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Old 07-10-2021, 01:37 AM   #1
The_Todd
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Default Akai AK5519UHDS TV LED backlight issue driving me Insaine

Hello, so I have a backlight issue thatís driving me insaine.

The tv is an AKAI 55Ē 4K ect ect, tv that turned on but no backlight.

Used a backlight tester and of course, no lights. So spent the good part of 2 millenniums taking off the panel, replacing a single blown LED, testing with my LED tester again, they all turned on, however slightly dim. Put the tv back together and .... nothing....

To make a long story short, the power supply ramped up to the 140-170v to run the backlights but after a second the voltage drops down to 0v, and the tv keeps starting as normal. (And no visible lighting up at all) it does the same weather the backlights are plugged in or not, all standby, power_on, adjustment , backlight ect voltages are all there and normal.

I even went as far as to get another similar power supply to test with, now itís ramps up to 150-180v and stays there (adjustment work too) however no visible backlight. Now In a dark room you can see in the holes in the back the lights are actually on but very dim. And Iím not sure why.....

Now initially I tested each strip Separately and all as a complete circuit and all worked (again, however it seem a little dim) after I replaced the one LED and all the lights were about the same, so why on earth is the backlight not getting to full brightness?

Could the new LED be causing the whole circuit to be dim? Or could there of been some kind of surge that damaged a lot more LEDís? At this point Iím going to assume that the backlight driver is fine and something is going on with the lights themselves, there was some slight burn (heat) marks where the backlight wire was but I test the voltage and resistance of the wire and it seems perfect.

So before I have to pull the bloody LCD off again does anyone have any idea whatís going on? Or have I missed somthing?

On a side question, how forgiving are LED strips and/or driver circuits if I was to pull a set of backlight strips off another tv To test? I mean if it was a few LEDs less or More Would it still work? And could I use a filament 240v globe to test the driver output of would the draw stress the driver too much?

Any help or ideas highly appreciated, still using a 32Ē Samsung plasma from colonial times as Iím poor, 4K would be very nice

Cheers,

The Todd
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Last edited by SMDFlea; 07-10-2021 at 06:11 AM.. Reason: Akai AK5519UHDS
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Old 07-10-2021, 01:38 AM   #2
The_Todd
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Default Re: TV LED backlight issue driving me Insaine

So I got another power supply from
Another tv that had similar output for the backlight, connected it up to the backlight LED circuit, plugged it in and “turned it on” is got around 140 -160v but the LEDs did not light up properly. And I’m assuming as long as you do the math and make sure they are 3v LED the strips can be quite interchangeable

i will also let you know there are 50 x 3v LED lights (5 x 10 LED stips) and each strip is actually 2 x 5 LED strips that connect together.

so 50 x 3v = 150v

so even if it briefly went over voltage to 3.5v each that would be 175v so i don't think that they got fried.
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Old 07-10-2021, 01:46 AM   #3
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Default Re: TV LED backlight issue driving me Insaine

It sounds like you still have issues with one or more LEDs.
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Old 07-11-2021, 09:42 AM   #4
RDC55
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Default Re: Akai AK5519UHDS TV LED backlight issue driving me Insaine

Quote:
Originally Posted by The_Todd View Post
Used a backlight tester and of course, no lights. So spent the good part of 2 millenniums taking off the panel, replacing a single blown LED, testing with my LED tester again, they all turned on, however slightly dim. Put the tv back together and .... nothing....So before I have to pull the bloody LCD off again does anyone have any idea what’s going on? Or have I missed something?
Some good advice after doing LED work is to test the LED's before reinstalling the screen. The TV should power up fine without the panel installed.

What voltage did you get on the backlight tester after replacing the one bad LED?

I agree with diif, and you still have a bad LED somewhere.

It is totally up to you but the consensus on this site is one bad LED means others are soon to follow, so the general rule to follow is to replace with new strips to avoid another teardown in the near future.

And what's the deal with capacitor E301 hanging off the board in the pic in post #1?

A good clear picture of the entire back showing the cable interconnect might be helpful.
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Old 07-11-2021, 10:13 AM   #5
dick_barton
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Default Re: Akai AK5519UHDS TV LED backlight issue driving me Insaine

Are you certain you changed the led(s) for the correct type. I see the power supply says that the leds run at 145 - 156V @ 480mA
I would suspect incorrect replacement led.
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Old 07-11-2021, 04:14 PM   #6
CapLeaker
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Default Re: Akai AK5519UHDS TV LED backlight issue driving me Insaine

Either wrong replacement led, or there is another bad one somewhere in the string.
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Old 07-16-2021, 02:59 AM   #7
The_Todd
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Default Re: Akai AK5519UHDS TV LED backlight issue driving me Insaine

Quote:
Originally Posted by RDC55 View Post
Some good advice after doing LED work is to test the LED's before reinstalling the screen. The TV should power up fine without the panel installed.

What voltage did you get on the backlight tester after replacing the one bad LED?

I agree with diif, and you still have a bad LED somewhere.

It is totally up to you but the consensus on this site is one bad LED means others are soon to follow, so the general rule to follow is to replace with new strips to avoid another teardown in the near future.

And what's the deal with capacitor E301 hanging off the board in the pic in post #1?

A good clear picture of the entire back showing the cable interconnect might be helpful.
Ignore the weird caps hanging off, i temp replaced them just in case that was the issue.

well actually the tester was coming up over 200v and the lights where still very dim. I did check the lights before re assembling, they were dim but ALL working, so i assumed they were going to fine when the board was powering them.

the replacement LED and other around it shine perfectly when tested singularly as well as other ones, same brightness same voltage.
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