Need help troubleshooting power circuit on Dell Lattitude E7450

[shaundh]

I just got a pretty nice Dell Lattitude E7450 laptop from a friend to try and fix. When the charger is plugged into the laptop, the light on the adapter immediately goes out. I disassembled it and took the board out. I put 3v to Vin at charge port and found that mosfet AO4435 was getting hot. That was the only component I found to get hot and there are no signs of damage to this chip or any component . There are also several caps that are grounded on both sides when testing for shorts but I can't tell if this could be by design, due to the bad mosfet, or other condition. Question is, how likely is it just this chip or should I be looking for other bad components?

[Spider1211]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

When the charger is plugged into the laptop, the light on the adapter immediately goes out.

Most likely a short on the 19V main rail. Provide schematic and we can dig further. Depending on which circuit the mosfet is from, that could also be an issue. Can't say for sure without schematic.

Do not apply voltage blindly, it can cause more harm. If you need to apply voltage, always start with a low voltage (Typically 1V).

[mon2]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

I think the schematic is posted here:

Lattitude E7450
https://www.badcaps.net/forum/showthread.php?t=70511

1) are you using the original power adapter?

2) voltage at dc_in_ss ?

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

I think the schematic is posted here:

Lattitude E7450
https://www.badcaps.net/forum/showthread.php?t=70511

1) are you using the original power adapter?

2) voltage at dc_in_ss ?

Yes, I'm using the original adapter and I found that schematic here but I don't know how to read it.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Here is the schematic

[Spider1211]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

No worries, you'll learn along the way. If I may ask, do you have some background in electronics/soldering/using multimeter, etc?

Our first step is to try to rule out any obvious faults through visual inspection. Since you mentioned adapter going off as soon as you connect it, I suspect it may be a shorted capacitor (that is a common fault). Using a magnifying glass (ideally a microscope if you have one) Look around on the board and see if any of the small capacitors (brown bricks) seems to have a crack on them or have obvious discoloration.

We'll move onto using schematics and multimeter at a later stage.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

No worries, you'll learn along the way. If I may ask, do you have some background in electronics/soldering/using multimeter, etc?

Our first step is to try to rule out any obvious faults through visual inspection. Since you mentioned adapter going off as soon as you connect it, I suspect it may be a shorted capacitor (that is a common fault). Using a magnifying glass (ideally a microscope if you have one) Look around on the board and see if any of the small capacitors (brown bricks) seems to have a crack on them or have obvious discoloration.

We'll move onto using schematics and multimeter at a later stage.

Yes. I have a soldering station and hot air with a 30v power supply. I can solder pretty good but if the components are too small…… Im using a fluke 117 but no microscope or o-scope. I did not see any visual damage to the board or components that were obvious. I'll try to get my hands on a microscope though.

[Spider1211]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Ok great! A good magnifying glass will do if you cannot find a microscope.

Before we concentrate on the main rail, we want to rule out the possibility the other power supplies are not shorted. Set you DMM to continuity mode, connect the black probe to ground (just find a screw hole with copper around it and hook it there).

Using the red probe, poke all the inductors (grey bricks) and note the resistance value and report back. Each inductor has a unique name, something like PL100, etc (usually written next to it). It does not matter much which side of the inductor you poke, just make sure all power source is disconnected first!

For example:
PL700= .....
PL600= .....

By the way, just to confirm, your 30V power supply is adjustable? Can you set the voltage and current limit independently.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Ok great! A good magnifying glass will do if you cannot find a microscope.

Before we concentrate on the main rail, we want to rule out the possibility the other power supplies are not shorted. Set you DMM to continuity mode, connect the black probe to ground (just find a screw hole with copper around it and hook it there).

Using the red probe, poke all the inductors (grey bricks) and note the resistance value and report back. Each inductor has a unique name, something like PL100, etc (usually written next to it). It does not matter much which side of the inductor you poke, just make sure all power source is disconnected first!

For example:
PL700= .....
PL600= .....

By the way, just to confirm, your 30V power supply is adjustable? Can you set the voltage and current limit independently.

Yes, it is adjustable. I just bought it about a month ago. Hanmatek HM310. I will report these values back later today. Thanx for your help.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Ok great! A good magnifying glass will do if you cannot find a microscope.

Before we concentrate on the main rail, we want to rule out the possibility the other power supplies are not shorted. Set you DMM to continuity mode, connect the black probe to ground (just find a screw hole with copper around it and hook it there).

Using the red probe, poke all the inductors (grey bricks) and note the resistance value and report back. Each inductor has a unique name, something like PL100, etc (usually written next to it). It does not matter much which side of the inductor you poke, just make sure all power source is disconnected first!

For example:
PL700= .....
PL600= .....

By the way, just to confirm, your 30V power supply is adjustable? Can you set the voltage and current limit independently.

These are all the square inductors I found on top of the board. Did not find any on the bottom:

• Inductor readings:
○ PL500 = 65.5 Ohms
○ PL700 = 0.2 Ohms
○ PL701 = 4.6 Ohms
○ PL101 = 3.69 Ohms
○ PL102 = (constantly moving, draining from 500k Ohms down and back up again)
○ PL200 = 446 Ohms
○ PL301 = 1.9k Ohms

[Spider1211]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Definitely have a short on main rail. PL101 seems low also. Let's figure out the mail short first.

Since you mentioned you did not see any physical damage, try injecting voltage on PL700. Start at 1V and 1A, see what gets hot. If you feel nothing, increase Ampere only (try 3A and move up to max you PSU can supply which I guess is 5A?).

If you still can feel anything, increase voltage to 2V and check again. In my opinion, its most likely either a capacitor or mosfet.

Edit:
Once you identify the shorted component and remove it, measure continuity on the inductors again. Make sure you do not apply power if you remove mosfets.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Definitely have a short on main rail. PL101 seems low also. Let's figure out the mail short first.

Since you mentioned you did not see any physical damage, try injecting voltage on PL700. Start at 1V and 1A, see what gets hot. If you feel nothing, increase Ampere only (try 3A and move up to max you PSU can supply which I guess is 5A?).

If you still can feel anything, increase voltage to 2V and check again. In my opinion, its most likely either a capacitor or mosfet.

Edit:
Once you identify the shorted component and remove it, measure continuity on the inductors again. Make sure you do not apply power if you remove mosfets.

I’m at 2v 9amps and I don’t feel anything getting hot when I inject voltage into PL700.

[Stevedb]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Are you certain that the heat not get lost in your wires. Are you using thick wires. But this 18w of heat you must feel it.

[mcplslg123]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Leave the mb with 2v,9a injected for atleast 5-10 minutes and then see which area is warm. Once you know the area, try to findout the culprit.

[Spider1211]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

After trying the above, try removing PL700 and see on which side the short is.

If you are comfortable with it, try removing PU800 and check if the short is still present or not. This chip can sometimes fail and give similar issue to yours. Just make sure NOT to power the board with the chip removed.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

After trying the above, try removing PL700 and see on which side the short is.

If you are comfortable with it, try removing PU800 and check if the short is still present or not. This chip can sometimes fail and give similar issue to yours. Just make sure NOT to power the board with the chip removed.

Lost internet here after the storm.

I can't even find PU800 on the board. I'm starting to think the schematic does not match my board. I will try the above again powering for a longer time. But I think I'm going to remove PQ700 which is the AO4435 mosfet.

[Spider1211]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

If you can't find PU800, try looking at the small black IC that has the following label: CD3301 (a quick google search should show you how it looks like).

Post a picture of your motherboard reference so we can crosscheck if its the correct schematic.

Removing PQ700 won't help much since it is just an input protection mosfet. You can try though. I have a feeling it might be the PU800.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

If you can't find PU800, try looking at the small black IC that has the following label: CD3301 (a quick google search should show you how it looks like).

Post a picture of your motherboard reference so we can crosscheck if its the correct schematic.

Removing PQ700 won't help much since it is just an input protection mosfet. You can try though. I have a feeling it might be the PU800.

I do see PU800 now. It does not seem to be getting hot. Only thing getting hot is PL701 near Pl700 after some time. Not too comfortable with removing PU800 though. I'm going to keep testing.

[Spider1211]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

Yes, that was the crazy part. The chip itself wasnt heating up but once removed the short was gone.

[shaundh]

Re: Need help troubleshooting power circuit on Dell Lattitude E7450

I removed PL700 from the circuit and it still only measures 0.2 Ohms. There are a couple of capacitors still shorted to ground on both sides though. But shouldn't PL700 have more resistance than 0.2?