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Trying to fix a lenovo l340-15api. 3.3V rail comes on, then immediately goes to zero.
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<blockquote data-quote="pcrepair" data-source="post: 290" data-attributes="member: 1"><p>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.</p><p></p><h3><strong>Key Observations and Analysis:</strong></h3> <ol> <li data-xf-list-type="ol"><strong>3.3V Rail Behavior:</strong><ul> <li data-xf-list-type="ul">The 3.3V rail briefly powers up and then drops to 0V. This could be due to:<ul> <li data-xf-list-type="ul">A protection circuit detecting an issue and shutting it down.</li> <li data-xf-list-type="ul">An intermittent fault that is only present under load.</li> </ul></li> </ul></li> <li data-xf-list-type="ol"><strong>Power LED Flashing White:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> <li data-xf-list-type="ol"><strong>CPU Heating Up:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> <li data-xf-list-type="ol"><strong>Resistance Measurements:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> <li data-xf-list-type="ol"><strong>Board Current Draw:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> </ol><h3><strong>Possible Causes and Steps Forward:</strong></h3> <ol> <li data-xf-list-type="ol"><strong>Check for Faulty Capacitors:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> <li data-xf-list-type="ol"><strong>Investigate Power ICs and MOSFETs:</strong><ul> <li data-xf-list-type="ul">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.</li> <li data-xf-list-type="ul">If possible, measure the gate voltage of the MOSFETs controlling the 3.3V rail to see if they're being properly activated.</li> </ul></li> <li data-xf-list-type="ol"><strong>BIOS/EC Firmware Issues:</strong><ul> <li data-xf-list-type="ul">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.</li> <li data-xf-list-type="ul">Consider resetting the BIOS/EC or re-flashing the firmware if you haven’t already.</li> </ul></li> <li data-xf-list-type="ol"><strong>Thermal Issues:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> <li data-xf-list-type="ol"><strong>Check for Signal Integrity on Key Rails:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> <li data-xf-list-type="ol"><strong>Peripheral Devices:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> <li data-xf-list-type="ol"><strong>Check the Charger Circuit:</strong><ul> <li data-xf-list-type="ul">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.</li> </ul></li> </ol><h3><strong>Next Steps:</strong></h3> <ul> <li data-xf-list-type="ul"><strong>If possible</strong>, try to find a schematic for the motherboard. This can provide more precise information on the 3.3V rail's pathways and connected components.</li> <li data-xf-list-type="ul"><strong>Consider probing</strong> 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.</li> </ul><p>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.</p></blockquote><p></p>
[QUOTE="pcrepair, post: 290, member: 1"] 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. [HEADING=2][B]Key Observations and Analysis:[/B][/HEADING] [LIST=1] [*][B]3.3V Rail Behavior:[/B] [LIST] [*]The 3.3V rail briefly powers up and then drops to 0V. This could be due to: [LIST] [*]A protection circuit detecting an issue and shutting it down. [*]An intermittent fault that is only present under load. [/LIST] [/LIST] [*][B]Power LED Flashing White:[/B] [LIST] [*]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. [/LIST] [*][B]CPU Heating Up:[/B] [LIST] [*]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. [/LIST] [*][B]Resistance Measurements:[/B] [LIST] [*]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. [/LIST] [*][B]Board Current Draw:[/B] [LIST] [*]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. [/LIST] [/LIST] [HEADING=2][B]Possible Causes and Steps Forward:[/B][/HEADING] [LIST=1] [*][B]Check for Faulty Capacitors:[/B] [LIST] [*]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. [/LIST] [*][B]Investigate Power ICs and MOSFETs:[/B] [LIST] [*]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. [/LIST] [*][B]BIOS/EC Firmware Issues:[/B] [LIST] [*]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. [/LIST] [*][B]Thermal Issues:[/B] [LIST] [*]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. [/LIST] [*][B]Check for Signal Integrity on Key Rails:[/B] [LIST] [*]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. [/LIST] [*][B]Peripheral Devices:[/B] [LIST] [*]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. [/LIST] [*][B]Check the Charger Circuit:[/B] [LIST] [*]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. [/LIST] [/LIST] [HEADING=2][B]Next Steps:[/B][/HEADING] [LIST] [*][B]If possible[/B], try to find a schematic for the motherboard. This can provide more precise information on the 3.3V rail's pathways and connected components. [*][B]Consider probing[/B] 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. [/LIST] 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. [/QUOTE]
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Trying to fix a lenovo l340-15api. 3.3V rail comes on, then immediately goes to zero.
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