Macbook Pro A1990 (820-01814) no backlight display

[mon2]

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

Yes - pin 17 of U8400.

[XVLZ]

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

done

[mon2]

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

Ok - and what is the voltage to ground of EDP_BKLT_EN = pin 17 on U8400 ? (with the display lid open)

[XVLZ]

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

yes - pin 17 of u8400.

0

upd
the display cover is open, the motherboard is powered by type c charging and connected to the display

[mon2]

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

See attached. Remove all power. Meter in resistance mode.

Measure the resistance to ground of this point.

[XVLZ]

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

done

[mon2]

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

Good reading and matches the 47k pull-down resistor shown in the schematic.

Meter in DC volts mode (10 volts or higher is ok). Power on. Measure the voltage to ground of this same test point. We are checking if the voltage to enable the backlight is originating at this MUX IC or not? This MUX IC outputs the logic '1' or '0' near this local resistor = ball # 12 of the MUX IC (BGA package). However, we are unable to probe the ball directly so this local resistor should suffice.

What is the voltage to ground @ R9833, pin # 2 ? Measure with care since the power is ON.

[XVLZ]

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

@ R9833, pin # 2 = 3.33 V

[mon2]

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

Very good. See attached.

With care - still using the DC volts mode on your meter. Measure the voltage to ground of R8442, pin #1 = the one with the YELLOW DOT is pin # 1.

Do you also see +3v3 here ?

We will check the resistance of R8442 next.

[XVLZ]

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

done

R8442, pin #1 = 3.3 V
and 47~ 200k ohm resistence mode

[mon2]

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

Very good but confirm again that with power off, R8442 is ~47kohms ??

This should be ~0 ohms so if accurate, R8442 is clearly defective. This is a tiny resistor but with care, you can flux it and remove it. You can actually blob with a sharp point on the soldering iron to short the PCB pads. That is, apply a bit of solder on your soldering iron and just dab the 0201 pads on the PCB.

Just be careful to NOT short to the cap that is missing beside this resistor. The cap is going to ground so we do not want this 3v3 signal going to ground. It has to reach pin # 17 of the backlight IC.

PS: Before you power up after this R8442 bypass - post a pic for a quick review of the work.

PS #2 - after you blob it -> you can apply IPA (alcohol from Costco works well here) & Q-tip to clean up the flux so you can actually see the soldered pads once again.

[XVLZ]

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

yes I checked again, really 47 kohms (all power off)
resistance mode 200k

So the backlight malfunction can only be in this resistor?

do you have the part number for this resistor? so i can buy it and replace it(R8442)

upd

or I just need to remove it and connect directly pin 1 and pin 2 of R8442?

[mon2]

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

It is a very common 0 ohm 0201 smd size resistor. It is meant to be a 'jumper'.

Remove all power -> meter in resistance mode -> check the resistance to ground of R8442 but measure the OTHER side of this 0 ohm resistor = pin # 2. This pin # 2 will be the side going to the backlight IC.

We want to know the resistance to ground of pin # 2 on R8442.

Yes, this high resistance can impact the operation of the backlight.

From your earlier posts, when you apply power, pin # 2 of R8442 = 0 volts ?

Inspect this part of the PCB to be sure there are no solder faults.

In summary, if the resistance to ground of pin # 2 of R8442 is not low, then you can just apply a small solder blob to bypass R8442. It is the same effect.

You will have to do this with care since the part is very small and will require a steady hand.

or I just need to remove it and connect directly pin 1 and pin 2 of R8442?

Yes.

[XVLZ]

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

2 pin R8442 also 47 kohm

[mon2]

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

Ok - work to bypass this resistor. In the end, you should have 0 ohms when measuring the resistance of pin # 1 to pin # 2 of R8442.

[XVLZ]

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

then what is the role of this resistor? is it okay if we directly connect two pins? Could this affect the performance of the laptop in any way? Or is it better to replace it with a new one?

[XVLZ]

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

i'm confused, just now i decided to check pin 2 of R8442 again and got 3v...

[mon2]

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

If the resistance is not ~0 ohms, then this resistor is defective.

Apply flux and reflow with low air pressure. Perhaps not soldered or was moved from earlier work. Use low air so the part does not go flying. Then allow it to cool and check the resistance again across the pins of this part.

Then confirm that 3v3 is stable.

[XVLZ]

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

I was going to solder it tomorrow, I haven't touched it yet, I just did a back test of R8442 and got 3.3 V on both sides, I'm confused

upd

but resistance mode R8442 still 47 kohm at both pins, this is normal?

upd 2

I thought if R8442 has a resistance of 47 kOhm, then it should not show 3.3 V

[mon2]

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

I think that you may be measuring incorrectly.

When checking the resistance of R8442, apply one meter probe on pin #1 and other meter probe on pin #2 of R8442. This reading should be ~0 ohms to match the schematic.

The 47k resistance you are reading is from the pull down resistor that exists on the same signal line.