31.8 VDC Is Not Good Enough To Power Up LED TV Backlight
This Singer 32″ LED TV came in for repair with the complaint of no display but have power with green power LED lighted up. For your information, no display fault can be caused by many factors such as improper dc output voltages, bad main board, corrupted memory in ICs, bad T-con board, bad LED backlight and etc. The fastest way to confirm which section that gave the problem was to shine light directly to the TV screen. If you could see very mild or dim picture this indicates problem in the LED backlight or the LED backlight driver issue. If there is totally no display then suspect T-con IC fail if the power LED light is green color. In the below photo, I could see very dimmed picture thus I knew where the faulty section was.
When the TV cover was removed I saw only two circuit boards ie; power supply board and the mainboard.
The LED backlight driver circuit is located in the power supply board.
Next, I powered up the boards and checked on the DC output of the LED backlight driver and found it was only 31.8 volt DC! From experience, for a 32″ LED TV the dc supply voltage should be higher. In other words, the bigger the size of LED TVs, the higher would be the DC output voltage.
Well, the lowered DC output voltage can be caused by few factors from bad e-caps to faulty driver IC. Obviously I would check on the capacitors first because it was the easiest components to check. I found two electrolytic capacitors were bad (22uf 100volt and 100uf 100 volt)
After the replacement with good caps, I need to make sure it would work fine otherwise I would continue to replace the driver IC or even check on all the SMD components in the corresponding circuit. Ha-ha it turned out to be good the dc output voltage which was 51.7 volt-please see the photo below:
I knew that the TV gonna work fine with the higher DC output. I connected back everything and powered it up and true enough the picture came back.
Conclusion- You can take my experience in this article to apply on other LED TV if those TV happens to have the same fault (No display). If you would like to enhance your LED TV repair experience, I can suggest that you get Kent’s ebook on Smart OLED,LED,LCD TV Repair tips.
This article was prepared for you by Suranga Bandara who owns an Electronics repair shop in Anuradapura, Sri Lanka.
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https://jestineyong.com/replacing-working-components-in-switch-mode-power-supply/
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Albert van Bemmelen
March 28, 2020 at 3:11 pm
Very useful information on checking the backlight voltage level that highly depends on the size of the TV screen.
Robert Samuel
March 28, 2020 at 4:05 pm
Thanks for the tip. Is there a chart or table that gives the DC voltage for different sizes of LED screen size?
Thanks.
Mark
March 28, 2020 at 4:18 pm
Thanks Suranga,
I have a monitor with a similar fault and although we shouldn't guess, I feel pretty confident that I will find faulty caps.
Parasuraman S
March 28, 2020 at 4:49 pm
Very useful information. Thanks for sharing. Very professional fault finding and solution!
Yogesh Panchal
March 29, 2020 at 3:51 pm
Good Job!Suranga
Gassan Ali
March 30, 2020 at 5:50 pm
Great work! How did you test the capacitors?
Humberto
April 1, 2020 at 2:15 am
Great repair Suranga. Good explanations and pictures.