Backlight Fix on Vizio E420 LCD TV

There are many, many things I’m saving my money for. Vacation. A wedding. A house. A boat? So a few weeks ago when my living room television began to malfunction, I was quite unhappy about the prospect of spending $400 on a replacement TV.

The TV, a 42-inch Vizio E420i-A0, was purchased a little less than three years ago. It worked reliably until one evening a few weeks ago, after a long day of work, errands and exercise, the middle horizontal 1/3 of the screen appeared faded. The top 1/3 and bottom 1/3 of the screen were as bright as ever, yet the middle was noticeably dimmer. This made watching dark scenes in movies and shows near-impossible (especially if you’ve just convinced your fiancée it’s time for her to binge watch Mad Men).

It seemed evident that it was a backlighting problem after some internet searching and reviewing a TV assembly diagram. Unfortunately there wasn’t much advice for my specific model.

Thanks to CR4, I’ve made some inroads with an electronics expert and DIYer, frankd20, a.k.a. Mr. Bad Capacitors Make Me Smile himself. With a strong electronics background, he’s rescued several TVs from the recycling heap. After quizzing him about how complicated a repair would be, he suggested getting some blog fodder for CR4 out of it as well.

We met up and set about removing the dozen or so screws holding the rear plastic housing that covers the power supply board, which receives power, and the main board, which receives input signals. frankd20 recommended checking out the power supply for loose connections, as power supply problems can manifest in so many different ways. We were quickly able to eliminate the power supply as the culprit for the malfunction.

Since it seemed like a panel or backlight problem, we needed to separate the back panel from the LCD panel, diffuser and prism films assembly. After a few screws and a few hidden clips, we were left with the metal back panel of the TV, with the boards located on its reverse. In the concave side of the assembly, we have five rows of ten ribbon LEDs, and a white background material to help reflect the light forward toward the LCD.

Unclipping the white background required us to remove the boards, which were added back so we could troubleshoot. Plugging in the power supply revealed that of the five rows of LEDs, only two—the top and bottom—were illuminating. The three rows running center were all out as they were connected in series. Looking at the LED layout online, I had guessed maybe that only the middle row was out, but had I investigated the wiring pattern better, I could have expected a larger outage.

Each individual LED required three volts, so a row of 10 LEDs required 30 volts. However, each row was made of two LED ribbons of five LEDs each. This actually worked to our favor, as all we had on hand was a spare 12 V power supply to test with. I shorted the left LED strips at the connection with a quarter while frankd20 applied power to the terminals. One by one, all LED strips illuminated, except for the last one. The bulb closest to the terminal had failed open, which caused the other four LEDs on the strip, and the other 25 LEDs wired to it, to not light up as well.

This was an opportunity for a part search, specifically LC420DUG-JFR1 LED strips. They are made in five and six LED strips. The ones I required, the five LED type, were out of stock everywhere I looked and also started at $50 for a whole set of 10.

It’s probably for the best anyway—shipping would have taken at least several days, and frankd20 came up with a solution the allowed me to bring the TV back home that day, not a week or more later.

After removing the malfunctioning strip, frankd20 scratched away the insulation between the broken LED and the subsequent LED. A little solder between the two conduits completed the connection, and when inserted back into the LED array, 49 of 50 LED bulbs turned on.

After some minor reassembly, I brought the TV back home where the fiancée was able to resume Mad Men, and I felt as though I saved myself a considerable sum with a little elbow grease and asking for help. Admittedly, frankd20 asked how far I would have got without his help—I reckon I would have gotten to LEDs, but I probably would still be searching for replacement strips. So frankd20, if you’re reading this, thanks!

One more note: two weeks later I reopened the TV, because when we put the LED strips back in, we probably didn’t push down hard enough for the adhesive to transfer the heat from the strips to metal backing. At least I had better confidence on that occasion.