820-02020 No backlight 0,8v on BKLT_EN_R

[hassie2001@gmail.com]

Hi,

I have no backlight on my screen, tested with 2 different known working screens. Logicboard is perfect clean no traces of liquid only on pin 17 en 18 of UP800. Got chip off and saw that pin 18 was corroded bad. Cleaned it and replaced UP800 with one from 820-01598 (according to PDF, same model) Chip should get 5v.

Diode mode readings are ok except for pin 17 and 4

Pin 17 (BKLT_EN_R) is 0.187 should be 0.374 and give 0,82v
Pin 4 (LCDBKLT_FET_DRV) is 0.422 should be 0.550 (According to PLDaniels database).

Could it be that UP800 has to be from another a2338 M1 donor. I used one from a a2159 intel based Mac?

And what else can I check..

[mon2]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Check the voltage to ground of IPD_LID_OPEN_1V8.

There is a hall effect sensor in the trackpad to sense if the lid is open or closed.

If the trackpad is connected, test without it.

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Thank you for the reply, I already checked IPD_LID_OPEN_1V8 As I read many of your posts on this forum! (great job by the way). it is 1.8V

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

board is out of housing, so no trackpad connected

https://photos.app.goo.gl/qvWHaGg9jyct71YH9

[mon2]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Remove carefully RP482.

Remove all power.

Meter in DIODE mode.

Red meter probe, yes red to GROUND.

Black meter probe to pin #17 (BKLT_EN_R) on UP800.

What is the reading ?

Black meter probe to BL_PWR_EN on UP800.

What is the reading ?

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

I guess you meant RP842.

Here are the Diode mode readings
RP842 pin 1 0.170 (BL_PWR_EN) I'm thinking something here is wrong...
RP842 pin 2 0.516 (BKLT_EN_R)

[mon2]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Yup, a resistor is a resistor ☺️

This seems to imply that the pin driving the enable pin is defective. Do wait for other comments but you can test to see if the driving side is going logic high or not without this 0 ohms series resistor.

That is, the led driver ic appears to be fine but driver side is with a low diode reading.

Are 100% sure the tested LCDs are fully compatible with this logic board? The backlight driver will not enable if the display is not detected.

Does the board boot ok with your OS and you can toggle the caps key led?

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Yup, a resistor is a resistor ☺️
This seems to imply that the pin driving the enable pin is defective. Do wait for other comments but you can test to see if the driving side is going logic high or not without this 0 ohms series resistor.

The enable signal (BL_PWR_EN) comes directly from u8100, does that mean that u8100 is failing? It produces 0.8v but not enough to logic high (1.2v-1.8v)? There was some corrosion between pin 18 and 17 of UP800 so maybe 5v went to pin 17 and directly to u8100... Can't I just boost the low signal u8100 is giving to 1.8v?

The board works perfectly on an external display even on internal perfect without backlight! So this is my only dilemma!

The tested screens are all a2338 Screens from previous repairs. Where working perfectly then. Can't test now, because I don't have another m1 board.

How can I be sure the screen is detected apart of lid sensor? It came in as "dropped damage" It has a small dent at the bottom case but no visual damage on board...

[mon2]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Besides this enable pin, there are:

BKLT_SCL
BKLT_SDA

lines that are used to dial up the backlight intensity.

Can you measure the voltage to ground of each of these lines ?

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

RP850 (BKLT_SCL) starts with 5.1v as soon as image apears goes to 4.9v, diode mode 0.491
RP851 (BKLT_SDA) starts with 5.1v as soon as image apears goes to 4.8v, diode mode 0.491

[mon2]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

These lines are not stuck. Good.

Measure the mosfet pins (source;gate;drain) - voltage to ground of QP800.

[mon2]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Based on the assumption that the logic board is able to detect this display, this enable pin should be a logic high. Locate a 3v3 rail and you can feed that voltage directly onto this enable pin to force the enable state. Do this with the 0 ohm resistor off the board as you will be driving the pin directly.

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Measure the mosfet pins (source;gate;drain) - voltage to ground of QP800.

Source 12v
Gate 11,9v
Drain 0v

No shorts

[mon2]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

This is a P-channel mosfet so the gate must be lower than the voltage being between source and drain.

The gate will be lower if the led backlight circuit creates the pull down on the gate resistor. Then gate will be lower from the created voltage divider.

To use the backlight IC, it must be enabled and the I2C must be working.

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Tried but no luck! What I noticed is that

“RP851 starts with 5.1v as soon as image apears goes to 4.8v” this is not 4.8v it stays 5.1v.

But the scl line still starts with 5.1v and goes to 4.8v. So as I understand there is no good communication…

What can I look at more?!?

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

Figured out something else! On the small lcd flexcable pin 24 is broken. On the connector on the logicboard I get a good diode measurement (0.507) but when I connect the small lcd flex cable I get a O/L on the other side... So the LCD doesn't get a connection!

Tried different Flex cables but when I use another one I don't get image. Is there a difference between the M1 small lcd flex cable and the once before the m1?

To bad I only have 1 M1 here, so can't check... Maybe I can try to repair the flex cable?!

Could this be the reason why BKLT_EN is low?

[radear]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

The signal BL_PWR_EN comes from u8100. Have you tryed another screen? reinstall bridgeOs by DFU?

If you take a look on schematic, the signal bl_pwr_en can have max 1,2v an min 0,4v.. so if you having 0,8 you be abble to trigger the enable..

[radear]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

[radear]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

i tried to upload an imag but i cant

[hassie2001@gmail.com]

Re: 820-02020 No backlight 0,8v on BKLT_EN_R

If you take a look on schematic, the signal bl_pwr_en can have max 1,2v an min 0,4v.. so if you having 0,8 you be abble to trigger the enable..

Don't think this is correct.
As per schematic, VIH=1.2V; so it comes from 1.8V powered logic.
0.82V is prohibited value, can't be correctly recognized.
0 logic is between 0-0.4V and 1 logic is 1.2-1.8V...