Vizio M70-D3 4K with dark spots backlight issue

[Yanleb]

I have a Vizio M70-D3 4K with a dark spots backlight issue. It is using the Y8387248A backlight driver board.

On this model each of the 64 transistor powers 2 LEDs. Using my TKDMR TD3H backlight tester I've determined that the backlight LED strips were the issue, and the backlight driver board was just fine. Some LED pairs required 5.9 volt at 155mA, some required 9.5 volt and some were completely open.

I've removed the LCD panel and once the LED strips were exposed I've tested them again by connecting my LED tester directly on the test pads. To my surprise all, I've said ALL, LEDs were OK. Every single pair of LEDs used 5.9 volt at 155mA and was bright and white.

I've reconnected my LED tester to the connector going to the LED driver board and surprise, the pair I tested was now asking 7.4 volt (instead of 5.9 volt) at 155mA.

I've found high resistance across the connector between the LED strip and the green power distribution board. On the picture I've connected the LED tester to the connector going to the LED driver board. My multimeter was connected using clips to the same trace on each side of the connector.

I used clips because I needed my hands to take the pictures. As you can see in the pictures I've measured 1.6 volt of drop across the connector. Some other ones were showing open circuit. A good connection is supposed to show zero voltage drop.

I've examined the connector terminals under a strong magnifier and they showed black spots due to bad contact. My theory is that the thermal cycles when the TV is on then off were making the LED strips to expand when hot then contract when cold. This has made a lot of wear on the terminals to the point of making bad contact.

Tomorrow I will hard wire the LED strips. I just don't want to clean the terminals as I know the problem will come back pretty soon. I will either solder wires over the connector, or simply remove the connectors and solder wires directly between the LED strips and the green power distribution boards.

And to stop the expansions and contractions due to thermal cycles I will glue down the end of the LED strips to the metal case. That way I will be sure that I will not experience cracked solder joints in the future and that my repair will be definitive.

I will post pictures of my repair later this week.

For reference my LED strips models are the following:
70" FBC A-Type REV03_2016.03.30
70" FBC B-Type REV02_2016.03.30

So, on the Vizio TV sets, don't simply replace the backlight strips without measuring voltage drop across the connectors under load first. Don't do resistance measurements, it will not show the problem. As I said, power on the TV set and measure the voltage drop across the connector.

Now I understand why some people reported to have receive bad LED strips from ShopJimmy. The LED strips were probably good, but the connectors on the green power distribution boards were probably bad.

[Nick's Tvs]

Re: Vizio M70-D3 4K with dark spots backlight issue

Thank you for posting this

[glaquerre]

Re: Vizio M70-D3 4K with dark spots backlight issue

Thanks for this very detailed analysis. I have a Vizio M60-C3 4K with dark spots backlight issue too. I was about to buy a new backlight driver board but instead I will wait for your results.

[Yanleb]

Re: Vizio M70-D3 4K with dark spots backlight issue

Repair finished. I ended up removing all the problematic connectors and as I said hard wiring the LED strips. I've attached pictures of the repair.

All connectors have been removed. All the LED strip ends near the green power distribution boards have been glued to the back panel (only one side of the LED strip, where the connectors were located). Solid wires have been soldered directly on the boards. I even put wires on the pads that secured the connectors for added rigidity.

The result is obvious. Every LED lights up at full and even brightness. I'm so happy to have removed those junk connectors.

I'm pretty sure the problem on these sets are caused by movement in the connectors and premature wear on the contacts due to thermal cycles (expansion due to heat when powered on, then contraction when cooling down). An after thought is that maybe multi-braided wires would have allowed some movement without breaking out. I'll see overtime.

Thanks for reading my post.

[jidping]

Thanks for the solution. I was able to fix the same issue on Vizio M50-C1. Have to solder enamel wires to bypass all the connectors.