Dell 200N with possible bad MOSFETS

[cbman]

I have this dell laptop that is a few years old that I bought cheap to try to repair. It has a P4 2.0GHz (non-M) processor and apparently they commonly have temperature problems. It would randomly boot, and I think I may have found the problem.

The Vcore is at ~0.4V and some of the mosfets are getting pretty hot. With power off I test some of the mosfets and three of them appear to be bad. With a multimeter I get 4 Ohms from drain-to-source in both directions. So, I'm assuming that these mosfets are bad. They are the low-side mosfets which have integral Shottky diodes. The part number is FDS6680S and I can't seem to find any in stock anywhere. So, can anyone recommend a replacement?

On another note, is this even worth trying to repair? Is there likely and even bigger problem that is causing the mosfets to fail? Can mosfet failures be intermittent where sometimes it would boot and other times it wouldn't?

Thanks for any help in advance.

[Th3_uN1Qu3]

Re: Dell 200N with possible bad MOSFETS

Measure the mosfets when removed from the board. If they still register as 4 ohms, they are shorted. Look for a datasheet and find equivalents based on case size, Vds, Rds(on), and maximum drain current.

[cbman]

Re: Dell 200N with possible bad MOSFETS

Well, after removing the mosfet things are a little different. When I was testing things before, the CPU was still installed. So with the mosfet removed everything appears OK, except the Vsd bias voltage is like 0.08. However, the Vds bias voltage doesn't register (open-circuit). That leads me to believe maybe the integral Schottky is bad, but I would have thought it would fail as a short which doesn't depend on polarity.

[Th3_uN1Qu3]

Re: Dell 200N with possible bad MOSFETS

Sometimes the CPU can make the power supply appear shorted... if the 'fets really were shorted they probably wouldn't work at all anyway.

Is it one of those laptops with desktop P4s? If so, no wonder it has heat issues. Have you ruled out the CPU? I've only fried one P4 in my life and that was intentional (i let it run at 2 volts for about an year, darn thing just wouldn't die, then it finally did).

0.4 volts Vcore just isn't right. Do you have an oscilloscope? If you can see a switching waveform on the mosfets then most likely there's nothing wrong with them.

[cbman]

Re: Dell 200N with possible bad MOSFETS

Yeah, it's a desktop P4. When it would still start up one time I ran prime95 for a few minutes just to see if the extra stress would cause any issues, and it ran fine. Now it won't start at all. The only thing that happens is the CPU fan comes on and the mosfets' heat spreader get hot.

Unfortunately I don't have an oscilloscope. I may have been measuring voltage on the wrong side of the inductor.

I'm still thinking that can't be good that I was getting 0.08 on "diode" setting on my meter from source to drain.

[severach]

Re: Dell 200N with possible bad MOSFETS

0.4 volts vCore must be a measurement error. The voltages in the ADP3180 VID table range from 0.8375 to 1.600.

[cbman]

Re: Dell 200N with possible bad MOSFETS

I figured out that my core voltage is 1.42V, which sounds a lot better but is probably lower than it should be.

The CPU fan spins and the DVD drive spins, but I don't get any lights or anything on the display. I've tried an external monitor and nothing on there either. Although, the CPU fan does not spin until it gets hot.

There is one possible clue as to what the problem might be. Several times when I tried to run memtest86 from a CD it would eventually freeze up and display incorrect ascii characters. The bar that shows progress uses "####" character and whenever it would freeze in memtest some of the character would be incorrect. So, maybe that rings a bell for someone.

I wonder if a failing processor could be "intermittent"?

[Th3_uN1Qu3]

Re: Dell 200N with possible bad MOSFETS

I wonder if a failing processor could be "intermittent"?

Well, it did happen to me. The P4 i fried would power up on another motherboard, but it would trick that one that the BIOS was broken... It would reload the BIOS when the right CD was supplied, reboot, then go all over again. Another CPU worked just fine.

[cbman]

Re: Dell 200N with possible bad MOSFETS

Well, from what I have read the Vcore should be at 1.525V but it is running at 1.42V.

Here's the scenario I'm guessing might be the problem. The "bad" CPU is drawing more current than it is supposed to and that is causing the Vcore regulation to be low, and it is also causing the mosfets to run hot since they are working too hard. Does that hypethesis sound reasonable to anyone?

I think I'll just try and get my hands on a CPU to test.

[cbman]

Re: Dell 200N with possible bad MOSFETS

I tried another CPU that I bought on ebay for $6, and I'm still having the same issues.

CPU fan comes on and CD-ROM drive spins, but no power light and nothing on the screen. The Vcore mosfets and inductors get hot. My Vcore is now running at 1.46

[cbman]

Re: Dell 200N with possible bad MOSFETS

Does anyone have any ideas where I might be able to go from here? I'm kind of out of ideas at this point. I'm debating whether to smash the laptop into pieces or sell it for parts on ebay.

[shovenose]

Re: Dell 200N with possible bad MOSFETS

smash it: thats my way of doin stuff. i dont recommend

[cbman]

Re: Dell 200N with possible bad MOSFETS

I haven't smashed the laptop yet, so I though I would post some observations and see if anyone has any possible suggestions.

As I mentioned before the power light doesn't come on.

There were a few times where I plugged the power cable in and the CPU fan and CD-ROM drive started spinning immediately without pressing the power button.