A2179 2020 MacBook Air

•The most likely cause for a 820-01958 to be stuck on 5 V and low current draw (≈0.00–0.02 A) on the USB-C current meter is a short to ground on PPBUS_G3H.

•Always check if both ports read the same. One port reading 5 V ≈0.12–0.25 A while the other reads 5 V ≈5 V 0.02 A is more indicative of a CD3217 issue, while both ports reading 5 V ≈0.00–0.02 A is more indicative of a PPBUS_G3H issue or T2 corruption.

Differential Diagnosis for 5 V and low current draw as indicated on the USB-C current meter

•Absent PPBUS_G3H voltage due to a short or creation problem.

•Device stuck in DFU mode due to corrupt T2/BridgeOs firmware.

•Absent or shorted PP3v3_G3H_RTC voltage. Failed CD3217 (U3100 or U3200)

•Short to ground on any of the power rails generated by U7800 that power the T2 chip, such as PP1v8_SLPS2R or PP1v1_SLPS2R.

•Non-specific areas of corrosion causing T2 or CD3217 communication issues.

➤Diagnostic Steps

1. Check if the device is stuck in DFU or Recovery mode.

- Plug the MacBook into another Mac or MacBook via its master port (Left side, bottom port, closest to the trackpad) and open Apple Configurator 2 to verify that it is or isn't in DFU mode. If the device is in DFU mode, Proceed to "Device stuck in DFU mode due to corrupt T2 firmware" in repair steps below.

2. Check voltage on PPBUS_G3H.

To check voltage on PPBUS_G3H, place your multimeter in DC voltage mode, and place the black probe on ground, and the red probe on PPBUS_G3H. A good spot to measure from on the 820-01958 is C6482.

≈12.60–12.65 V = Normal

≈12.28–12.35 V = T2 communication issue with the ISL9240. The likely cause for this is the device is stuck in DFU mode due to corrupt T2 firmware. Proceed to repair steps below for corrupt T2 firmware.

≈0 V = Short to ground or absent voltage due to a creation problem. Proceed to repair steps below for shorted or absent PPBUS_G3H voltage.

3. Check PP3v3_G3H_RTC Voltage.

- Put your multimeter in DC voltage mode. Place the black probe on ground and the red probe on PP3v3_G3H_RTC. It is recommended to measure PP3v3_G3H_RTC on a capacitor or resistor near one of the CD3217s to rule out a trace/via issue. C3100 or C3200 are both good places to measure this line.

≈3.3 V (3.290–3.425 V) = Normal.

Values below the above spec should be considered abnormal. Proceed "PP3v3_G3H_RTC low or absent" in the repair steps below.

➤Repair Steps

Device stuck in DFU mode due to corrupt T2 firmware:

- Revive T2 firmware via Apple Configurator 2.

NOTE: You must be running the latest version of MacOs for this to work consistently. Check for MacOs updates prior to reviving/restoring T2 firmware.

How to revive T2 firmware: Plug the device you are working on to another Mac or MacBook via its master port. The master port on the A2179 is the bottom left side USB-C port (closest to the trackpad.) If you are confused, please see the diagram listed on Apple's how to page — [1]

Once plugged in, open Apple Configurator 2. You should see a big square icon pop up that says "DFU" or rarely, "RECOVERY". Click the icon, Navigate to the top menu bar click "Actions" then "Advanced". Select Revive device. You will see a progress bar appear. This process can take anywhere from 2 minutes to over 30 minutes in some cases.

SELECTING RESTORE WILL WIPE ALL USER DATA!

PPBUS_G3H Short to ground:

Inject around 1 V with a 5 amp limit into PPBUS_G3H with a DC power supply. A larger tantalum capacitor is preferred to inject voltage to, as you will often need a lot of current to be pushed into the line to cause the shorted component to heat up. 1 V is usually sufficient. It is not recommended to inject over 1 V as if one of the CPU VRMs is shorted, you will end up killing the CPU.

With voltage being injected, perform thermal imaging of the board. If thermal imaging is not available, feel around the board to see where it is getting warm. Once the area is localized, add a small amount of isopropyl alcohol to the area to localize the shorted component.

Once the shorted component is localized, replace the shorted component.

For more info on finding short circuits, please see - [2]

PPBUS_G3H Absent or low with no short:

PPBUS_G3H is created by the Intersil/Renesas ISL9240 (U7000) which is a buck or boost converter depending on its input voltage. If USB-C voltage is 5 V, the ISL9240 will boost the 5 V input to ≈12 V. If USB-C voltage is 20 V, the ISL9240 will buck (lower) the 20 V input to ≈12 V.

•When PPBUS_G3H is absent with no short to ground, we need to first make sure that PPDCIN_G3H is making it to the ISL9240 (U7000). C7024 or C7026 is a good place to measure from. Measure PPDCIN_G3H with your multimeter in voltage mode. You should get the same voltage that you are getting on the USB-C amp meter (5 V in this case, because that is what we are troubleshooting.) If you get 0v on PPDCIN_G3H, check for a short, and if no short is found, move down to "CD3217 troubleshooting" in the repair steps below.

•If you are getting 5 V on PPDCIN_G3H, the ISL9240 is likely the cause of the problem, however we still need to check a few things before replacing the chip. The ISL9240 relies on a series of current sensing resistors to measure the amount of power being used by its output rail, PPBUS_G3H. If one of these resistors is blown, the chip will think that something on the output is pulling too much power, and it will disable itself as a protection measure.

To check the current sensing resistors, put your multimeter on resistance mode. Check each resistor listed below by placing your probes on each side of the resistors. The values should be within 5–10% of the reference listed below. Values slightly outside of the reference should be considered insignificant and ignored. If you find a resistor out of spec, replace the resistor AND the ISL9240, as the resistor likely blew as a result of the ISL9240 internally shorting.

R7021 — 1.00 Ω

R7022 — 1.00 Ω

R7061 — 1.00 Ω

R7062 — 1.00 Ω

If the current sensing values are normal, replace the ISL9240.

PP3v3_G3H_RTC low or absent:

PP3v3_G3H_RTC powers the "brain" functionality of the CD3217 USB-C controllers which allows negotiation with the USB-C charger to allow 20 V. When PP3v3_G3H_RTC is shorted, low, or absent, this communication cannot take place, and the device will be stuck at 5 V. On the 820-01958, U6960 generates PP3v3_G3H_RTC. U6960 is a buck converter which lowers the voltage from PPBUS_G3H (≈12 V) to 3.3 V. Since PPBUS_G3H is the VIN (Voltage Input), we must get PPBUS_G3H before we get PP3v3_G3H_RTC.

•The most common reason for PP3v3_G3H_RTC to be missing or low, is a short to ground, usually on a capacitor around one of the CD3217s, or from a CD3217 itself.

•If PP3v3_G3H_RTC is not shorted, check for its enable signal, GHGR_EN_MVR. CHGR_EN_MVR is produced by the ISL9240 (U7000). The ISL9240 commonly fails in a way that prevents CHGR_EN_MVR from being produced. If the enable signal above is missing, replace the ISL9240 (U7000.)

•PP3v3_G3H_RTC can also be pulled low by PP3v3_UPC_T_LDO or PP3v3_UPC_X_LDO, so if you have no measurable short to ground, and your enable/VIN is present, check both the above rails for a short or low resistance to ground. Low resistance to ground/short on the above LDO lines will usually be caused by a bad CD3217.

If you found and resolved a short circuit, and still have low voltage on PP3v3_G3H_RTC, check voltage on both sides of R6979. This resistor can blow in response to a short circuit.

NOTE: This is an evolving issue, seen on many T2 series boards, and there is no fix for many of these cases as of 12/2/21. This article will be updated if/when a fix becomes available. See below for possible causes.

"Differential diagnosis for no power, with 20v and ~0.03-0.06 as measured on a USB-C amp meter.

• Failed T2 chip (Most likely diagnosis for most cases in the absence of the below causes.

• Short to ground on PP3v3_S5.

• Corrupt T2/BridgeOs firmware.

• Failed PMIC (Rare.)

• Failed NAND.

➤Diagnostic Steps

1. Check if the device is stuck in DFU or Recovery mode.

- Plug the MacBook into another Mac or MacBook via its master port (Left side, bottom port, closest to the trackpad) and open Apple Configurator 2 to verify that it is or isn't in DFU mode. If the device is in DFU mode, Proceed to "Device stuck in DFU mode due to corrupt T2 firmware" in repair steps below.

2. Check for a short to ground on PP3v3_S5.

- If a short to ground is found, proceed to the "PP3v3_S5 short to ground" repair steps below.

➤Repair Steps

Device stuck in DFU mode due to corrupt T2 firmware:

- Revive T2 firmware via Apple Configurator 2.

NOTE: You must be running the latest version of MacOs for this to work consistently. Check for MacOs updates prior to reviving/restoring T2 firmware.

How to revive T2 firmware: Plug the device you are working on to another Mac or MacBook via its master port. The master port on the A2179 is the bottom left side USB-C port (closest to the trackpad.) If you are confused, please see the diagram listed on Apple's how to page — [3]

Once plugged in, open Apple Configurator 2. You should see a big square icon pop up that says "DFU" or rarely, "RECOVERY". Click the icon, Navigate to the top menu bar click "Actions" then "Advanced". Select Revive device. You will see a progress bar appear. This process can take anywhere from 2 minutes to over 30 minutes in some cases.

SELECTING RESTORE WILL WIPE ALL USER DATA!

PP3v3_S5 shorted to ground.

- Locate and replace the shorted component.

If PP3v3_S5 is shorted to ground, pin 5 of U4885 is a good place to inject voltage to.

1v at 5 amps is an appropriate voltage to inject into the line.

With voltage being injected, perform thermal imaging of the board. If thermal imaging is not available, feel around the board to see where it is getting warm. Once the area is localized, add a small amount of isopropyl alcohol to the area to localize the shorted component.

Once the shorted component is localized, replace the shorted component.

For more info on finding short circuits, please see - [4]

If no short is found on PP3v3_S5, and the device is not in DFU or recovery mode, you can try forcing a firmware revive.

• Attempt to restore BridgeOs firmware via Apple Configurator 2 by placing the device into DFU mode.

- STOP! Before you begin, is your Mac on the LATEST VERSION of MacOs? If not, update your system before proceeding. Forcing a MacBook into DFU mode, and attempting to restore BridgeOs firmware on a old version of MacOs may result in a bricked device.

NOTE: Selecting restore will wipe all user data.

➣ Follow the instructions on this Apple support article on how to revive or restore T2/BridgeOs firmware, including on how to force a Intel based MacBook into DFU mode by using a key combination. [5]

If the device fails the firmware revive or restore, the T2 chip or one of the NANDs is likely dead or is receiving unstable power from the PMIC (U7800) causing it to crash. If the device fails the firmware revive, you can try empirically replacing U7800.

Replacing U7800 should be seen as a last ditch effort, as it only works in a small amount of cases, and does not have a definitive diagnostic test to determine if it is bad.

Differential diagnosis for cycling SLPS2R and AWAKE voltages/signals:

• Short on any of the G3S power rails.

• Failed PMU (U7800).

• Short on PP3v_G3H.

➤Diagnostic Steps

1. Check voltage and resistance to ground on PP5v_G3S.

- The resistance to ground on PP5v_G3S should fairly high, well within the kilo-ohms. Voltage should be around 5v. Values less than 4.9v should be considered abnormal.

2. Check voltage and resistance to ground on PP3v3_G3S.

- The resistance to ground on PP3v3_G3S should fairly high, well within the kilo-ohms. Voltage should be around 3.3v. Values less than 3.215 should be considered abnormal.

3. Check voltage and resistance to ground on PP1v8_G3S.

- The resistance to ground on PP1v8_G3S should fairly high, well within the kilo-ohms. Voltage should be around 1.8v. Values less than 1.70v should be considered abnormal.

4. Check voltage and resistance to ground on PP3v_G3H.

- The resistance to ground on PP3v_G3H should fairly high, well within the kilo-ohms. Voltage should be around 3.0v. Values less than 2.915 should be considered abnormal.

➤ Repair Steps

• If no shorts were found on the listed rails above, replace U7800.

• If you found a short on the listed rails above, inject 1v into the rail and perform thermal imaging. Replace the shorted component.

Differential diagnosis for high fan speed / slow or hanging at the Apple logo on boot.

• Battery unplugged or not fully seated into the connector.

• Failed battery pulling down the SMBUS communication lines to/from the battery, T2 and ISL9240.

• Failed trackpad, causing the palmrest thermal sensor to misread.

• Other non-specific thermal sensor related issue.

• Non-specific areas of corrosion causing T2 data line communication issues.

➤Diagnostic Steps

1. Double check that the battery cable is fully seated into the battery connector. The cable should "click" into the connector. A partially seated battery connector is a common occurrence among technicians unaccustomed to working on the newer MacBook Air devices.

2. Attempt to boot into Apple Diagnostics by holding the D key on startup.

- Apple Diagnostics may display a "There may be an issue with the System Management Controller (SMC)" error. This error seldom indicates a board issue on A1932 devices and usually indicates an issue with the battery, or any of the device's thermal sensors, most commonly the palmrest sensor located in the trackpad.

- If Apple Diagnostics displays any battery related error, replace the battery.

Occasionally, you will be unable to boot into Apple Diagnostics as the device will hang after you enter your WiFi password to boot into diagnostics. This can be caused by any of the above causes listed in the differential diagnosis. If this occurs, perform a thorough visual inspection of the logic board, Audio I/O board, trackpad (you will need to remove it from the enclosure.) and trackpad cable, and the battery management board (you will need to pull up the protective cover and foam).

3. If you are unable to boot into Apple Diagnostics, or Apple Diagnostics displays no error, and your visual inspection is unremarkable, test with a KNOWN GOOD battery. Battery issues presenting with high fan speed/hanging on boot are very common on A2179 devices.

➤Repair Steps

Perform necessary repairs based on your diagnostic findings.

- Replace battery as needed if diagnostic findings suggest a battery issue.

- Replace trackpad / trackpad cable if your visual inspection reveals any corrosion or damaged/knocked off components.

- Replace any corroded components on the system board per visual inspection.

Differential diagnosis for P5VG3S_EN missing:

• Device stuck in DFU or Recovery mode.

• Corrupted T2 Firmware or corrupt T2 ROM.

• Failed T2 chip.

• Failed PMU (U7800).

➤Diagnostic Steps:

1. Check if the device is stuck in DFU or Recovery mode.

- Plug the MacBook into another Mac or MacBook via its master port (Left side, bottom port, closest to the trackpad) and open Apple Configurator 2 to verify that it is or isn't in DFU mode. If the device is in DFU mode, Proceed to "Device stuck in DFU mode due to corrupt T2 firmware" in repair steps below.

2. Check voltages that U7800 outputs.

- PP1v8_SLPS2R - Normal voltage ~1.8v "- PP1v1_SLPS2R - Normal voltage ~1.1v "- PP0v9_SLPDDR - Normal voltage ~ 0.9v "- PPVDDCPU_AWAKE ~ Normal voltage ~ 1.06v - PP0v82_SLPDDR ~ Normal voltage ~ 0.82v

If any of the above rails is missing, U7800 may be bad, the rail may be shorted, or the T2 may not be communicating with U7800. Many of these rails have very low resistance to ground as they power the CPU portion of the T2. Do not assume you have a short until you have compared it to a known good board. Many of these rails will read 50-100 Ω to ground. PP1v8_SLPS2R is an exception, with its resistance normally being well within kilo-ohms.

A missing SLP or AWAKE voltage listed above without a short to ground (Less than 10 ohms usually), with P5VG3S_EN missing is suggestive of an issue with U7800.

3. If all SLP and AWAKE voltages are present at their normal voltages, the T2 may not be communicating properly with U7800. Attempt to force the device into DFU mode to reflash the T2's firmware. Proceed to repair steps below for "Forcing device into DFU mode to reflash T2 firmware"

➤Repair Steps

Device stuck in DFU mode due to corrupt T2 firmware:

- Revive T2 firmware via Apple Configurator 2.

NOTE: You must be running the latest version of MacOS for this to work consistently. Check for MacOS updates prior to reviving/restoring T2 firmware.

How to revive T2 firmware: Plug the device you are working on to another Mac or MacBook via its master port. The master port on the A2179 is the bottom left side USB-C port (closest to the trackpad.) If you are confused, please see the diagram listed on Apple's how to page - [6]

Once plugged in, open Apple Configurator 2. You should see a big square icon pop up that says "DFU" or rarely, "RECOVERY". Click the icon, Navigate to the top menu bar click "Actions" then "Advanced". Select Revive device. You will see a progress bar appear. This process can take anywhere from 2 minutes to over 30 minutes in some cases.

SELECTING RESTORE WILL WIPE ALL USER DATA!

Missing SLP or AWAKE voltage without the presence of a short to ground

REMEMBER, Many of the SLP and AWAKE lines will naturally have low resistance to ground !!

- Replace U7800.

Missing SLP or AWAKE voltage with the presence of a short to ground.

Unfortunately, many times when a SLP or AWAKE voltage is shorted to ground, it will be due to a failed T2 chip, which would render the board beyond repair, however in some cases, a capacitor, or U7800 itself will be the cause of the short circuit.

- Use short detection strategies to find the short circuit and replace the shorted component. Do not inject a higher voltage than the line creates. Example: Do not inject higher than 0.82v on PP0v82_SLPDDR

To learn how to use various short detection strategies, see this page - [7]

Forcing the device into DFU mode to reflash T2/BridgeOs firmware

• Attempt to restore BridgeOs firmware via Apple Configurator 2 by placing the device into DFU mode.

- STOP! Before you begin, is your Mac on the LATEST VERSION of MacOs? If not, update your system before proceeding. Forcing a MacBook into DFU mode, and attempting to restore BridgeOs firmware on a old version of MacOs may result in a bricked device.

➣ Follow the instructions on this Apple support article on how to revive or restore T2/BridgeOs firmware, including on how to force a Intel based MacBook into DFU mode by using a key combination. [8]

If the device fails the firmware revive or restore, the T2 chip or U7800 may be the cause of the failure.

If all SLP and AWAKE voltages are present, and a DFU revive fails, replace U7800 empirically. If the problem persists after replacing U7800, the T2 chip is the likely cause of the fault, and a diagnosis of a failed T2 chip can be made.

Differential diagnosis for camera detection issues on the 820-01958 logic board:

• Failed webcam on display assembly.

• Failed TCON board, resulting in PP5v_MAIN_ALSCAM being shorted to ground, which usually presents as a catastrophic failure of L8570. Usually, when this fault occurs, the TCON board is seen with a burned camera connector with occasional PCB layer damage.

• Corrupt T2/BridgeOs firmware. This is rare.

• Failed camera secure disable chip (U8595). This is very rare.

• Corroded display connector/cable.

➤ Diagnostic Steps:

1. Check continuity on L8570.

- Place your multimeter in continuity mode and measure across the component. L8570 is a filter, so it is essentially a wire. Your reading should be close to 0.01. You will often find this component visually exploded/burned with adjacent trace damage. If L8570 measures normally, proceed to the next steps. If L8570 is blown or burned, proceed to the "L8570 blown or visibly burned repair steps below."

2. Check camera function with a KNOWN GOOD display.

- You don't have a board issue until you know you don't have a parts issue. If L8570 measures normally, test with a known good display, or place the board in a known good housing. If the camera works normally with a different screen/enclosure, the display is the issue and will need to be replaced. BE SURE TO CHECK L8570 FIRST!

➤Repair Steps

Camera works with a known good test display

• Replace the display assembly.

L8570 blown or visibly burned

Understand that this fault occurred because of a catastrophic short within the TCON board of the display. If you repair L8570 without addressing the display issue, L8570 will fail again.

• Generally, when L8570 blows, there will be corresponding pad and trace damage and jumper wires will usually be required.

- 36 or 37 AWG enamel coated copper wire is recommended for jumper wires in this application due to the power requirements of the webcam and ambient light sensor, which are both powered off this filter.

- The closest recommended jumper point for the input side of the filter (PP5v_G3S) is CC775/CC774 if the traces and vias for PP5v_G3S are not damaged in that area. If damage is seen on and around CC775/CC774, use L6703 as a jumper point. Ensure you have conductivity between PP5v_G3S and CC775/CC774 if you are using those capacitors as a jumper point.

- Using a conformal coating on the jumper wire is strongly recommended.

- Use a 120 Ohm ferrite filter rated for 1.5 amps in a 402 package size only, preferably from a donor board. Do not substitute a resistor, wire or fuse.

- After L8570 is replaced, and jumper wires are run as needed, replace the display assembly.

No camera function, L8570 measures normally, and the camera still does not work when tested with another display.

• Attempt to restore BridgeOs firmware via Apple Configurator 2 by placing the device into DFU mode.

- STOP! Before you begin, is your Mac on the LATEST VERSION of MacOs? If not, update your system before proceeding. Forcing a MacBook into DFU mode, and attempting to restore BridgeOs firmware on a old version of MacOs may result in a bricked device.

➣ Follow the instructions on this Apple support article on how to revive or restore T2/BridgeOs firmware, including on how to force a Intel based MacBook into DFU mode by using a key combination. [9]

• If you still have no camera function after a T2/BridgeOs firmware revive/restore, and you have ruled out other causes listed above, the camera secure element (U8595) may be bad, or you may have a non-specific trace/resistor issue between the camera connector/U8595 or between U8595 and the T2 chip. A hardware issue with the T2 chip can also cause no camera detection issues.

Differential diagnosis for no audio or no audio on right side speaker.

• Corroded/failed speaker amplifier on audio I/O board. Can present as no audio on right speaker or no audio at all if SPKAMP_RESET_L or any of the codec lines are pulled down. (Most common, and will be the cause of sound issues in most cases.)

• Failed audio codec (Located on audio I/O board.)

• Audio I/O board or audio I/O board cable fault.

• Failed T2 chip. (No audio at all)

The vast majority of audio issues a A2179 will be related to the audio I/O board after liquid exposure. Audio issues in the absence of liquid exposure is rare.

➤ Diagnostic Steps:

- Perform a detailed visual inspection of the audio I/O board, paying special attention to the right speaker amplifier and the audio I/O cable and connector. The right speaker amplifier is located adjacent to the right speaker connector and is marked "TI TAS5770". If the right speaker amplifier is corroded, proceed to the "Right speaker amplifier corroded" repair steps below.

- If there is no liquid damage or corrosion seen to the audio I/O board, try with a known good part.

- Replacing the audio codec and surrounding components is not practical as the chip and adjacent components are encased in a hard epoxy like resin, which is extremely difficult to remove.

➤ Repair Steps:

Right speaker amplifier corroded

- Replace the right speaker amplifier (Texas Instruments TI TAS5770) which can be pulled and reballed from any T2 generation donor board. The ball pitch is relatively standard, and many stencils will be compatible. Rarely, the VIN trace which is sourced from PPBUS_G3H will require a jumper.

For all other audio issues excluding blown speakers, replace the audio I/O board as necessary.

Differential diagnosis for no display backlight on the 820-01958 logic board:

• Failed display TCON board.

• Failed display LED strip.

• Short to ground on backlight output caused by a failed output decoupling capacitor.

• Failed backlight driver (U8400)

• Backlight driver circuit fault (resistors, traces, capacitors, MOSFETs.)

• SMC_LID_LEFT or SMC_LID_RIGHT voltage too low due to a failed hall effect sensor or a issue within the SMC_LID circuit.

The most common cause for a 820-01521 to have no backlight in the absence of liquid damage, is a short to ground on backlight output due to a failed capacitor.'

➤Diagnostic Steps

1. Check for a short to ground on backlight output (PPVOUT_LCDBKLT_F). C8469 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. If the short is persistent with the screen unplugged, proceed to the "Backlight output shorted to ground" repair steps below.

2. If no short to ground is measured, and there is no signs or history of infiltrative liquid damage to the system board, test the device with a known working display. If the device has backlight with a known working display, proceed to the repair steps below for "Device has backlight output on a known good display, but not on the originally installed display" below.

3. The following section will focus on determining if U8400 is the cause of the backlight issue.

Failure of U8400 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, U8400 may be bad or you may have another circuit fault such as a bad resistor or broken trace within the U8400/backlight generation circuit.

If U8400 is corroded, replace it along with any other corroded resistors or capacitors adjacent to it.

It is also common for current sensing pins (pins 9 and 10) of U8400 to be very corroded with corresponding pad/trace damage. If this is the case, run a wire for each pad and connect them to U5620 (Which is a omitted/no-stuff component.) Pin 9 (ISNS_LCDBKLT_N) can be run to Pin 5 of U5620 and Pin 10 (ISNS_LCDBKLT_P) can be run to pin 4 of U5620.

①. Ensure that the backlight circuit has its input voltage (VIN) by measuring on BOTH SIDES of F8400, which is the backlight fuse. Voltage on both sides of the fuse should be equivalent to PPBUS_G3H. If voltage is normal on both sides of the backlight fuse, proceed to the next step. The backlight fuse can blow in response to a previous short to ground in the backlight circuit, or due to the screen being plugged in without the battery disconnected. Rarely, the backlight fuse can become open (blown) due to development of sulfur deposits on the fuses' filament , or due to micro manufacturing defects on the fuses filament. If the backlight fuse is blown, check for a short on backlight output (PPVOUT_S0_LCDBKLT_F) and on backlight input (PPVIN_S0SW_LCDBKLT_F). If the fuse is blown, proceed to the "Backlight fuse (F8400) blown in the absence of a short to ground" repair steps below.

② Check that the backlight circuit is being enabled. The screen will need to be connected for the enable signal to be sent out. With the board on your bench, with a known good screen connected, measure voltage on pins 1 and 2 of R8442. You should have ~3.3v. Low voltage on pin 2 of R8442 is suggestive of an issue with U8400. If you have 0v on pin 1 of R8442, the backlight circuit is not being told to turn on, which raises the possibility of a CPU issue, or an issue with the display cable/connector.

③ Check the 5v input to U8400 (PP5v_S0_BKLT_D).

PP5v_S0_LCDKLT_D can be measured on R8444. Be sure to check voltage on both sides of the resistor. If voltage is lower on one side of the resistor, R8444 is likely blown, probably due to a short within U8400.

If PP5v_S0_LCDBKLT_D is low (Substantially lower than 5v), replace U8400 AND R8444 as R8444 acts as a current limiting resistor to protect PP5v_G3S, and likely blew as a result of an internal short within U8400.

If you have low voltage on pin 2 of R8442 with normal voltage on pin 1 of R8842, replace U8400.

If VIN, EN, and PP5v_S0_LCDBKLT_D are all present at normal levels, and you do not have backlight, replace U8400.

If EDP_BKLT_EN is missing, U8400 is NOT your issue.

➤ Repair Steps

Backlight output shorted to ground:

- Find and replace shorted component.

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. If you have a short on backlight output, solder a wire to a sturdy component that can handle a good amount of current, usually one of the backlight output capacitors (C8468 or C8461 is usually a good spot.) Inject 10v at 5 amps and thermally image the system board or deploy comparable short detection methods. For more information on short circuit detection, please see the following page - [10]

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

- Replace the display assembly. To retain keyboard backlight function, be sure to use the same year display.

Backlight fuse (F8400) 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.