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Old 02-08-2019, 05:43 PM   #1
Daveatfernie
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Default Hisense LTDN55K321 Purple backlight?

Hi Everyone,
Hot on the repair of my plasma I found a 55" 4K LED backlit hisense tv for £40 on facebook that would be great for a garage TV if I can get it working correctly. The listing said it had a purple tint (and it does indeed).

When I've powered it up and looked through the vents in the rear the backlight can indeed be seen to be purple rather than the usual crisp white I'm used to seeing. Is this a thing? Can all the LEDs change colour like that? I use high power LEDs where I work and am used to seeing a few nm drift in colour when they get hot but to go from white to purple? I Imagine that if I replace the backlights it would give me a nice picture as this is it's only problem but is it something that's seen often?

Thanks in advance once again.
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Old 02-08-2019, 05:47 PM   #2
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Default Re: Hisense LTDN55K321 Purple backlight?

I'm used to HPS going from white to a vivid pink and I suspect that's normal for some HPS light bulb manufacturers and for some, it means a faulty HPS bulb or ballast, LOL.

Purple for fluorescent, like the earlier LCDs, is typical if failing or a bad inverter.

And for the frosted mercury vapour light bulbs used for street lights and gymnasiums, they're purple when not warmed up. (a.k.a. colour-corrected mercury vapour)

For LEDs, I dunno...
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Last edited by RJARRRPCGP; 02-08-2019 at 05:53 PM..
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Old 02-08-2019, 06:34 PM   #3
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Default Re: Hisense LTDN55K321 Purple backlight?

there are threads here about "purple leds" problem... basically, you need to replace them.
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Old 02-11-2019, 07:47 AM   #4
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Default Re: Hisense LTDN55K321 Purple backlight?

So indeed the back light had turned blue/purple. It seems that there's no real thermal junction between the LED boards and the back panel other than periodic screws to hold them in place. Something I'll remedy if I replace the LEDs.

While the swap out would normally be a cut and dry affair during disassembly I found that there was a strip of adhesive connecting a small part of the front bezel to the LCD panel itself. Had it been all around I'd have been fine as I'd know to expect it but it wasn't and I didn't.

I managed to damage the very bottom right corner of the panel as a result. While the panel is still working there's a small amount of bleed through in this location. I was hoping to give my mother in law the screen to replace her aged 32" Polaroid but I imagine it will be a screen for the kids to use instead. It may be that if I made a frame I could lose the bottom inch of the TV in the frame to hide it.

So note of caution, beware the odd piece of randomly placed tape.
Also the screen on this TV is sooo thin. I assume to minimise loss through the glass but the amount of flex was unreal. I had to remove the panel by turning the TV up on it's side and carrying vertically.
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Old 02-11-2019, 09:54 AM   #5
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Default Re: Hisense LTDN55K321 Purple backlight?

Grab some suction cups from harbor freight or ebay for your next one.
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Old 02-11-2019, 09:55 AM   #6
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Default Re: Hisense LTDN55K321 Purple backlight?

this is not as you thought Purple LED. you have one shorted or open LED which cause the PSU drop down the current to the LED strips so some of them lid in this way. i didn't find you were measure the current out of he LED from PSU Socket. just the eyes judgments
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Old 02-11-2019, 11:00 AM   #7
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Default Re: Hisense LTDN55K321 Purple backlight?

Quote:
Originally Posted by budwich View Post
there are threads here about "purple leds" problem... Basically, you need to replace them.
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Old 02-11-2019, 12:01 PM   #8
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Default Re: Hisense LTDN55K321 Purple backlight?

just for the record, in case some one is searching.... but agree that maybe just a string in this case.

https://www.badcaps.net/forum/showth...t=weird+purple

Last edited by budwich; 02-11-2019 at 12:03 PM..
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Old 02-11-2019, 12:49 PM   #9
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Default Re: Hisense LTDN55K321 Purple backlight?

There is another thread too about purple LED's glow, I just cannot remember whom beside Michale32086.
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Old 02-11-2019, 02:06 PM   #10
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Default Re: Hisense LTDN55K321 Purple backlight?

It was Dannyx with blue leds.

https://www.badcaps.net/forum/showthread.php?t=74697
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Old 02-11-2019, 03:26 PM   #11
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Default Re: Hisense LTDN55K321 Purple backlight?

Over time, the yellow phosphor will degrade and all you have left is blue light, which can look purple too. THAT is the science of it, those that think otherwise need to look it up.
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Old 02-11-2019, 03:48 PM   #12
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Default Re: Hisense LTDN55K321 Purple backlight?

Quote:
Originally Posted by nomoresonys View Post
Over time, the yellow phosphor will degrade and all you have left is blue light, which can look purple too. THAT is the science of it, those that think otherwise need to look it up.
sorry... what are you talking about ? yes Hisense LED wasn't pure white since beginning , they draw black circle over the strips to enhance black level only , also the black some time look little to blue. but this not the case over all links provided over here.(2018-2019 ). in 2017 i had Hisense under my hand and i was very deep searched ( https://www.badcaps.net/forum/showthread.php?t=62117 ) till the LED back light. .. let us talk sense.. not stories of fiction
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Old 02-11-2019, 03:51 PM   #13
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Default Re: Hisense LTDN55K321 Purple backlight?

NO FICTION HERE, Just look it up, it's not rocket science just led science. Pretty basic shit, not hard to understand at all. If you don't know the facts of something try doing some research before getting frustrated and showing ignorance by running your mouth about fiction.

Last edited by nomoresonys; 02-11-2019 at 04:08 PM..
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Old 03-08-2020, 05:20 AM   #14
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Default Re: Hisense LTDN55K321 Purple backlight?

Quote:
Originally Posted by Daveatfernie View Post
So indeed the back light had turned blue/purple. It seems that there's no real thermal junction between the LED boards and the back panel other than periodic screws to hold them in place. Something I'll remedy if I replace the LEDs.

While the swap out would normally be a cut and dry affair during disassembly I found that there was a strip of adhesive connecting a small part of the front bezel to the LCD panel itself. Had it been all around I'd have been fine as I'd know to expect it but it wasn't and I didn't.

I managed to damage the very bottom right corner of the panel as a result. While the panel is still working there's a small amount of bleed through in this location. I was hoping to give my mother in law the screen to replace her aged 32" Polaroid but I imagine it will be a screen for the kids to use instead. It may be that if I made a frame I could lose the bottom inch of the TV in the frame to hide it.

So note of caution, beware the odd piece of randomly placed tape.
Also the screen on this TV is sooo thin. I assume to minimise loss through the glass but the amount of flex was unreal. I had to remove the panel by turning the TV up on it's side and carrying vertically.
Thanks Daveatfernie for the warning, I have the same TV and need to replace the LEDs too (not blue, simply no backlight at all), so I will make sure to look for this piece of adhesive...

Can you please tell me the reference of the LED strips so that I can buy them before opening the TV ? As I don't like to open multiple times or leave it open collecting dust while waiting for the LED strips to come.
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Old 03-09-2020, 01:40 AM   #15
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Default Re: Hisense LTDN55K321 Purple backlight?

In fact I have opened the back of the TV and tested the backlight with GJ2C tester, and the LEDs are working but they are all blue too! The tester shows the same voltage for both LEDs channels, and multimeter shows only 1V coming out of the LED connector on the PSU, so additionally to replacing the LED strips I have to repair/replace the PSU... After a bit of testing I couldn't find something wrong on the board, so I suspect it's the LED driver (LX27901ID) that is faulty, I will try replace it just for fun, the board is only 15€ on aliexpress
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Old 03-09-2020, 07:25 AM   #16
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Default Re: Hisense LTDN55K321 Purple backlight?

Quote:
Originally Posted by guix View Post
In fact I have opened the back of the TV and tested the backlight with GJ2C tester, and the LEDs are working but they are all blue too! The tester shows the same voltage for both LEDs channels, and multimeter shows only 1V coming out of the LED connector on the PSU, so additionally to replacing the LED strips I have to repair/replace the PSU... After a bit of testing I couldn't find something wrong on the board, so I suspect it's the LED driver (LX27901ID) that is faulty, I will try replace it just for fun, the board is only 15€ on aliexpress
Its the Leds no the board. I have encountered this problem a dozen times.
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Old 03-09-2020, 07:59 AM   #17
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Default Re: Hisense LTDN55K321 Purple backlight?

https://www.badcaps.net/forum/showthread.php?t=80139

Yellow filter crumbles away causing blue/purple color.
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Old 03-09-2020, 12:41 PM   #18
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Default Re: Hisense LTDN55K321 Purple backlight?

Quote:
Originally Posted by ducky29 View Post
Its the Leds no the board. I have encountered this problem a dozen times.
But the output voltage stay at ~1V when I turn on the TV with the LEDs disconnected, it should be around 170V normally, so there is a problem on PSU, no ?
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