Samsung QN82Q70RA backlight mystery

[EazyBone]

I picked this up and it had picture but no backlight. I was somewhat familiar with his TV as the LED driver fails a lot on this model. But when I tested the LED driver it had all the voltages necessary. I even had a spare one to try out just in case and that one did not solve the problem either.

I tested all the LEDs before pulling the screen apart but ended up pulling the screen apart anyways to further test them. Every one of them tests at 90 volts with the tester. They all light up. I replaced a couple as seen in the picture because I had extra and I accidentally popped the cap off the old one.

The power board sends the proper 84 volts to the LED driver. It also sends the other proper voltages.

I plugged in the TV with The screen off in the main board plugged in and each of the lights is getting 84 volts on the positive side. Yet none of them light up.
I have even disconnected the main board and none of them light up.

I do not have an extra power board but it seems I'm getting the correct voltages from it anyways so my gut instinct says that's not it but I am not sure.

Anyone have any clue on what I should do or familiar with what might be going on here?

[petehall347]

bad connection maybe

[Storyteller12]

This tv is a pain in the ass. Those connections tend to go bad as mentioned. Also check it's not grounded out from the led strip or that strip that connects all the leds.
sometimes you can disconnect a few strips and power on until you narrow it down to a section/ strip that's bad

[Diah]

I accidentally popped the cap off the old one.

it could be the strips you replaced was i wrong position + - ,
you have on each strips zener diode ... test it in diode mode if one shorted.

[EazyBone]

This tv is a pain in the ass. Those connections tend to go bad as mentioned. Also check it's not grounded out from the led strip or that strip that connects all the leds.
sometimes you can disconnect a few strips and power on until you narrow it down to a section/ strip that's bad

Tried this just now, doesn't work for this situation anyway.

[EazyBone]

it could be the strips you replaced was i wrong position + - ,
you have on each strips zener diode ... test it in diode mode if one shorted.

Each light has its own diode, test all 400 diodes? Shiiiiiiiiiiiiiit. That's seems painful. So far all read OL many to go though which will be little later.

I had a fire TV once where I replaced all the LEDs and the screen was still black. I used led tester and kept them on for a while and wouldn't fix it. Eventually I thought it must be the PSU, EVEN though the PSU gave the correct voltage to the lights. I replaced the PSU and started like a champ. I have no idea why it worked.. any reason this could be that you know?


Still searching for bad connection. Or something.

[Diah]

I had a fire TV once where I replaced all the LEDs and the screen was still black. I used led tester and kept them on for a while and wouldn't fix it. Eventually I thought it must be the PSU, EVEN though the PSU gave the correct voltage to the lights. I replaced the PSU and started like a champ. I have no idea why it worked.. any reason this could be that you know?

when the TV had function at setting "Audio Only" mean the BL chips on PSU this function hold in place forever, you will have correct VCC but BL are off. replace chips will solve it.

about the zener i mean the external one on each strips

[EazyBone]

Okay, what you have circled (squared) there is also that same diode in-between each led making one for every led. Only the inner one is which I should check?

[Diah]

there are no inner one, what you are think its inner, in fact this one out side LOL

[EazyBone]

Every led has one in-between. That same diode is between every single led on every strip. There's 400 lights and there is 400 of these little diodes. Im not at home or id take a pic

[EazyBone]

In the pic there is one between each light. And two in-between the last one

[Storyteller12]

What are you grounding to when measuring the 84v with dmm?

some of these TVs always send the 84v to the leds even when off but it's the ground that's not connected that essentially powers them on.

If the screen is still out measure with ground from the led strip and each led on 1 strip just for good measure. And then measure with ground on the chassis

[KYBOSH]

.... Eventually I thought it must be the PSU, EVEN though the PSU gave the correct voltage to the lights. I replaced the PSU and started like a champ. I have no idea why it worked.. any reason this could be that you know?

PSU voltages reads true but fails under load.
I have heard stories of this happening to due to failing (dead but he doesnt know it yet) components on the board.

[Diah]

In the pic there is one between each light. And two in-between the last one

don't be confused , each LED will have one zener.

[nomoresonys]

when the TV had function at setting "Audio Only" mean the BL chips on PSU this function hold in place forever, you will have correct VCC but BL are off. replace chips will solve it.

about the zener i mean the external one on each strips

Yep, this here is often the issue, it's not called the backlight driver chip for no reason.

[KYBOSH]

A set of these BLs are $150 on Shopjimmy....
Are those grains of sand Samsung uses for drivers straightforward to test and replace?

[EazyBone]

I'll get back to this soon, wife had to leave town and now I can't work on it.

I did find an broken power board for this tv that was in my scrap pile. I had taken stuff off it, going to try and repair it see if there is any difference with a PSU

I'll report all those readings early this week to continue this thread.