820-2936 – No Backlight

So we have a 2936 Motherboard infront us with no backlight on the screen.

Step 1:
Do we have image on the screen, with no backlight?
Or is it no image & no backlight?

  • Shine a light through the Apple logo on the rear of the computer to lighten up the LCD. See if you can see an image or not.

Step 2:
What voltage do you have on backlight output?
We know that different voltages on the backlight output will determine different things.

  • 0 volts will mean there’s a break in the circuit. Either it’s not being told to send backlight through, or maybe the fuse is blown.
  • 12v would mean that the line is simply not boosting. As this line begins with PPBUS_LCDBKLT which is a 12v rail.
  • 27v~ would mean the backlight should be working but the screen is likely the issue.
  • 48-50 v~ would mean that either the LVDS cable is broken, or the light in the screen is broken. It is recieving the backlight output, it is boosting, but there’s no load present.

In the particular test case we have here today, we have 11.3 volts. Which is likely PPBUS_G3H being passed through the circuit. Which is a slightly low reading but that’s either because the batteries in my multimeter haven’t been changed in a while, OR the MacBooks battery itself is low.

Now, for the LED driver (which is a DC to DC boost circuit) we need Backlight Enable (BKL_EN) to be present. In the particular case im working on now, this machines backlight enable is 2.78v. Therefore this chip is being told to turn on.

SO, let’s check BK_PWM
Wait..Ben how do you know to check PWM?
Well if you Google the datasheet for this particular chip (which im not going to do right now) but we know that PWM sets the brightness of the backlight. So the LED driver needs PWM present in order to know what voltage/brightness to set the backlight to.

In our case study machine today we have 3.095v. which is good. So let’s check to make sure the LED driver is actually receiving it’s supply voltage. We can check the V-IN line. Which should be 5v..and it is.

So our chip is being supplied it’s supply voltage, It’s being supplied backlight enable, and it’s being supplied with the brightness level. So it’s being supplied with all the correct things but it’s still not outputing what I want it to.
At this point, we can blame the chip – the LED driver.
So now I’ll replace this

And we have light !