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What would cause over current on a backlight circuit?

so the un65j6200afxza power supply….. dead set, no backlight, no standby. unplug main board…. proper voltages, no backlight. unplug backlights, proper voltages, no standby. im looking at the power supply at this point. did backlight check with direct current and they lit up, so power supply troubleshooting starts. checked live and cold for shorted diodes, transistors….all good. caps… good. focused in on backlight circuit of main board and am reading over 300v on a 220v rated output. I removed the mosfets from the board and they test good. dont have an esr meter but pulled caps and they seem to be in spec. im not finding what the issue is. is there a give away here that im missing? any help is appreciated.

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late post wow…sry.

Final Resoulution: I replaced the main board and it worked like a charm.

Reason for this: Once tearing the set apart and getting the backlights to magically come on and working…..voltages were fine going to leds and main board. once plugging in the main board, voltages would drop to leds and main board voltages were fine up to the lvds out to tcon. There wasnt a schematic i had so i just used the printed readings at vdd and 12v in on the tcon from the main board. all voltages were present and correct from power supply to main, and somehow got lost from main to tcon. So, i decided to give that board replacement a go and it worked in this case.

Hope this helps, and thank you oldturkey and others that were coaching.

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Thanks I think it has an issue to do with the dimming function on the main board.

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Hi @jostewcrew ,

I would try a different tack.

You may have 2 problems

1. why is there no standby light when the power is connected?

There should be approx 8V on the standby lead from the power board to the mainboard. Is this correct? If it reads 13V then the power board thinks that the TV is On and is providing full power

2. Agree that with Samsung LED TV when the mainboard is disconnected from the power board then backlights should turn on if backlights connected and power on the powerboard.

So assuming that the backlights are connected to the powerboard and this I don't know if it is the case with your model or not, then there is an open between the backlights and the power board.

Reading 300V is indicative of a "no load" output on the LED driver. If the LED circuit was there you would most probably read 220V.

just some thoughts

Update (11/14/2019)

Hi @jostewcrew ,

Here’s a link to the Un65j6200afxza-troubleshooting-manual.

It gives the Power Up sequence testing which should help to narrow down the problem further, at least to a board perhaps.

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Well, i would say the power board is fine then. In a no load circumstance, there is 13v on standby, i have all my 12v lines to mainboard and 300v on both lines to backlight. The issue though is all voltages drop when both backlight and main board are plugged in. Being the cases previously mentioned in last post.... My open has to be a shorted led. I still get no stby with backlight unplugged, even with proper voltage, but the main boards optical drive lights up indicating life. Usually it one thing or another but this one is trolling me a bit

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Hi @jostewcrew ,

You say " The issue though is all voltages drop when both backlight and main board are plugged in"

What happens to the backlight voltage when the mainboard is unplugged?

It should drop to 220V and the backlights should be on.

They're not and that is one problem.

The second problem is why is there no standby light when there is 8V on the standby lead and that problem may be in the mainboard.

As I suggested perhaps 2 different problems

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ok so i took a different tack and started troubleshooting two seperate symptoms. First, no backlight when mainboard unplugged: I broke down and thought my gut was right ( which was wrong) and took the panel apart to get to the backlights. all strips and bulbs are fine. put back together and voila...backlight works with no mainboard attached. plug in main, lights go out....unpug main lights come on. Voltages were indeed brought down to closer 220v rating when finally loaded down by led strips. First symptom solved, but may present itself later as an issue as i have no clue as to why they work now. ( faulty harness?)

So, second issue , no standby begins: Unplug all periphrials except for mainboard connected to power supply. Get first initial red light of life at standby, then five flashes to nothing. Plug each periphrial ( tcon, speakers..etc) and get the same 5 or 6 flashes depending on if you count the first initial flash as power is applied.

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i have inspected the main board for any obvious signs of broken joints or blown caps or anything that would stand out. nothing. started testing caps for possible shorts on the 12v lines...anything that was shorted was supposed to be if im following the tracks correctly. there are a few diodes on the board that are reading forwards and back wards, but theres multiple that do so and not on the same lines. ( i thought that diodes were specifically only allowed flow in on direction?) im willing to replace the main board but just want to be sure its the issue. i dont think its the tcon as i get the same flashes with or without any other peripherals attached on the standby light. any other thoughts?

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So as far as stby voltage.... It does call for 8v but only getting 5v. But it is a steady voltage....im curious as if this is the issue? Everything ive "researched " states to look for a 5v standby however... So not sure. I will look for a schematic tomorrow as i only searched for the board via part number on the sticker. Can't believe i didnt think to search for the layout. Keeping my fingers crossed.

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jostewcrew さん、ありがとうございました!
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