820-3330-B : LCD_PWR_EN at 0.011V, Q9706 output at 0.1V

[stef4n]

Good evening,

I have a MBP 2012 with 820-3330-B logic board without Backlight but with image; if I apply some light from the back of the lid, I can distinguish a "question mark folder".

The LVDS J9000 connector had obviously been damaged; pins 38, 39 and 40 burnt.
Connector now removed. 0V on pads 38/39/40
Then, 0V on both side of L9701
PPBUS_SW_BKL is at 0V but F9700 is not burnt: 12.68V on both side of the fuse

Q9706 is weird ;it's a P-Type Mosfet, but I have 0.1V on pins 5 and pins 6. Regarding pin 2, it starts at 7V (when I start measuring it with the multimeter), but it relatively quickly drops to 4->3.2->2.5->2->1.6->1.2->0.9->0.7->0.6V->....
Referring to the specifications, pin 3 is the gate and pin 4 the source. If V-Gate <V-Source, then P-Channel is open, current flows.
On pin 3 I have 12.32V (G)
On pin 4 I have 12.68V (S)

I would have expected 12.68V on pins 2,5,6, not 0.1V


The circuitryR9788->R9789->Q9707 acts like a voltage divider if Q9707 fully opened, which seems to be the case here.
Whatever LCD_BKLT_EN and BKLT_PLT_RST_L are, the source being 0, it will always be opened, unless something burns in this circuitry (am I wrong there ?)



Regarding J9000, I do have 3.3V on pins 4 and 6; and I measure 0.01V on pins 1, 2, 3, is it because the LCD is not plugged in ?

On U9000, I do have 3.3V on pins 2 and 3. However, pin 0 shows 0.011V
I measured the same value on pin 1 of R9678, pin 1 of R9094 and TP[1]LCD_PWR_EN
I measured voltage on pin 2 of R9678, there is 2.69V. The resistance itself is at 4.4kOhm. So far it looks normal.

Howeverm U9600 should give something on "pin 112" for LCD_PWR_EN ("PB19B" on the schematics"). I wonder if the missing PPBUS_SW_LCDBKLT_PWR (0.1V instead of12.68) is the cause.
Also, LCD_BKLT_EN is at 0V, which is an input for Q9707

PP3V3_S0_GMUX_ULC_VCCPLL is at 3.37V
PP3V3_S0_GMUX_LRC_VCCPLL is at 3.37V
But it's probaby useless to investigate in this direction.

Shall I remove Q9706 ?

KR to the readers

S.

 

[2informaticos]

"Connector now removed. 0V on pads 38/39/40"
Why do you expect backlight voltage, if no LCD connected?

 

[stef4n]

woops...
Soldered a new LVDS connector (for A1286 from 2009 to 2011, since display assembly can be swapped I take it for granted that connector are functionnaly compatible, despite small physical differences)
Plugged in a 2011 screen.

I now get backlight (32.3V on pins 38/39/40). However, I do not have image anymore (no "?" folder on my white screen).
I do have 0V on both side of L9000, which led me to U9000

I measured 3.3V on pins 1/2/3 of U9000, but 0V on pins 4/5
Resistane on pins 4 of U9000 is 0.
Furthermore, if I apply isopropanol on this U9000, it instantenously turns into smoke. Seems clear that this U9000 is shorted to ground.

 

[2informaticos]

Pin 6 is ground, normal to have short.
However, if pin 4 (and 5) shows short to ground and only the chip overheats, then you've found the culprit (U9000).
When have doubts, in case like this, just remove the chip and see if short disappeared.

Missing 5V output also explains why you get pure white screen, not classic Apple gray; TFT matrix is not polarized.

 

[stef4n]

Yes, editted my post in the meantime regardin pin6;
Besides, since I had an image initially but no backlight, I checked my solder joints.
It appears I made a bridge between pin 1 and 2/3. I'm therefore sending the3V3 to gound....working on it.
Will have a look after on what you mean by missing 5V output.

 

[stef4n]

Bridge removed ; I now have 3.3V on pin 1/2/3/4/5 of U900, 3.3V on both side of L9000
Now the screen is white-grey, let's say grey.
However when I start measuring voltage on pins 4,5...of LVDS connectore, the screen suddenly but occasionnaly turns pure white. Redoing my soldering on these pins, one must be poorly soldered.

 

[2informaticos]

"Will have a look after on what you mean by missing 5V output."
My error, I was looking at 820-3332 schematic.

Do you get image now?

 

[stef4n]

I removed the LVDS connector, cleaned the area and soldered a new one again, but it didn't change anything at first and I had no image.

When i turned the logic board on, with a 2011 LCD assembly plugged, fans started spinning full speed, then stabilized at low speed after 20 sec.

I had 3.3V on pins 2&3; 32.3V on pins 38/39/40, pins 31 to 36 reads between 0.5V and 0.8V..
Pin 37, assumed "not connected" reads 15V
U9000 was normal; 3.3V on pins 1/2/3/4/5

However, screen was still grey.

I turned it off, put back the logic board in its case, plugged the original 15" 2012 in the connector ; poor connexion, i had a blue band onthe left side and 50% of the screen looking like a "grid".Unplugged this screen.

I plugged a 17" 2011 display assembly instead. Got no image, greyish screen forever. The laptop did boot, though. I heard the chime, etc.
I unplugged the 17" 2011, turned it off.

I plugged an external monitor (displayPort). I got a question mark folder. Unplugged this monitor. Unplugged the charger, and took out the Logic board; back on the desk.

I plugged a 17" 2011 screen. This time, question mark. No clue what happened in the meantime.
I unplugged the 17" 2011, put back a 15" 2011. Question mark folder.

So...it looks like it's working. But i do not know why the screen stayed greyish until i tried to plug an external monitor to see what would happen.