Trying to fix a lenovo l340-15api. 3.3V rail comes on, then immediately goes to zero.

[dazz]

Hi everyone. I think the title sums it up pretty well. The main voltage rail seems fine, the 5V rail is fine, the 1.1V rail for the CPU looks fine as well, but the 3.3V rail shuts down a split second after power up. I can measure it, just barely, before it shuts down. Meanwhile the power LED flashes white continuously. According to the laptop manual, a slow blinking white power LED means the lapto is in sleep mode. The board is drawing some 600mA in the process and the fan is not spinning. The CPU gets pretty toasty to the touch after a while if I leave it like that, so I don't leave it on for too long. Any pointers, please?

Thanks.

There are no shorts, as far as I can tell. The 3 coils next to the CPU measure 2 ohms and they read 1.1V, which seems consistent with the correct Vcore for the CPU. The coil next to the battery reads 22K ohms and when I plug the charger there are initially 8V there (the battery voltage, I guess) then goes to 0V after a few seconds. The 5V rail is present, reads 240 ohms. The 3.3V rail shown in the pictures reads 29 ohms and measures 3.3V for a split second before going to 0V. On the other side of the board there are two more coils, both not shorted. One of them reads 230 ohms and 0.9V and the other one 45 ohms and 1.2V

 

[pcrepair]

It seems like you've already done a thorough diagnostic of the power rails and associated components. The fact that the 3.3V rail drops shortly after power-on, while other rails remain stable, suggests a deeper issue related to either a protection mechanism being triggered or a component failure that’s not causing an obvious short.

Key Observations and Analysis:​

1. 3.3V Rail Behavior:
   • The 3.3V rail briefly powers up and then drops to 0V. This could be due to:
     • A protection circuit detecting an issue and shutting it down.
     • An intermittent fault that is only present under load.
2. Power LED Flashing White:
   • A slow blinking white LED usually indicates sleep mode, but in your case, it might be a sign that the system is unable to fully transition out of sleep mode due to the power issue.
3. CPU Heating Up:
   • The CPU getting warm suggests it's receiving power (likely from the 1.1V rail) and may be stuck in some form of loop or hang due to the unstable 3.3V rail.
4. Resistance Measurements:
   • Your resistance measurements suggest there’s no hard short on the 3.3V rail, but the low resistance (29 ohms) might still be indicative of a potential issue, depending on the specific circuit design.
5. Board Current Draw:
   • 600mA is a relatively high current draw, especially if the system is not fully booting. This might suggest a problem with power distribution or excessive consumption by a component.

Possible Causes and Steps Forward:​

1. Check for Faulty Capacitors:
   • Even if no obvious shorts are detected, a leaky or degraded capacitor on the 3.3V rail could cause instability. Consider checking or replacing capacitors connected to this rail.
2. Investigate Power ICs and MOSFETs:
   • The power IC or associated MOSFETs responsible for the 3.3V rail might be malfunctioning. Inspect these components for any signs of damage or overheating.
   • If possible, measure the gate voltage of the MOSFETs controlling the 3.3V rail to see if they're being properly activated.
3. BIOS/EC Firmware Issues:
   • The issue could be related to the Embedded Controller (EC) or BIOS, especially since the laptop appears to be stuck in a semi-active state. If the EC isn’t functioning correctly, it might not be properly managing the power rails.
   • Consider resetting the BIOS/EC or re-flashing the firmware if you haven’t already.
4. Thermal Issues:
   • The CPU heating up without the fan spinning is concerning. Ensure that the thermal management system (including thermal sensors) is functioning correctly. A thermal fault could potentially cause the system to shut down the 3.3V rail.
5. Check for Signal Integrity on Key Rails:
   • Look at the signals related to the 3.3V rail (enable signals, feedback loops) using an oscilloscope if available. This could reveal whether the rail is being actively shut down or if it’s collapsing due to a fault.
6. Peripheral Devices:
   • Disconnect any non-essential peripherals, including the battery, keyboard, and touchpad, to see if the 3.3V rail stabilizes. A fault in one of these devices could potentially cause issues with the rail.
7. Check the Charger Circuit:
   • Since the battery voltage appears momentarily on the coil before dropping, there might be an issue with the battery charging circuit that’s interfering with the power rails.

Next Steps:​

• If possible, try to find a schematic for the motherboard. This can provide more precise information on the 3.3V rail's pathways and connected components.
• Consider probing the motherboard with an oscilloscope to check the behavior of the 3.3V rail, enabling signals, and other power rails during the power-up sequence.

Given the complexity of the issue, if you are unable to pinpoint the exact cause with these steps, it might be worth consulting a professional technician with advanced diagnostic tools, such as a thermal camera or specialized motherboard tester.

 

[dazz]

It seems like you've already done a thorough diagnostic of the power rails and associated components. The fact that the 3.3V rail drops shortly after power-on, while other rails remain stable, suggests a deeper issue related to either a protection mechanism being triggered or a component failure that’s not causing an obvious short.

Key Observations and Analysis:​

1. 3.3V Rail Behavior:
   • The 3.3V rail briefly powers up and then drops to 0V. This could be due to:
     • A protection circuit detecting an issue and shutting it down.
     • An intermittent fault that is only present under load.
2. Power LED Flashing White:
   • A slow blinking white LED usually indicates sleep mode, but in your case, it might be a sign that the system is unable to fully transition out of sleep mode due to the power issue.
3. CPU Heating Up:
   • The CPU getting warm suggests it's receiving power (likely from the 1.1V rail) and may be stuck in some form of loop or hang due to the unstable 3.3V rail.
4. Resistance Measurements:
   • Your resistance measurements suggest there’s no hard short on the 3.3V rail, but the low resistance (29 ohms) might still be indicative of a potential issue, depending on the specific circuit design.
5. Board Current Draw:
   • 600mA is a relatively high current draw, especially if the system is not fully booting. This might suggest a problem with power distribution or excessive consumption by a component.

Possible Causes and Steps Forward:​

1. Check for Faulty Capacitors:
   • Even if no obvious shorts are detected, a leaky or degraded capacitor on the 3.3V rail could cause instability. Consider checking or replacing capacitors connected to this rail.
2. Investigate Power ICs and MOSFETs:
   • The power IC or associated MOSFETs responsible for the 3.3V rail might be malfunctioning. Inspect these components for any signs of damage or overheating.
   • If possible, measure the gate voltage of the MOSFETs controlling the 3.3V rail to see if they're being properly activated.
3. BIOS/EC Firmware Issues:
   • The issue could be related to the Embedded Controller (EC) or BIOS, especially since the laptop appears to be stuck in a semi-active state. If the EC isn’t functioning correctly, it might not be properly managing the power rails.
   • Consider resetting the BIOS/EC or re-flashing the firmware if you haven’t already.
4. Thermal Issues:
   • The CPU heating up without the fan spinning is concerning. Ensure that the thermal management system (including thermal sensors) is functioning correctly. A thermal fault could potentially cause the system to shut down the 3.3V rail.
5. Check for Signal Integrity on Key Rails:
   • Look at the signals related to the 3.3V rail (enable signals, feedback loops) using an oscilloscope if available. This could reveal whether the rail is being actively shut down or if it’s collapsing due to a fault.
6. Peripheral Devices:
   • Disconnect any non-essential peripherals, including the battery, keyboard, and touchpad, to see if the 3.3V rail stabilizes. A fault in one of these devices could potentially cause issues with the rail.
7. Check the Charger Circuit:
   • Since the battery voltage appears momentarily on the coil before dropping, there might be an issue with the battery charging circuit that’s interfering with the power rails.

Next Steps:​

• If possible, try to find a schematic for the motherboard. This can provide more precise information on the 3.3V rail's pathways and connected components.
• Consider probing the motherboard with an oscilloscope to check the behavior of the 3.3V rail, enabling signals, and other power rails during the power-up sequence.

Given the complexity of the issue, if you are unable to pinpoint the exact cause with these steps, it might be worth consulting a professional technician with advanced diagnostic tools, such as a thermal camera or specialized motherboard tester.

Thank you. Reseting the BIOS is one of the things I thought might solve the problem, I assumed unplugging the battery and the coin cell would reset it, but I can't be sure if that's the case. I also tried using the reset button in the mobo to no avail. I have already tried desconnecting all peripherals, in fact, the problem persists even with nothing but the charger plugged to the disassembled motherboard. I'll see if I can probe the gate signal in the 3.3V rail mosfet and find out what's driving it. Thank you again.