How to fix HP 15 laptop DC Jack LED Not Glowing

Introduction

The HP 15 laptop, model U81, has encountered a major motherboard issue where the DC jack LED is not glowing. This issue often stems from power-related faults, and in this case, the motherboard appears to have been damaged due to liquid spillage. A spilled liquid can cause severe short circuits, corroded connections, or failed components, leading to multiple failures on the board. This guide will take you through a step-by-step process of diagnosing and repairing this motherboard, starting from DC-IN analysis to full power restoration.

Step 1: Identifying the Motherboard and Power Issue

Before beginning the repair, it is crucial to identify the exact part number of the motherboard. The motherboard in question is the HP 15 U81, and our primary concern is that the DC jack LED is not glowing, indicating a power failure at the entry level.

Step 2: Checking the 24V Power Line from the Charging IC

1. Measuring the DC Jack Voltage: Use a multimeter to check whether the DC-IN port is receiving the correct voltage. If not, the issue may lie in the adapter, charging circuit, or the DC jack itself.
2. Checking the MOSFET Switching: In our case, we checked the gate voltage of the N-channel MOSFET responsible for regulating power input. The absence of voltage at the gate indicates that the switching mechanism is not functioning properly.
3. Reference to Switching Circuits: If you are unfamiliar with how MOSFETs, BJTs, or diodes switch, refer to our dedicated switching circuit guide to understand their operation in HP laptop motherboards.

Step 3: Checking for 19V Presence on the Motherboard

The expected voltage at the DC input should be 19V. This voltage is crucial as it powers the voltage regulators that generate 3.3V and 5V, necessary for system operation.

1. Locate the 3.3V and 5V Regulators: These regulators start working only when they receive the correct 19V input.
2. Confirm Voltage Flow Using a Multimeter: Check whether the charging IC is receiving power at pin 20. In this case, pin 20 receives power via a 22-ohm resistor.
3. Understanding Resistance Variations: Depending on different HP motherboard designs, this resistor can vary between 10 ohms to 50 ohms, so always refer to board schematics for accuracy.

Step 4: Examining the Charging IC and AC-IN Signal

1. Pin 20 Voltage Check: Measuring pin 20, we confirm that 20V is entering the charging IC, indicating no issue with DC-IN power transfer.
2. Verifying AC-IN and AC-OK Signals: The AC-IN signal plays a crucial role in detecting adapter presence and allowing power regulation.
3. Testing the Pull-up Resistor: The AC-OK signal should be properly pulled up to indicate that the adapter is supplying the correct voltage. If this is missing, the motherboard will fail to turn on.

Step 5: Diagnosing the 6V LDO Voltage Issue

1. Checking Pin 16 Output Voltage: The 6V regulator output should be stable at exactly 6V. If this voltage is lower, the charging IC could be malfunctioning.
2. Replacing the Faulty Charging IC: Since the capacitor associated with pin 16 is intact, replacing the charging IC should resolve the issue.

Step 6: Replacing the Charging IC

1. Removing the Faulty IC: Using a hot air rework station, carefully desolder the faulty charging IC.
2. Installing the New IC: Ensure proper alignment of pins before soldering the new IC in place.
3. Checking the New IC’s Functionality: Once replaced, measure pin 16 again to confirm it outputs a steady 6V.

Step 7: Confirming Power Stability

1. Testing the Power Flow Post-Repair: Injecting 18V at the power input, we confirm that the same voltage is present at the HP motherboard power rails.
2. Checking the Power Button Functionality: Pressing the power button should result in a stable power-on state, with fan movement indicating successful power sequencing.
3. Monitoring Current Load: A stable current draw of around 0.15A in standby mode and 0.5A post-power-on suggests that power distribution is functioning correctly.

Conclusion

This repair process successfully restored the power functionality of the HP 15 U81 motherboard, allowing the DC jack LED to glow once again. Diagnosing power issues systematically, from the DC jack to the charging IC and beyond, ensures effective repairs and minimizes unnecessary part replacements. If additional failures persist, further troubleshooting of core voltage regulators and signal pathways may be required.

Stay tuned for our next guide, where we will discuss resolving unstable power sequencing and system shutdown issues.

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