LCD Inverter Test - Laptop Testing for Backlight or Inverter Failure

Computer Repair with Diagnostic Flowcharts

I thought about non-invasive ways to test a laptop inverter for years before I finally came up with this test. Before you begin, make sure that your laptop LCD doesn't have an LED backlight, because this test is for the CCFL backlight and inverter combination. The test takes advantage of the fact that inverters are RF emitters, so by testing for the presence of an RF field in the proper frequency range, you can determine if the inverter is operating. My first attempts to test the LCD inverter were done with a cheap Harbor Freight meter,which has the advantage of a large display. Please not that the multimeter probes never touch the laptop or the inverter in this test, I'm just holding them close to pick up the field.
	LCD inverters that drive the CCFL tubes used for backlights and PC modding but out around 600 volts in a frequency somewhere in the tens of kilohertz (KHz). The particular meter I started with is unfortunately limited to 20KHz, but my hope was it would actually see RF to about twice that frequency (typical for measurement equipment) and just report overscale. But in my first attempt, with the probes held with a small air gap about a thumbs width from the inverter output, it measured 11.76 KHz! That's shown in the picture to the left. By not allowing the probes to touch the inverter outputs during the test, you eliminate the possibility of the probes touching each other while on the output and shorting out the inverter.
I knew the 11.76 KHz was an odd measurement, so I moved the probes closer to the inverter and as the picture to the right shows, the meter reported a "1" in the left significant digit, which means the reading is overscale. So even this crude test is much better than the idea of setting your multimeter on the highest voltage scale and seeing it the inverter output will bump the reading up for a fraction of a second before it shuts down, a good way to ruin either the inverter or the multimeter. While it's been a good two decades since I worked with network and spectrum analyzers testing RF networks and antennas, some of it began coming back to me while checking different inverters.
	After confirming that the meter could pick up the RF field from the inverter, I decided to try it without taking the plastic bezel of the screen, a truly non-invasive test. Because the field is weak and the meter isn't designed for the frequency range, it took me a few minutes of searching to find the hot spot where the inverter was located on a different laptop. But sure enough, as the picture to the left shows, I picked up a field from the live inverter right through the plastic. In the picture below, when I homes in on the exact location of the inverter output and got the separation between the probes correct, the meter again went overscale, showing that the inverter output was higher than the meter could measure.
But when I tried to expand my test across a greater variety of laptops and LCD screens, I ran into a problem with my cheap meter not picking up anything at all. That even included a PC modding tube and inverter that I keep around as a plug in test. So I tried it again with a Fluke 110 true RMS meter. Sure enough, it showed that both of the Toshibas I used for test laptops had inverter outputs in the low 30 KHz range, overscale for the cheap meter but within the 2X bandwidth multiple, one octave. Higher quality multimeters are useful for testing up to 100 KHz, which is higher than I've seen for and LCD inverter outputs.

	The great thing about the non-invasive inverter test is that once you learn the inverter location on various laptop models, you can save a lot of time on taking them apart and swapping parts. And the test has advantages over using a generic inverter for testing the backlight, which may not strike the plasma if it doesn't ramp up properly. If the inverter tests good, either it's the backlight, the wiring to the backlight from the inverter, or a more serious display problem. Rather than guess at the air gap between the probes for your multimeter, you can use a little piece of insulation, like a torn business card, or design a loop probe. But at the frequencies used, the wavelengths are so long that the gap is all that really matters.
The Laptop Repair Workbook is 100% focused on troubleshooting laptop hardware and helping you make cost effective decisions about repairs or work-arounds. it includes an introductory section on laptop display technology so you'll have an idea what's going on, and an advanced diagnostic flowchart for LCDs. You can purchase the 191 page printable eBook version of The Laptop Repair Workbook for instant download anywhere in the world. It's cheaper than a multimeter that's capable of measuring high frequency fields, and you won't have to wait for the postman to bring it to your home.

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