Saturday, January 16, 2010

Acer Aspire 3680 (Dead)

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ACER Aspire 3680-xxxx

This unit comes in Wednesday with symptoms: DEAD
Will Not Charge
Will Not Work With Good Battery

Diagnosis begins by following the DC IN from the DC Jack. The voltage would drop to 3v and not enough to power laptop or close any gates on the MOSFETs.

Taking the MOSFET [FDS6675] source side out of circuit shows the direction of the shorted components. Since the power branches to the
charge section or the run section with the other MOSFET shown below it. Checking to voltage at the source shows +19v. When in soldered on the board, the MOSFET DRAIN voltage is +3v. After replacing the FDS6675 the DRAIN side is still shorted.

Further testing indicates larger capacitor shown (in circle picture 1) is also shorted, removing it from circuit voltage on Drain side is now +19v. But the laptop would not turn on.

Still further testing and following the circuit path leads to the top side (facing technician when opened) to a series of three capacitors. Removing one side of this bank and testing each capacitor finds the shorted capacitor. As it turn out it was the topmost cap (marked with X in image), and when removed it was horizontally spit into 2 pieces.


Replace Capacitor.

Unit lights up!
Sorry for the picture orientation, but it appears blogger puts them in reverse sequence


You can read up on how MOSFET works in WIKIPEDIA
But in simpler terms -
There are 3 basic parts of a MOSFET. Source, Gate and Drain.

It acts as a doorway. When power is supplied to the GATE current or voltage crosses to the other side. In most laptops, common MOSFETs are 8 pins.
Pins 1,2,3 are SOURCE (power or current in)
Pin 4 is the gate
Pins 5,6,7,8 is the DRAIN (power or current out)


There are more complex MOSFETs such as Dual Channel with 2 gates, 2 sources, or 2 drain it is large matrix of numbers and combinations. P-Channel, N-Channel and others. Knowing what is what requires a good memory, manual or cross reference index.

20 comments:

  1. I have a Gateway MA2. Adapter tests fine jack test fine. I removed the jack and get 19v at the positive lead but when I solder the jack onto the board I only get .5 volts. I'm lost.

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  2. This is a great site!! I'm trying to fix my wife's laptop. Gateway MA2, I have some soldering experience from workking at Symbol Technologies but never had to trace until now that I cannot afford to buy another laptop and trying my best to revive this one. I bought a spare MA2 motherboard on eBay so I have parts to use.

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  3. Finally narrowed it down to a cracked capacitor. One of those little green ones that look just like the ones pictured on this post. Thanks for your help.

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  4. Great job Gil, I had a slow day Valentines Day and was not checking email.




    I may also be found on Second Life as 'Hoosier Newman'

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  5. You said

    [Taking the MOSFET [FDS6675] source side out of circuit shows the direction of the shorted components]

    What do you mean by that? Do you mean to leave the mosfet in place, but just disconnect the source pins? How does that show the direction of the shorted components?

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  6. @Rob: I raised one side of the chip so that I could test it 'out of circuit' with a meter. then I had two points I could check. Either the pads on the board, or the leads on the chip that were not connected.

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  7. Hi there, just found your blog, it's awesome! reading about so many things I've come across (and some I haven't). I fix laptops as a bit of a hobby but i'm purely self taught so pretty crap at it.. I've got an Acer here that has no power/charge/lights, the socket's fine, and I've traced through one 4835B mosfet, seems OK, 19v eac side, but the second 4835B it comes to is only outputting 10-12V and it's fluctuating about 5 times a second as well as making a clicking sound... I'm looking at getting a replacement 4835B but just thought I'd see if you think that's a good idea or I'm wasting money... there's another 4835B near the battery charge point, could I pull that and test it first to make sure it IS actually the 4835B at fault? All the caps seem OK but I haven't taken them out. Do bad mosfets normally tick?? Thanks so much - Craig

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  8. Craig: that pulsing is a cap gone bad somewhere in the circuit, or feeding the 4835. If you have a meter, check the ceramics caps in the area and you will find a short. Another tip is to use a Crayola crayon. The crayon will melt when it gets to the shorted cap. Or you can use your fingers until you find one too hot.

    Hold that helps

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  9. Hmm nope, I tried pulling the first ceramic cap and replacing it but that made no difference and none seem to be heating up at all. Will advise once the MF's arrive. Thanks ":)

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  10. Well this is odd... replacing the mosfets made no difference so I gave up on the unit.. no sooner had I done so that I received another notebook (Toshiba P200) with the exact same issue.. no power, faint ticking sound (not as loud as the acer but definitely there). This one isn't in as good condition as the acer was, heavily used, I've done a cursory inspection of the caps and other components and all seem ok. Any idea?

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  11. Great pic with mosfet function

    http://circuits.datasheetdir.com/95/FDS6675-pinout.jpg

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  12. Sir i got 1 motherboard acer aspire 3683 the problem is like your picture in circle the capacitor there is too hot. Also my power adapter is blinking. What the problem need replace new ceramic capacitor or check other component. The power drop to 2.0v from 19v after i plug in the power adapter

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    Replies
    1. Replace the cap. If it hot is most likely shorted.

      You can test the power section by removing all 3 caps and see if the power light stops blinking. But do not try to run or operate the system. It is only a test to see if the short is solved. The caps are needed. Do Replace!

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  13. Hello, how do you test for shorted SMD capacitors?

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  14. You can test them with the capacitance setting on Most FLUKE ohm meters, or you can use a basic Ohm Meter and if it is short, (0 ohms) will show. If it is working should be INFINITY.

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    Replies
    1. Can I do that in circuit? Without desoldering them off the board?

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    2. Yes- but if you get strange results on a ceramic cap, it would be best to take out of circuit. Reverse the leads also when you test. One direction will give you infinity, the other will show some resistance. If infinity both directions it is open, if 0 ohms both directions, it is shorted.

      Delete
    3. Thank you kind sir for responding to me, I've been working on this Acer and the adapter light blinks, so I assume a capacitor it's shorted.

      Also, what if one direction is infinity and another direction show some resistance, does that mean the capacitor is good?

      Delete
  15. Another way to find shorted ceramic caps. If your fingers are not finding the overheated caps, you can use a crayon. If a cap is shorted and overheating it will melt the crayon. You can also test this method on MOSFET shorts. I used this shortcut since the Commodore 64/Vic-20 days to find shorted memory chips.

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I cannot repair laptops for you via the internet, I can only answer questions related to posts. This is because I have not had experience on every motherboard that is out there. Bear with me.

If you do need it repaired. Contact me for quote (US/CDN Only)

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