q7801 so may as well keep learning while I wait for it to arrive.

Thanks.
 

gcain

New member
I checked a working board and the output is really 1.5V. Gate voltage on Q7801 is 5.4V to turn it on. So you either have a dead Q7801 or a very heavy load on 1.5V_S3 pulling it down.
Check for low resistance to GND on 1.5V_S3RS0, if that is ok I suspect a dead Q7801.


Resistance to ground from R7954 (PP1v5_S3RS0) is 270ohms. Is this right, I am not sure what I should be expecting.
From Probe 587 is 299ohms.
From R7850 is 295ohms.
 
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