Wednesday, September 2, 2009

Thinkpad T40

Customer arrives with IBM Thinkpad T40 stating that it will work from a charged battery. But it will not work with known good AC Adapter.

Works on battery, will not work on AC Adapter

The problem is caused by 2 things in this case. First the Toshiba TPC8014 MOSFET was reading bad when I checked it with a meter. After I replaced it, the unit was only supplying 10.4v to the charge section of the board. Which is not enough to trigger the adjacent mosfet to supply power to the charge section. So then; checking the Fuse [F2] found that it was blown (open circuit), most likely by the MOSFET being shorted. click on picture to see larger view

Replace MOSFET TPC8014, replace fuse.

30V N-Channel MOSFET ~11a max rating

Exact Replacement:
Toshiba TPC8014
comment: The funniest wording of what happens under constant use.
Using continuously under heavy loads (e.g. the application of high temperature/current/voltage and the significant change in temperature, etc.) may cause this product to decrease in the reliability significantly even if the operating conditions (i.e.operating temperature/current/voltage, etc.) are within the absolute maximum ratings.
To me this translates as. If this MOSFET is used continously under heavy loads it will stop working or properly (like: your computer being ON all day). In technical terms "It Will Break".

Cross Reference:
Fairchild FDS6690AS

Fairchild Semiconductors FDS6690

When replacing the MOSFET, be careful not to use too much solder or you may short exposed pads under the chip (This is a typical practice of the Chinese board manufacturers to keep unskilled techs oblivious and discourage them from repairing at the component level. Computer techs eventually buy another motherboard where electronic techs fix them). Again, this is my opinion because that is not a test point under the chip and you do not have access to it under a normal repair situation. If you are not be skilled enough at micro soldering, you can pre-insulate the holes with a small piece of mylar tape to prevent solder from bridging the holes before replacing the MOSFET.

Since it was an easy solution, I am trying to be as clear as possible with the solution and the diagnosis.


  1. Thanks for the info just done it on aboard and repaired it :)

    Just a quick one for you if you have a baord will not charge and when u try to power it on a battery it just shuts off and turns on repeatidly what woul dyou this is gone.

  2. Hi!
    I had the same problem, and this website really was a life saver! Replaced mosfet and voila! it works!

    Thanks a ton!!!

  3. Do you have any suggestions or comments for replacing an F2 fuse? It's been a long time (10 years or so) since I've done any soldering projects, and wanted to hear from an expert about this. I'm just a young man who hasn't done any hobby work in a while and some guidance would be much appreciated...


  4. First you will need to verify that the fuse is bad with a ohmmeter.

    If you have to replace the fuse, then make sure you have some tweezers, flux, and a small tip soldering iron. If you cannot get that close, you may want to try a heat gun ~500 degrees (no higher). The motherboard will have to be removed from the case, so you don't melt plastic, and don't go above the temp above, because the DC IN connector will melt.

    There is no easy way, just need a steady hand and the right tools.

  5. Much appreciated, and thanks for the quick reply. Cheers.

  6. This seems to align with the symptoms I'm experiencing. Would anyone know where I can buy the MOSFET and fuse for an IBM T43? If you could provide a URL link to someone selling the parts online.

  7. You can get the MOSFET and fuse at Newark Electronics.

    But they do usually sell in larger quantities.
    If not you may have to try small orders for hobbyist.

  8. The fuse is small and I had trouble getting it, so I just took one off of another T40 parts board. You know, the ones you keep that are 'dogs' and no chance of repairing. Like spills and other maladies.

  9. I have similar problem with R50.
    The board looks the same, but i found out thet the cmos batt was empty, and as soon as you plug the power nothing happens.
    On MOSFET that was here replaced is on both sides 16,5V from power supply and on the one on right which connects battery is also on one side 16,5V and on other is battery voltage which is 11,2V. Now i dont know what is the charged battery voltage (batt is rated cca 10,8V).
    So the notebook is dead at the moment and i dont know what to do.
    Any suggestions?
    thanks Dusan

  10. Dušan:
    1:did you check the fuse on the front side?

    2: You should get voltage on both sides of the MOSFET (both of them). The RUN mosfet, and the CHARGE mosfet.

    3: Were the MOSFET's that were replaced the same value as the original ones?

    You should at least see the charge light on the keyboard come on. Also remove the laptop battery and see if it will run on AC only. Other than that I cannot think of much else right now.

  11. Thanks for quick reply, since my post i did nothing with it but i will have to:)
    Another thing i noticed. The bios battery was empty. Could this be the reason that i dont see the charging light?


  12. My T43 is facing some similar issue recently.

    - Laptop works perfectly using the battery
    - When used with the power adapter with or w/o batt, it failed to boot sometime. If it did boot to Windows, it will hang after awhile.
    - The batt can charge when inserted into the laptop
    - F2 fuse is OK

    Suspect issue when the MOSFET, but not too sure
    Seek your advice


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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