Macbook Pro A1990 (820-01814) no backlight display

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

In diode mode testing, the RED lead, yes red meter probe to ground.

Black meter probe to point to test.

in diode mode ~545 on both testpoints J0200
and continuity on VLED J0803

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

If the EN of u8400 (backlight) IC is 3v3 then this is good. The logic is wanting to enable the backlight.

Confirm the voltage of each side of R8402 which is the resistor on the gate of the mosfet for the backlight.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

R8400 has 12.5V on all four pins

UPD

my bad, I thought you were talking about R8400
now I see that you are talking about R8402

UPD 2

R8402 have 11.8 V on 1 pin and 10.9 on 2 pin

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

No, I did update the post. It is correct as you have just posted.

Is the EN pin at 0v or 3v3 (earlier post)?

If 3v3 then backlight is enabled else disabled.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

EN is 0

my webcam and key backlight also do not work. maybe this info will help

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

Do you still see an image with a flashlight on the display?

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

yes

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

What is the voltage to ground for the 'S' pin on the mux (multiplexer)?

You can measure at the resistor pads.

Update - also check the voltage to ground of the i2C lines as per new attachment.

PS: It is very late for you there - best to do this when you are rested. Can continue this tomorrow. My excuse is that my wife is watching a movie in a dialogue that I am unable to understand and there are no sub-titles to follow along. She has hijacked my netflix viewing time.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

the 'S' pin is 1.7 V (I measured 2 pin R8505)

upd I saw your upd (xd), so I will do it, I will continue tomorrow, thanks again for wasting your time on my problem

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

What is the voltage of PP3V3_G3H_RTC_X (measure anywhere you can on the board). This rail powers the mux IC you just measured.

Adding to your checklist for tomorrow. See attached.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

PP3V3_G3H_RTC_X
R6935 and R6934 test 1 pin = 3.3 V

ps really time to sleep before i screw up something

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

hello mr mon2
I checked U5210, all pins are 0 V
and R5218 & R5219 is 0 V

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

See attached. Confirm the voltage to ground readings for each of these points of interest.

The I2C lines should NOT be @ 0v unless they are being pullled down due to some even OR the pull-up voltage rails are missing.

You are checking for the presence of the pull-up voltage rails.

I2C is an open-drain design. This means the logic IC can drive the pin LOW = 0v but not a logic HIGH. Instead, when it is a 'high', the trace is floating or high impedance. Respectively, the pull-up resistor(s) must be present AND they must be pulled to an active voltage like you see in the attached diagram.

If the voltages are missing, the I2C lines will be parked into 0v state and the interfaced logic will fail to operate (ie. webcam, backlight, etc.).

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

i just checked all the lines you mentioned, on all lines 0 v

the motherboard was connected, including the charging was connected, but the display cover was closed

upd I measured the voltage on the pins in the attachment

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

Having the display cover closed will cause the backlight to be off. Can you test with the display cover open?

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

my bad..
see attachment


upd maybe some previous measurements should also be done with the display cover open?

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

Much better readings. Can you test the I2C lines again with the display lid open? That is, now you are measuring the voltage to ground on the OTHER side of each resistor.

It is important that they are NOT @ 0v when the pull-up voltages are active.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

done

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

These are good readings. These set of I2C lines are ok.

Can you check the EN pin again on the backlight (with the display lid open) ? Do you see ~3v on the EN pin or is it still 0 volts.

If 0 volts, continue to check the mux pins..

See attached. Check the voltage to ground of these lines which are linked to the MUX (multiplexer) that is used to enable / disable the backlight.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

These are good readings. These set of I2C lines are ok.

Can you check the EN pin again on the backlight (with the display lid open) ? Do you see ~3v on the EN pin or is it still 0 volts.

If 0 volts, continue to check the mux pins..

See attached. Check the voltage to ground of these lines which are linked to the MUX (multiplexer) that is used to enable / disable the backlight.

EN at 17 pin U8400?
do you mean R8442 and C8442?
and can you tell me which mux you mean? can you give the boardview element number?

upd I saw which mux you mean, thanks)