MacBook Pro A2338 No backlight on display repair

MacBook Pro A2338 No backlight on display repair
Device	MacBook Pro A2338
Affects part(s)	display board, main board
Needs equipment	multimeter, soldering iron, soldering station
Difficulty	◉◉◉◌ Hard
Type	Soldering, Part replacement

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Problem description

No backlight on display on the 820-02020 Logic Board, The laptop turns on, LCD is displaying a picture but without backlight.

Symptoms

Detail all measurable or observable symptoms in this section.

No Backlight
All voltage rails are normal
Possible liquid damage/corrosion

Solution

Diagnostic Steps

Failed display TCON board or a failed display LED strip

Check for a short to ground on backlight output (PPVOUT_LCDBKLT)
- CP860 or adjacent capacitors in the series is an appropriate area to measure, since it does not require removal of the system board.
- If a short to ground is measured, unplug the display connector to rule out a potential short to ground within the TCON board or LED strip of the display.

Short to ground on backlight output caused by a failed output decoupling capacitor (Most common)

Check for a short on the backlight output while the screen is unplugged
- CP860 or adjacent capacitors in the series is an appropriate area to measure, since it does not require removal of the system board.
If the short is persistent with the screen unplugged, proceed to the "Backlight output shorted to ground" repair steps below.

Failed backlight driver (UP800)

UP800 is in a prime spot to be hit with liquid on the 820-02020 and is very commonly seen to be corroded after history of liquid damage.
- Failure of UP800 without the presence of liquid damage is uncommon, however if you have no backlight with a known good display, and do not have a short on backlight output, UP800 may be bad or you may have another circuit fault such as a bad resistor or broken trace within the UP800/backlight generation circuit.
- If UP800 is corroded, replace it along with any other corroded resistors or capacitors adjacent to it.
It is common for current sensing pins (pins 9 and 10) of UP800 to be very corroded with corresponding pad/trace damage.
- gently scrape the coating off of the traces going to pins 9 and 10 of UP800 and run jumper wires from the traces to pins 9 and 10 of UP800.
- 0.02mm jumper wire is recommended.
Ensure that the backlight circuit has its input voltage (VIN) by measuring on BOTH SIDES of FP800, which is the backlight fuse.
- Voltage on both sides of the fuse should be equivalent to PPBUS_AON.
- If the backlight fuse is blown, check for a short on backlight output (PPVOUT_LCDBKLT) and on backlight input (PPVIN_LCDBKLT_F).
- If the fuse is blown without a short, proceed to the "Backlight fuse (FP800) blown in the absence of a short to ground" repair steps below.
Check that the backlight circuit is being enabled, the screen will need to be connected for the enable signal to be sent out.
- measure voltage on RP842. BKLT_EN_R / BL_PWR_EN should measure around 1.8v while a GOOD known display is connected.
  - If backlight EN is missing, be sure that you do not have a parts issue with your display or logic board to display cable.
  - BKLT_EN missing on this board due to a board issue is very rare, but may be cause by a failed PMIC (U8100) or failed iGPU in the M1 SoC itself.
  - If BL_PWR_EN is shorted due to U8100. Remove resistor RP842 and run a jumper wire from BKLT_EN_R to Pin 5 on UP900. All screen functions are working and you don't have to replace U8100 only because the BL_PWR_EN line was shorted.
Ensure you have continuity between pin 2 of RP831 (LCDBKLT_FB) and PPVOUT_LCDBKLT.
- Although rare on this board, it is worth checking to see if the backlight feedback trace is blown. Without feedback, the backlight driver will not produce its output.
Check the 5v input to UP800 (PP5v_BKLT_A and PP5v_BKLT_D).
- RP845 or RP844 may blow if UP800 fails. Ensure you have 5v on both of the above listed lines. If low, replace the blown resistor(s) and UP800.
If you have VIN (PPVIN_LCDBKLT_F), Enable (BKLT_EN), and both 5v lines (PP5v_BKLT_A and PP5v_BKLT_D), and no backlight output with the absence of a short to ground, replace the backlight driver (UP800).

NOTE: Some 820-02020s may present with unresolvable no backlight after liquid damage, even after replacement of all corroded components. These devices will typically have no short on backlight output, have normal backlight input voltage, enable and normal PP5v_BKLT A and D voltages. These devices will also have normal diode mode readings pin by pin on UP800 when compared to known good. At this time there is no known solution for this problem. When a solution becomes available, this section will be updated.

Repair Steps

Backlight output shorted to ground

Find and replace shorted component How to find short circuits
- Ensure the short is not caused by the display. Be sure to measure with and without the display connected.
- Most commonly, a short on backlight output is caused by a shorted capacitor.

Device has backlight output on a known good display, but not on the originally installed display

Replace the display assembly. Use only a display for a A2338 MacBook Pro. Earlier Intel MacBook displays will not work.

Backlight fuse (FP800) blown in the absence of a short to ground

Replace the backlight fuse with a fuse from a donor board or a compatible replacement.
- 0603 package size, 3 amp, 32v fuse. Brand does not matter.
Before applying power be sure to double check for a short to ground. Check for a short to ground on backlight output and backlight input.

PP5v_BKLT_A or PP5v_BKLT_D absent or low

Ensure PP5v_S2 is present to both resistors prior to replacing any components
- If PP5v_BKLT_A is absent or low, replace RP845 and UP800.
- If PP5v_BKLT_D is absent or low, replace RP844 and UP800.