Dell Precision M3800 won't turn on

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The laptop wouldn't turn on and the charger would shut off as soon as it was plugged in to the laptop suggesting a short on the motherboard some place. I found capacitor PC205 was cracked and shorted. I replaced that. Now the charger doesn't shut off when plugged in to the laptop but the laptop still doesn't turn on. After some more probing I think I found that MOSFET PQ200 is shorted. Seems that the source, drain, and gate are all connected. I did try shorting pins 3 and 4 of the MOSFET which I think is supposed to dissipate any charge that might be in the MOSFET. If there is a charge then the gate might be open giving one the impression of a short when there really isn't one. If I have that wrong somehow then please let me know. Looks like PQ200 and PQ201 are connected to PU200. So I guess 19V was being dumped on PU200. Is PU200 probably fried? Is there a way I can check if it is OK before replacing MOSFET PQ200?

[mon2]

Re: Dell Precision M3800 won't turn on

Can you link or post the schematic?

What charger IC is onboard? Is it BQ715 BQ24715?

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Re: Dell Precision M3800 won't turn on

Can you link or post the schematic?

What charger IC is onboard? Is it BQ715 BQ24715?

Sorry! I felt like I was forgetting something when I finished the opening post but couldn't remember what.

Looks like I have the BQ715.

MOD Edit: Link to Schematic and Boardview -> https://www.badcaps.net/forum/showthread.php?p=1037478

[mon2]

Re: Dell Precision M3800 won't turn on

1) the battery charger operation sticky by @piernov posted above is a must read.

2) meter in dc volts mode (20v or higher). Measure the voltage to ground of the following points:

a) REGN

b) ACOK

c) ACDRV

On pu200. Post each measurement.

[TopPop]

Re: Dell Precision M3800 won't turn on

I'm reading piernov's sticky. I'll be honest. I'm not getting all of it but Google is helping me some.

I plugged in the AC adapter. All three of those showed as 0V which I thought was weird so I backed up to the PQ200 MOSFET and it had no voltage which I thought meant something new broke. I just forgot to plug the AC adapter port back in to the motherboard.

REGN is 5.85V

ACOK is 3.27V

ACDRV is starting at 19.9V when I first connect then dropping to a range of 18.9V to 18.6V

I guess I'm missing something basic. How do you know what the voltages are supposed to be at those three pins? I tried searching the PDF for various things but only found the diagram on page 51 that shows PU200 outputting +5VALW and +3VALW.

[TopPop]

Re: Dell Precision M3800 won't turn on

I see now in the sticky it mention voltages for REGN, ACOK, and ACDRV. Looks like my ACDRV voltage might be low.

[mon2]

Re: Dell Precision M3800 won't turn on

ACDRV should be ~25v if all is normal and this boosted voltage will enable the DCin mosfets.

ACDRV voltage = adapter voltage + REGN voltage.

Carefully remove the ~4k resistor at PR208. Use flux to help. Do not lose this part.

Then power up again. Measure the ACDRV pin voltage to ground on pu200 charger IC.

What is the ACDRV pin voltage now?

Be sure that you are using the original and suitable power adapter to power this board. Dell uses a single wire communication with the adapter for identification. If this adapter line is faulty, the logic board will throttle the speed and halt battery charging.

What is the voltage to ground of PS_ID ?

[TopPop]

Re: Dell Precision M3800 won't turn on

Might be a few days before I can get back to you about this due to other things going on right now. I had to put away my laptops and soldering gear.

Looks like I probably have a compatible AC adapter. Mine has the Dell part number M1MYR which is listed at https://www.parts-people.com/index.p...=item&id=19684 as compatible with the M3800.

Thank you for your help!

[mcplslg123]

Re: Dell Precision M3800 won't turn on

When 19V rail is shorted, usually 1st dc-in mosfet is also shorted. Before troubleshooting charging IC, i think mosfet should be checked in resistance mode(without any power source) between
pin1-4
pin1-8
pin4-8

[piernov]

Re: Dell Precision M3800 won't turn on

OP already said it is shorted, no point in wasting time elsewhere.

[TopPop]

Re: Dell Precision M3800 won't turn on

Prior to this laptop mosfets were something I'd heard and read of but not actually identified and probed so this is my first experience working with them. The nice thing about this board is I have two identical mosfets on the board and another M3800 laptop that will power on and POST ok though for some reason the second M3800 doesn't charge its battery. So even more identical model mosfets to look at if I need to. Initially I thought PQ200 was shorted but then suspected it wasn't always that simple to figure out so I went to Youtube and found a video of someone checking 8 pin mosfets on a laptop motherboard. I did what he did, being sure there was no residual charge and checked the diode that is built in, and found my PQ200 was still not behaving right and concluded it really is shorted. However, in my lack of experience I still might have missed something so went ahead with mcplslg123's request.

With the black probe on the first pin and red on the second and meter set at 200 Ohms

PQ200
pin1-4 1 Ohm
pin1-8 0.5 Ohm
pin4-8 1.2 Ohm

Looks like the source, gate, and drain are all shorted.

PQ201
pin1-4 1.1 Ohm
pin1-8 not connected
pin4-8 not connected

Do I have a partly shorted PQ201? I guess I missed that earlier or maybe it is a new short that recently developed.

A couple days earlier I ordered some identical replacement mosfets from China through eBay. I tried looking for a replacement from a couple USA suppliers but couldn't find anything that matched just right. Besides that, I'm a noob and wasn't sure I should be daring to use substitutions for power mosfets in a laptop at this point.

[mon2]

Re: Dell Precision M3800 won't turn on

The mosfets can often be replaced by others that are available from Digikey / Mouser / Arrow in North America. For example, TI and other brands can work here. If there is an interest, can check around..before Magnum PI starts.

On the mosfet checking, you can check the resistance of the points as follows.

Meter in resistance mode. Remove ALL power.

One meter probe to one set of pins; other probe to the other side.

What is the resistance ? If you see 'OL' = over limit, so select the next higher meter resistance scale...unless you have an auto-ranging meter.

If you see a low resistance reading, the mosfet is shorted and must be replaced.

A good mosfet should be hundreds of kilo-ohms in resistance.

So as per above, check:

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)


Post each measurement AND the units of measurement shown on your meter face.

Here is a thorough webpage on the mosfet checking with resistance mode:

https://electronicsbeliever.com/how-...-is-defective/

scroll down to the resistance chart / table method.

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Re: Dell Precision M3800 won't turn on

I had a whole thing typed out and lost it to a request to log in. So here's the abbreviated version. I've probably been typing too much anyway.

PQ201

source (1-2-3) & drain (5-6-7-8) mm set at 2000M, 12
source (1-2-3) & gate (4) mm set at 200, 1.3
gate (4) & drain (5-6-7-8) mm set at 2000M, 12

I tried the 20M and lower ranges but got nothing.

Attached is a pic of the multimeter in case that helps inform every one of what I'm working with. I don't trust the 2000M range setting. If I touch the probes together I get a 9 and if I touch them to my thumbs I get a 13.

[TopPop]

Re: Dell Precision M3800 won't turn on

@mon2, yes that'd be great if you could recommend a substitute mosfet. I tried looking at the datasheet for the mosfet that is on the board but wasn't sure what specs were important to match and what were OK to have a bit different.

[mon2]

Re: Dell Precision M3800 won't turn on

From the shared measurements, this mosfet does not appear to be shorted.

BTW - for your meter, if you see the left side '1' = the meter scale is too low to measure the device under test. Then select a higher resistance scale.

'1' = over limit for your meter.

Proceed to perform the same measurements on mosfet PQ200.

Also, check the resistance to ground of pins 5-6-7-8 (drain) of PQ201. Need to confirm that the main power rail is not shorting to ground.

[mon2]

Re: Dell Precision M3800 won't turn on

Here are some options to replace this mosfet:

LCSC (Shenzhen, CN) - we have used them often over the years. They are like a Digikey / Mouser out of China.

https://www.lcsc.com/product-detail/..._C4355053.html

Digikey (USA)

https://www.digikey.com/en/products/...AO4576/3603553

https://www.digikey.com/en/products/...AO4354/3603552

[mcplslg123]

Re: Dell Precision M3800 won't turn on

Prior to this laptop mosfets were something I'd heard and read of but not actually identified and probed so this is my first experience working with them. The nice thing about this board is I have two identical mosfets on the board and another M3800 laptop that will power on and POST ok though for some reason the second M3800 doesn't charge its battery. So even more identical model mosfets to look at if I need to. Initially I thought PQ200 was shorted but then suspected it wasn't always that simple to figure out so I went to Youtube and found a video of someone checking 8 pin mosfets on a laptop motherboard. I did what he did, being sure there was no residual charge and checked the diode that is built in, and found my PQ200 was still not behaving right and concluded it really is shorted. However, in my lack of experience I still might have missed something so went ahead with mcplslg123's request.

With the black probe on the first pin and red on the second and meter set at 200 Ohms

PQ200
pin1-4 1 Ohm
pin1-8 0.5 Ohm
pin4-8 1.2 Ohm

Looks like the source, gate, and drain are all shorted.

PQ201
pin1-4 1.1 Ohm
pin1-8 not connected
pin4-8 not connected

Do I have a partly shorted PQ201? I guess I missed that earlier or maybe it is a new short that recently developed.

A couple days earlier I ordered some identical replacement mosfets from China through eBay. I tried looking for a replacement from a couple USA suppliers but couldn't find anything that matched just right. Besides that, I'm a noob and wasn't sure I should be daring to use substitutions for power mosfets in a laptop at this point.

PQ200 is clearly shorted and needs to be replaced. PQ201 is not shorted.However, you can re-measure resistance on PQ201 after removing PQ200 and that'll prove that PQ201 is not shorted.

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Re: Dell Precision M3800 won't turn on

I figured the left sided 1 basically meant OL.

Right, PQ201 sort of is shorted but the short isn't in PQ201. The source to gate short is most likely in PQ200 and I'd have to separate them to be able to check for sure.

PQ200

source (1-2-3) & drain (5-6-7-8) mm set at 200, 0.5
source (1-2-3) & gate (4) mm set at 200, 1.1
gate (4) & drain (5-6-7-8) mm set at 200, 1.3

The drain of PQ201 is not shorted to ground but it would have been if I hadn't replaced PC205 earlier as I mentioned in my opening post. The mm showed a left 1 until I set it 2000M and then I got a 10. So, basically, no short there.

mon2, thank you for the substitute mosfet recommendations.

If I replace one mosfet with a substitution would it be better to replace the other too? I ask because they work as a pair and I thought maybe the one that is a substitution might behave slightly different than the other.

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Re: Dell Precision M3800 won't turn on

I replaced PQ200. Now the laptop will turn on, shows a power button light, then turns itself off after about 4 seconds. So, some progress was made but there is still something wrong with it.

[TopPop]

Re: Dell Precision M3800 won't turn on

PU200

REGN is 6V

ACOK is 3.3V

ACDRV is 25V