Toshiba Satellite C850 Motherboard MOSFET Repair

ID: 219948

Description: [youtubevideo|W7sPTIJKgXk]Use this guide to...

Steps:

  1. Shut down the laptop, unplug the charger, and remove the battery before exposing the motherboard.
  2. Hold the power button for a few seconds to drain residual power from the board.
  3. Wear safety glasses and an ESD wrist strap while handling or soldering the motherboard.
  4. Connect power only during live voltage checks, and disconnect the charger and battery before soldering or moving components.
  5. Connect the laptop charger and set a multimeter to DC voltage.
  6. Place the black probe on motherboard ground and the red probe on the positive charger input point.
  7. Confirm that the charger input measures about 19 V.
  8. If 19 V is missing at the charger input, troubleshoot the charger, DC jack, or input wiring before testing the MOSFETs.
  9. Keep the probes steady so they don't short nearby pins or pads.
  10. Disconnect the charger and battery, and expose the motherboard enough to access the DC-in power area.
  11. Follow the 19 V path from the charger input to the first small N-channel MOSFET.
  12. Use this power-path map as a reference: charger jack → first N-channel MOSFET → second N-channel MOSFET → system and battery switch.
  13. Identify the MOSFET drain, source, and gate from the board markings, part markings, or datasheet before measuring.
  14. Reconnect the charger only, and leave the battery disconnected unless you are testing battery switching.
  15. Measure the voltage on the MOSFET control terminal and on both power terminals.
  16. A fault found on this board showed 19 V on one side of the MOSFET and only about 0.93 V on the opposite source or drain side.
  17. An N-channel MOSFET must receive the correct gate drive before it can pass voltage from drain to source.
  18. Suspect the MOSFET or its gate-drive path if 19 V reaches the MOSFET but the output side stays near 0.93 V.
  19. Disconnect the charger after taking the readings.
  20. Use a temporary jumper only as a short diagnostic bridge, and never leave a bridged MOSFET in service.
  21. If bypassing the first MOSFET lets the laptop run from the charger only, the charger-to-battery switching path is still faulty.
  22. Test the second N-channel MOSFET in the same power path because it controls switching between the battery and charger.
  23. Replace the second N-channel MOSFET if input voltage reaches it but the board still won't switch correctly between charger and battery.
  24. This small N-channel MOSFET is a common failure point in Toshiba Satellite C850 no-power and no-charge faults.
  25. Disconnect the charger and battery before heating the board.
  26. Apply flux around the faulty N-channel MOSFET.
  27. Protect nearby connectors and plastic parts with heat-resistant tape if they are close to the work area.
  28. Use a hot air rework station to heat the MOSFET evenly until the solder melts.
  29. Lift the MOSFET with ESD-safe tweezers only after the solder is fully molten.
  30. Use a soldering iron and solder wick to clean old solder from the pads.
  31. Use a matching N-channel MOSFET with the same package, pinout, and suitable electrical rating.
  32. A compatible Toshiba 1650 donor board can supply a working N-channel MOSFET if the markings and package match.
  33. Apply flux to the pads and place the replacement MOSFET in the same orientation as the original.
  34. Use a soldering iron and solder to tack one pin, then align the MOSFET before soldering the remaining pads.
  35. Use hot air to reflow the pads evenly if the package needs it.
  36. Clean the area with isopropyl alcohol and a lint-free swab.
  37. Inspect the pads for solder bridges, missing joints, or shifted pins.
  38. Connect the charger and measure the charger input again to confirm about 19 V is present.
  39. Measure the replaced MOSFET and confirm voltage now passes through the drain-to-source path instead of dropping near 0.93 V.
  40. Press the power button and confirm the laptop starts normally from the charger.
  41. Reconnect the battery and confirm the laptop switches correctly between battery power and charger power.
  42. If the laptop works only from the charger, recheck the second N-channel MOSFET and the switching path.
  43. Disconnect power before completing reassembly.
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