Thomson 50UD6306 (TCL) Backlight Issue, starts then stops, led changed

[Cornito]

Hello Team,

I'm back with another backlight issue that I thought was simple, but it turned out to be more complex than expected.

Let me provide a brief history: The TV was working well, then one day, the backlight started and shut down. I thought I had an LED issue leading to this behavior, so I replaced the LEDs with brand new ones. However, the issue remains the same.

Next, I replaced the electrolytic capacitors attached to the LED power supply, but the results were the same. I then replaced the driver, a OB3365RP, and did it twice due to the same result.

Using an LED tester, everything seems good, with 73.5V per line (2 lines on this TV). The power supply delivers around 83V per line, but there is no backlight, or just a brief backlight at startup for about 1 second.

The only way to have a kind of working backlight is to use my multimeter in resistor mode and test between the return of the LED and the chassis.

Regarding the main capacitor in the hot area, the voltage is 320V, with a brief spike up to 400V. Could this be the root cause of the issue?

Before buying a new power supply, do you have any advice on how I can resolve this backlight problem?

Thank you for your assistance.

Best regards,

[CapLeaker]

The LEDs are current controlled, not voltage controlled. The OBwhatever number IC is detecting a fault somehow and turning the backlights off. Download the data sheet and it will tell you how it works and what voltage range you need to see on what pin. I would start with the enable, ovp / uvp and the current sense pins.

[Cornito]

Hello Capleaker, thank you for the advice.

Do you know where i can find the data sheet with correct level of detail ?

I'll need to change the pull up resistor to correct this protection ?

[CapLeaker]

Look at alldatasheet.com OB3362RP from On Bright.
i think it is a OB3362F datasheet, but that doesn’t matter much, just different casing still 16pin

[Davi.p]

???... there's no minimal reason that it can reach 400v on hot side with no PFC present.. sounds like a joke...maybe a joke of your MM

[Diah]

Hello Team,
The only way to have a kind of working backlight is to use my multimeter in resistor mode and test between the return of the LED and the chassis

this tell you have one SMT R open / or changed Value at the BL circuit, the SMT R are located at the downside of the Power Boards

[Cornito]

Thank you all for the advice.

Combining the advice received, the potential root cause may be a faulty resistor in the protection circuit. This is linked to pin 3 according to the TDS.

Davi.p, the 400V appears very briefly when the backlight starts before being shut down.

I'll check the resitor values and let you know.

[nomoresonys]

Did you test the new leds before and after installation, sometimes they arrive faulty, maybe go back and check over your work. Dirty test for electrolytic caps if no capacitor tester at hand, can use a multimeter set at about 20k ohms, tv UNPLUGGED, reading should gradually rise or fall, switch leads and should go the other way. You will know if you do the test correctly by the readings, can try it on a good cap that isn't installed also.

[Davi.p]

400v does not exist in a circuit without PFC.. check your tester...

[Cornito]

Back on this story, I found for 12$ a PSU on Aliexpress, I bought it and it works.
Unfortunately it's not exactly the same as mine, a compatible model anyway, so i'm not able to measure and compare values to find the issue on mine.

I'll try with a colleague to check the 2 remaining possible component that may have failed => MOSFET and resistor on control pin.

I'll keep you updated

[nomoresonys]

Glad to hear it, for 12 dollars vs hours of troubleshooting, I'll spend the 12 dollars every time.