How to fix Toshiba C850 Laptop Not Turning On

Motherboard Repair Tips & Tricks – Toshiba C850 Laptop Not Charging, Not Turning On

Introduction

Repairing older laptops can be a rewarding challenge, and today, we are tackling a Toshiba Satellite C850 that is not charging and not turning on. Many customers describe this issue simply as “not charging,” but in reality, the laptop is completely dead. In this guide, we will walk through a step-by-step diagnostic and repair process, including checking common faults and replacing faulty components.

Preliminary Checks

1. Visual Inspection:
   • Ensure the charging port is intact, as broken center pins are a common fault in this model.
   • Check for any visible burn marks, damaged components, or corrosion.
2. Power Input Check:
   • Plug in the charger and measure the voltage at the charging port. It should read approximately 19.8V.
   • If 19.8V is present, proceed to checking the motherboard’s power circuit.
3. Diagnosing the Main Power Rail:
   • Locate the capacitors near the power input.
   • Measure the voltage across them. A healthy main power rail should have 19V.
   • If the voltage is significantly lower (e.g., 0.9V as in this case), then the power is not reaching the system properly.

Troubleshooting Power Input Circuit

Step 1: Checking the MOSFETs

The power passes through two MOSFETs before reaching the motherboard components. The first MOSFET should pass the 19.8V, while the second one regulates further.

1. Measure Input and Output Voltage:
   • The first MOSFET should have 19.8V at the input and output.
   • The second MOSFET should also output 19.8V.
   • If the second MOSFET only outputs 0.9V, it is likely shorted.
2. Confirming a Shorted MOSFET:
   • Using a multimeter, check the resistance of the second MOSFET.
   • A normal MOSFET should not read a very low resistance.
   • If resistance is around 2 ohms or lower, the MOSFET is shorted.

Step 2: Replacing the Faulty MOSFET

1. Desolder the defective MOSFET from the board.
2. Find a suitable replacement:
   • If you have spare boards, look for a similar MOSFET used in power circuits.
   • Since these MOSFETs are not switching types, any compatible model can work.
3. Solder the new MOSFET in place.
4. Reassemble the motherboard and test the input voltage again.

Verifying the Repair

1. Check the Main Power Rail:
   • After replacing the MOSFET, the main power rail should now show 19.8V instead of 0.9V.
2. Test the Laptop with Charger Plugged In:
   • Power on the laptop; it should now respond to the power button.
   • The current draw should be around 700mA when turned on.
3. Check Battery Charging:
   • Insert the battery and plug in the charger.
   • If charging is functional, the orange LED should turn on, and current draw should increase.

Final Thoughts

• Over 50% of laptop motherboard failures are due to power input circuit faults, especially MOSFETs.
• If replacing the MOSFET does not fix the issue, check the BQ chip, which controls power regulation.
• Gaming laptops often have the first MOSFET shorted, whereas in this case, the second MOSFET was faulty.
• Understanding these basic troubleshooting techniques can save money and extend the life of many laptops.

By following these steps, we successfully repaired a dead Toshiba C850 laptop, bringing it back to life. If you’re encountering similar issues, this guide should help you diagnose and fix common motherboard power problems efficiently.

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