Electronics > Repair
Tv PSU LED backlight rail at wrong voltage yet stable
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Thebitwielder:
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.
MathWizard:
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.
--- End quote ---
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.
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