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Old 11-16-2021, 02:12 AM   #1
flashcoder
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Default Westinghouse DWM40F1G1 backlight blinks and turns off

Hello,
I got this TV (Westinghouse DWM40F1G1) that did not came on one day, blinked for a split second and turned off. I looked at it closer with flashlight and figured that the TV actually works, its just the backlight that does not.
There is one PCB board - all of the components are on it: TP.MS3393.P85

I took my oscilloscope and looked what is coming to the MOSFET that is switching current to LEDs - I could see that the PWM signal comes in but it cuts off almost instantly. I thought its the driver chip OB3350CP and so I replaced it.

After that the TV starter working again. For one week. And then exsctly the same problem came back.

I thought OK now I know what needs to be replaced and I replaced the driver chip AND the MOSFET thinking that maybe it gets some kind of "backlash" that kills the driver chip or whatever so I replaced both because I had ordered several of these parts for Aliexpress already. This time it did not help, the TV still blinks the backlight for a split second and thats it.

So I suspected the LEDs themselves, I got a LED strip tester (XY284-GHB) connected it straight to the wire going to LEDs and the backling came on. It takes about 50V and 40mA and stays on no problems. I can turn on the TV and operate it with the LED tester driving the backlight. I tried to lower the brightness setting to the minimum, then plugged in the backlight into the board but that still does not help.

At this point I dont know how to continue. I am guessing the LED driver chip (OB3350CP) mistakenly detects LED short or something and turns off. How do I troubleshoot from this point on?

Could not find a decent service manual for this board, all I could find is this link: https://elektrotanya.com/umc_tp.ms33.../download.html
But the silkscreen designations do not match with schematics.

Any help welcome.
Thanks
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Old 11-16-2021, 02:36 AM   #2
angelos
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Default Re: Westinghouse DWM40F1G1 backlight blinks and turns off

Take photos of the board or boards so that we can also see.

I had a set also shutting down like that and it turned out to be the smoothing cap for that led supply voltage. It would go through a DC to DC converter. The cap was slightly swollen on the top.
Also testing the backlights with a tester is not a bullet proof method. Thee tester do not supply 250mA to thoroughly stress the led strings. Sometimes all you notice is that once the led tester is driving the leds constantly you will see the output voltage displayed on the led tester fluctuates a bit like say 45.7V then 46.2V then perhaps 46.7V. This is an indication that some leds are failing but not always. Another thing that i did to avoid opening up the set but of course i had previous knowledge of the leds inside the panel is that i built a circuit outside the tv with brand new strips and then noticed the leds stayed on so i knew that the leds in the panel were to blame even if the tester got them to come on and stay on. Hope this helps you.
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Old 11-16-2021, 02:17 PM   #3
dskall
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Default Re: Westinghouse DWM40F1G1 backlight blinks and turns off

I had an edge lit with several bad led's while working. Could never get them to fail with the backlight tester. Majority of backlight problems are the led's.
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Old 11-16-2021, 03:13 PM   #4
vinceroger69
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Default Re: Westinghouse DWM40F1G1 backlight blinks and turns off

I have that same meter and also got it from China its actually a good tester does all I need it too.
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