Retina A1398 No Backlight

[ACS-Repair]

Hello. This is a continuation of another thread about an A1398, but is new to be cleaner.
I have an 2015 A1398 820-00138 that has no backlight. Everything else on the computer is good. Starts up, the display initializes and has the PP3v3 and PP5v0 going to it. There is no backlight voltage but if I force the SD on U7701, 12.59v passes to the display. I tried to pass 3.3v to the enable through a 10k resistor but the signal stayed low.

These are the voltages I have on U7701:

pin 5 (VDDA), 6 (SW2), 18 (VDDA) = 5.05
pins 9 (VSENSE_N), 10 (VSENSE_P) = 12.59
pin 17 = 0v (EN)

The lid switch is working and provides 3.3v normally, then 0v if a magnet is passed over it. The control for the backlight comes from the same place as the screen power so I find it strange that the screen gets power but no backlight. There must be something else that is required for backlight

Does anyone have any experience on these A1398 or know if U7701 is a common failure?

[mon2]

Re: Retina A1398 No Backlight

Measure & post the voltages on:

I2C_BKLT_SCL
I2C_BKLT_SDA

Also confirm that the LCD panel is being powered. Check the voltage of PP5VR3V3_SW_LCD.

[ACS-Repair]

Re: Retina A1398 No Backlight

Thanks mon2, I do have that voltage at the display panel.
I forgot to mention that if I connect an external display, the panel backlight comes on just as the login prompt appears. The internal display stays black as if it it is not active.
This may not go anywhere because I think this is a larger problem than the backlight itself. The external display is herky-jerky when both are connected. If I disconnect the internal display, movement is smooth. I don't know that this board is repairable.

[ACS-Repair]

Re: Retina A1398 No Backlight

I found a working 820-00138 on ebay for about the same price as a parts board so I decided to buy it. It's going to take about 3 weeks to arrive.
With this much time I decided that I was not going to give up on the old board just yet. I focused on the 8 channels of DP_INT_ML_F_N/P to see if was DP communication between the GPU and the display. I was fortunate to find that FL8303 was broken in half, with half of it missing. I removed the broken coil and replaced it with one from a parts board. Once I did this, the display and backlight came up as it should. So the answer to this was to get the DP communication working.

[Simplebeans]

Re: Retina A1398 No Backlight

I found a working 820-00138 on ebay for about the same price as a parts board so I decided to buy it. It's going to take about 3 weeks to arrive.
With this much time I decided that I was not going to give up on the old board just yet. I focused on the 8 channels of DP_INT_ML_F_N/P to see if was DP communication between the GPU and the display. I was fortunate to find that FL8303 was broken in half, with half of it missing. I removed the broken coil and replaced it with one from a parts board. Once I did this, the display and backlight came up as it should. So the answer to this was to get the DP communication working.

Nice find ACS-Repair, well done!!!