3V/5V Troubleshooting: How to Troubleshoot Missing Rails on Laptop Motherboard

Ajouté : 2026-05-12

[00:00:00]The 3 volt and 5 volt are missing here.

[00:00:04]Let's troubleshoot this motherboard and find out why 3 volt and 5 volt are not here. We got to track the voltage until we find the problem using the multimeter of course. So let's get started. This is basically the 3V 5V circuit. Here we have the first channel, the second channel and we have the control IC. If we go to the back of this motherboard. So this is the back of the power jack. Of course, this is the current sense resistor. If we check, we have the main voltage in both sides. So this is good. So the B+ is good. From this current sense resistor, the voltage the B+ will be distributed to all circuits as you can see of the motherboard. also for the CPU circuit.

[00:00:56]But for us, the most important is the 3V 5V circuit. This is basically the back of 3V 5V circuit. Okay, if you pay attention here, this is the two coils for 3V 5V circuit.

[00:01:12]And in the back we have as you can see these four MOSFETs. So let's troubleshoot this circuit. Okay. Always as I told you the input is identified by this coil over here we have 19 volt. All circuits you will find coils or ceramic capacitors in the input. Here for example we have for CPU circuit we have coil as you can see 19 volt and we have this filtering ceramic capacitors. All capacitors will hold 19 volt as you can see. The same for this circuit. As you can see for the graphic card, we have the input over here and ceramic capacitors that hold 19 volt here. The same for 3 volt 5 vol circuit. We have the input here. Okay. And we have seven capacitors. These three serum capacitors over here are for the first channel 5 volt channel. And we have another three here for 3.3 volt channel. If you we check those you will find 19 volt in all of them. Three capacitors hold 19 volt for 3 volts always channel. Okay. Do the same for those capacitors also. Here as you can see we have 19 volt. Okay. All capacitors hold 19 volt as you can see. So the input voltage for this circuit is good.

[00:02:41]No problem with the input voltage. But we still don't get 3 volt and 5V circuit. Before we continue, a quick thank you to the sponsor of this video, Next PCB. Next PCB is a reliable multi-layer PCB manufacturer offering everything from PCB prototyping to full turnkey PCB assembly. The handle PCB fabrication, component sourcing, asmd stencils, and even mass production. all in one place. Whether you are building a prototype or scaling to production, Nex PCB makes the process simple and professional. Check them out using the link below. What we should do next? Once we have the input signals, we have to detect here the two higher MOSFET because for 3 volt and 5V circuit, we have always to begin with the higher MOSFETs. Okay, the higher MOSFET is always connected with the input or with the coil and capacitors. So let's follow this part as you can see here. So we have the coil here 19 volt as you can see and we have this path goes here.

[00:03:58]Okay. So we have 19 volt here as you can see. So this is the higher MOSFET and this is the lower one. If we check here, we will not get 19 volt in both sides.

[00:04:10]Always we get 19 volt into one side in the drain of the higher MOSFET. As you can see also the higher MOSFET is connected with this filtering capacitors. So let's check the higher MOSFET here for this channel also. We have the same path here connected to this to the drain here. If we check we have 19 volt. Do you see? But for this one here, the lower MOSFET, as you can see, no voltage here. Okay. So, we detect the input, the filtering capacitors, the higher MOSFET, this one and this one. Here we have the two lower MOSFETs. What's next? The next is to check the control signal in the gate of the two higher MOSFET. So, we have here 19 volt. This is good. And we have the dot here means here we have source.

[00:05:04]These three pins are for source. So let's check the gate. We have to find the control signal. To get three volt we have to find here control signal. Let's check.

[00:05:17]Here we have zero. Let's check the second MOSFET to higher MOSFET for 5 volt. So here we have 19 volt. Here we have the dot means this is the source three for source. Here we have the gate also zero volt means the two MOSFETs here are not activated. So the problem here is the control signal doesn't generated by this IC but we know that every IC has inputs without the inputs we cannot get the outputs. It could be do IC could be the failed one about 80% because it doesn't generate the control signal but also do I see if it doesn't receive all inputs of course it will not generate the output means the control signal. So let's check this IC inputs to confirm if the IC receive all inputs or not. If we find all inputs are good means the IC is the bad one. But if we find that one or more inputs are not here means the the problem is not the IC. We have to check the super IO. This is also the IC that control the charge IC and the 3V 5V circuit. So let's check this IC inputs and outputs.