How to Check a TV Thermistor

Checking a thermistor is almost as easy as checking a fuse.

Intense heat can damage sensitive electronics in just a few seconds. To help prevent heat damage, TVs have thermistors installed in heat-sensitive areas. As heat rises, the thermistor loses its ability to conduct voltage until, eventually, it burns out like a fuse. The burned out thermistor shuts off voltage to the TV, forcing it to cool down. This protects more expensive components from damage. Once you gain access to it, it’s easy to check a TV thermistor.

Things You'll Need

• Phillips-head screwdriver
• Flat-head screwdriver
• Flashlight
• Digital multimeter
• Soldering iron

Instructions

1. Unplug the TV from the power outlet or surge suppressor.

2. Remove the screws holding the rear shell or service panel on the TV. Pull the shell or panel off the TV to access the circuit boards inside.

3. Identify the circuit board with the thermistor on it. You may need a circuit diagram or technical drawing of your television to locate the correct board. Locate the thermistor on the circuit board. Use a flashlight to help you see the thermistor without having to remove the board from the TV.

4. Set a digital multimeter to the ohms scale. The ohms scale tests resistance, and is usually indicated with a Greek “omega” symbol, similar to an upside down horseshoe.

5. Place a lead from the multimeter on each end of the thermistor. Check the reading on the multimeter display. A working thermistor should show little or no resistance when cold and, if equipped with an audible alert, the multimeter should beep while you’re testing the thermistor. If the multimeter shows high resistance, the thermistor no longer conducts current properly and you should replace it.

6. Touch a hot soldering iron to the thermistor. Watch the reading on the multimeter. The resistance should increase rapidly as you heat the thermistor. Remove the soldering iron and watch the reading as the thermistor cools. The reading should steadily fall until it returns to the original cold reading. If the reading does not rise while heating, or doesn’t fall steadily while cooling, the thermistor is defective.

Tips & Warnings

• Circuit diagrams and service manuals for many TVs are available online.

• Thermistors are very inexpensive. If you remove one for testing, you should go ahead and replace it with a new unit.

• While the TV is open, you should clean any dust or debris out of the cabinet.

• Before replacing a thermistor, identify and repair the problem that caused the TV to overheat. This is usually a sign of a clogged filter or dust buildup inside the TV cabinet.

• Some components inside the TV can hold voltages even when unplugged. Be careful not to touch any bare metal components or solder traces while you check the thermistor.

References

• Techlore: Circuits of the Basic TV Set and What They Do