TV LED backlight issue driving me Insaine

[The_Todd]

Hello, so I have a backlight issue that’s driving me insaine.

The tv is an AKAI 55” 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 it’s 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 I’m 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 LED’s? At this point I’m 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 what’s 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 32” Samsung plasma from colonial times as I’m poor, 4K would be very nice

Cheers,

The Todd

[The_Todd]

In case you wanted the tv details

[RayRay]

Could you kindly elaborate on what you meant with "I even went as far as to get another similar power supply to test with"? And no, there's a lot of differences between different TV led strips, they're not interchangeable, and you shouldn't attempt it.

[The_Todd]

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

[The_Todd]

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. 

[RayRay]

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

That sounds a bit concerning. How exactly did you connect this other board? And could you provide a picture of it? I hope you haven't induced any damage to the LEDs by doing this. Also, is there any significant difference between that one LED you've replaced to the others? As for compatibility for LED strips in general, if it's the same number of LEDs and are of the same voltage, they'd prob be compatible from an electrical standpoint, but not always (as their current draw could be different) then there's their physical size that could also be different, but that's a different story of course. IMO, it's best practice to stick with the same kind of strips and not expirement with others.

[The_Todd]

I can’t get a picture atm, but I’m not sure why you are concerned? It bad the same conector as the original board , made sure the polarities are the same and made sure it was the same voltage and amp as original (I repair hisense tv so I have plenty of spare boards) and same for the LED, it was the same
Size, voltage, draw and polarity, I have a box of about 30 different types.

Also the other strips will only be used as a load test. Again, get exactly the same electrical characteristics but from a known working source. As I know LED driver circuits on power board are annoyingly picky and will shut off for any reason.

[robert.rozee]

first photo, right side of PCB there appears to be a 47uF 400v capacitor hanging off the side of the board with the leads almost shorted. this could be you issue. at the top edge of the photo there appears to be part of a white plastic socket ripped off of something - where did that come from?

you could consider bringing the wiring for each strip of LEDs out the back of the tv and powering them from an external power supply. 5x3v = 15v supply required. use 18vand a series resistor for each strip to limit current.


cheers,
rob   :-)

[fzabkar]

According to the user manual, the power consumption of the TV is 108W:

https://hndgwww2.blob.core.windows.net/www2/products/060%E2%80%93audio-visual/tvs/AK5519UHDS%20-%20AKAI%2055%20INCH%20UHD%20SMART%20TV%20IM.pdf

I would power up the TV with the LEDs disconnected and measure the power consumption with an AC power meter. You can buy these for AU$25 or less. The difference between this number and the total rated power would give you the normal power consumption for the backlight.

The rating sticker on the PCB in the photo shows "145V-156V/480mA", so 150V would seem to be the backlight voltage. A simple P=VI calculation should then give you the typical operating current for the LED array.

I expect there would be a low value, current sense resistor for the backlight. Measuring the voltage drop across this resistor should tell you the actual current being drawn by the array.

[The_Todd]

sorry I forgot to say I replaced temporarily some of the capacitors just to completely remove them, as a possible problem.

well ,I didn't actually want to open it again unless 100% sure i needed to. and you know what, it has never occurred
to me to run the strips in parallel, damn good idea, there I was formulating on how to get a 150v low amp power supply.

if i cant fix this issue, i might do that, thanks.

[PKTKS]

Comments above about power boil down to the fact that supervisor of led drivers are CURRENT MODE ONLY

the current is sampled and any different value from expected will cause issues

Common fix is to adjust current back to nominal expected value by tweaking sample  resistor

Paul

[The_Todd]

Is the correct current the 450ma written on the board?
but still at the corect voltage the LED are not getting bright enough?

[PKTKS]

There is no correct voltage

Only correct current...

Usually wo schematics a good
method is to add a pot very carefully
so to tweak the current as far as the
result is ok

Trimming with a smart pot should
give you some clue

Paul

[The_Todd]

Trimmer pot, not a bad idea at all, in the case I can’t find the schematics, any clues on where I may find the resistor. Like next to octocoupling, before/after secondary capacitor ect..

[PKTKS]

Trimmer pot, not a bad idea at all, in the case I can’t find the schematics, any clues on where I may find the resistor. Like next to octocoupling, before/after secondary capacitor ect..

I would start removing those heatsinks...

Searching ALL  supervisor part numbers and datasheets.

90% of cases the datasheet is close enough to the
implemented circuit so to help finding the sensor resistor(s)

My basic clue of this board attached

pretty sure there is more stuff (SMD) on the back side...
most cases (from my exp.) the supervisors are under side.
Heatsinks should house the transistors and/or hot stuff


Need the chip numbers to start.
Paul

[The_Todd]

ah perfect, thank you for your help. ill do some checking and testing and report back.