Author Topic: Tv PSU LED backlight rail at wrong voltage yet stable  (Read 392 times)

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Offline ThebitwielderTopic starter

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Tv PSU LED backlight rail at wrong voltage yet stable
« on: December 02, 2024, 02:21:56 pm »
Hello everyone, I've been working on fixing a TV that has been gifted to me broken , it kept power cycling so i replaced the smoothing capacitor that was blown to smithereens :-BROKE , I've put in a 47uF 450V one as it was one that seemed to make sense to put in due to the first one not really existing anymore , now my issue is the LCD itself is working as i could see the no signal marker when putting light though it , but the backlight refuses to work with the built in PSU , I've powered the led bars externally with my adjustable PSU and they work , to note is that they only turned on at ~31V , more probing around later i found the PSU even if its marked as 32V 220mA on the label which checks out with the led bar when powered externally but the rail when measured with a multimeter it gives me  ~12V , which at first i would have blamed on a faulty MOSFET for PWM control of the brightness ( NCE6050K as attached below) , but for the sake of it ive tried powering a 12V filament bulb off of it and while it wasn't bright it was definitely providing power and at a stable 12V output and now I'm lost on what could be wrong and how to fix it.

Im not particularly new to electronics but I still have plenty to learn and im just now geting into the nitty gritty of it.

I've also provided photos of both sides of the board (pretty unpopulated i must say :-// ) and close up shots of the 2 visible IC's on the back side.
 

Offline MathWizard

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Re: Tv PSU LED backlight rail at wrong voltage yet stable
« Reply #1 on: December 09, 2024, 08:02:59 am »
It looks like it has a 12V output, the 5V rail is missing, and then the 12V output goes through an inductor, and off into the section w/ the SMD IC, that must control the current to the LED's. I don't see any other inductor, so is that series inductor, the big black one, part of a step-up converter from 12V to 32V ? What is that IC, I'd want to map out that circuit, see if that chip is happy or not, and if the parts around it have a problem.

Or is that IC just doing PWM for the brightness, and it really is only 12V max ??.......Ok it's the BIT3260, a PWM boost converter IC. So what voltage is on pin 2?
Quote
The EA pin controls the state of BIT3260. When EA  2V,
BIT3260 is enabled. And EA  0.8V, BIT3260 enters into
shutdown mode operation.
https://u.dianyuan.com/upload/space/2012/02/16/1329362322-600862.pdf

So with no load attached, what DC voltages do you get out ? The main switching transformer, the yellow thing in the middle, should step down the rectified mains voltage, to about 12VDC after those 2 big diodes. If there was a problem on the primary side, that voltage would be off, or it would go off once a load is on the output.

IDK if your replacement cap is the right size, but maybe it's too small if the output side is lacking current.
« Last Edit: December 09, 2024, 08:16:19 am by MathWizard »
 


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