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MacBook Air A2337 No backlight on display repair

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MacBook Air A2337 No backlight on display repair
Device MacBook Pro A2337
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 820-02016 Logic Board, The laptop turns on, LCD is displaying a picture but without backlight.

Symptoms

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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 820-02016 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.
  • Ensure you have continuity between pin 1 of RP831 (LCDBKLT_FB_XWR) 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).

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 A2337 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.
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