Samsung QE82Q70RAT no backlight

[sansanpartida]

Hi everyone!

Hope you're all having a great day, and thank you very much for stopping by and taking the time to read this post to help me with my parents' Samsung TV.

My issue: No backlight.

Model: QE82Q70RAT
Type No: QE82Q70RAT

The TV was already serviced by Samsung once under warranty for the exact same issue, but six months after the fix, the backlight went out again. Now, Samsung is asking for nearly the same price as a brand new TV to fix this with the same 6-month guarantee.

So hopefully, I can fix this cheaper.

I figured out that unplugging the power board from the LED driver board occasionally made the backlight work again.

My initial thought was that the driver was bad, so I found a cheap replacement, but that didn't fix it.

Maybe the power board is bad, but I believe I've checked all the Vdry and A13V output, and they seem to be fine, so I'm kinda lost.

The PSU board is: BN44-00983B
The LED driver board is: BN44-00991A
The main board is: BN94-13999B

If someone can tell me what to look for or measure, I have all the tools needed for the job. I'm just clueless about what to check.

I've noticed a pin named BLU_ON/OFF on the LED driver board, and it's at 5V when the TV is on.

There is a FAULT pin coming from the PSU board, and it's at 5V. I don't know if it's an active low pin; also, I can't see any visual damages.

On the PSU board, I've tested some dual common cathode Schottky diodes in a TO-220F-3L package; they seem to be fine. STB/ON is at 12.8V instead of 12.35V, Vdry is at 84.7V instead of 84V, and Vamp is at 17.8V instead of 17.1V. I don't think those discrepancies are significant.

If you need something else (high-resolution pictures, measurements, scope traces), just tell me what to do.

Again, thanks to all for considering helping me.

[EazyBone]

I just fixed my 82qn70ra about a week ago, maybe it's American model vs Europe since is basically the same. I had to change backlights, so here's what you need to do.

Get a LED tester, unplug everything from led driver board (except light ribbons) put led tester ground somewhere in the middle of the 70 pin ribbon and then the positive on the first 3/4 pins on top and bottom of that same ribbon. Measure voltage on each one and they should all light up. If one led voltage like runs too high or too low compared to others it's probably LEDs.

Just have to test one by one the led strips from the LED driver

Also try unplugging the PSU to main, not the PSU to led driver to test lights

[sansanpartida]

I don't have a Backlight LED tester... I'll try to get one. This model seems to be really common on AliExpress:

Does it seem OK to you?

In the meantime, I do have a bench power supply, but it will only get up to 30V. Maybe I can get the LED to light up even if it's not full brightness?

[1cvbnm]

iIt's good. I have one too.

[sansanpartida]

One quick question:
I don't have 70 pin ribbon but 68 ones. I also have a cryptic table next each of them, here is a random one (next to CN9001 on the BN44-00991A board)

I believe I have to read the 82' column since this applies to the 82 inches TV model right?
Also, what are those numbers with a minus suffix (-) representing? I imagine + mean the Vdry voltage? (84V)

Here is a picture and a table easier to read.

68	14-	13-
67	17-	14-
66	18-	15-
65	21-	16-
64	22-	19-
63	25-	20-
62	26-	21-
61	NC	22-
60 ~ 49	NC	NC
48 ~ 22	+	+
21 ~ 9	NC	NC
8	NC	10-
7	13-	9-
6	10-	8-
5	9-	7-
4	6-	4-
3	5-	3-
2	2-	2-
1	1-	1-
	75'	82'

[Diah]

wrong way to start diagnose. check the socket on PSU which it goes to MB if there are BL_ON signal >3V

[EazyBone]

I'm not sure what's first, but this is a good solid way for me. Yes the 68 pins, I threw random number out because I didn't remember. And yes that led tester is great, I use it. Never let me down yet.

And yes the 9-60 can be one side of the probe and then measure each pin individually with the led tester the first 8 and the last 8 should all light up a row of LEDs.

I'd definitely start at what diah said since you don't have the tester and can do it before you get a tester.

Also not sure about the power supply, the led tester will tell you if for example (I think mine all tested at 88v per line or something ) each line is using 88volts or if one is using 82 volts or 98volts. So you want each line to read the same voltage.

I knew to check here though because I had a darkness in the front screen and the TV still worked though. This is why I started here. I also had a bad 75 which used the same board and the led driver is what failed, but you have two so figured look at lights/PSU first

[sansanpartida]

wrong way to start diagnose. check the socket on PSU which it goes to MB if there are BL_ON signal >3V

I don't have BL_ON (or perhaps I missed it). The closest thing I have is BLU_ON/OFF, which I measured at 5V when the TV is powered on, as mentioned in my initial post. Additionally, I measured the LED[+] test point on the driver board, and it read 84V.



This suggests a faulty LED strip. However, it's odd that I would expect at least some of the strips to work instead of all the LEDs being out.

30V does nothing, unfortunately. I'll wait for the tester to arrive.

Does anyone know what all the values on the table mentioned above are supposed to be? Are those voltage drops? I'm really puzzled...

[Diah]

post photo of the print chart on the PSU socket... not the LED driver boards..

[sansanpartida]

post photo of the print chart on the PSU socket... not the LED driver boards..

I hope the picture is OK



Also, here is a picture of the LED driver board. It might be useful.

[Diah]

wait for yours tester arriving

[sansanpartida]

Tester just arrived today, and the LEDs seems to be fine. I’m clueless now…

[EazyBone]

You tested all ribbons and all lights read what voltage?

You tested all 16 test lines on all 4 connectors right?

[sansanpartida]

You tested all ribbons and all lights read what voltage?

You tested all 16 test lines on all 4 connectors right?

Yes, I set up the tester to 84V and 1mA, put the positive lead in the center of the ribbon, and I got 76.8V on the 8 outermost connectors of the ribbons.

I also tested the voltage on the ribbon when the TV is turned on and found no voltage present. Even if the LED[+] test point show 84V.

BLU_ON/OFF measures approximately 3.2V.

[Diah]

1- post photo please on the whole back of tv.
2- did you hear sound?
3- did you see image on screen with torch light ?
4 please listed all voltage on the socket of PSU which it go to MB by name specially FB pin

[EazyBone]

Yes, I set up the tester to 84V and 1mA, put the positive lead in the center of the ribbon, and I got 76.8V on the 8 outermost connectors of the ribbons.

I also tested the voltage on the ribbon when the TV is turned on and found no voltage present. Even if the LED[+] test point show 84V.

BLU_ON/OFF measures approximately 3.2V.

Set the tester to max voltage/amps. It has a safety on it. Set it to 330v or max.

I have a thread here that diagnosed a 75 like this model in America version, but as you can see the led driver is for 75/82 version. The led driver was bad in that TV but it gives good reference to a specific chip you can check that is supposed to have 3 volts. Maybe it's a lead to two bad led driver boards you have.

Look at post 56 picture that shows 3v on the multimeter and test those 5 points voltages with everything plugged in as normal and the TV on. Look at the picture and there is 5 copper pads 1-5 what are those voltages?

https://www.badcaps.net/forum/troubl...samsungs/page3

This may be the wrong direction but it'd take 30 seconds to check if it's potentially a same fault. There should be 3v, I originally had 1.4v before I got one that wasn't broken.

Interesting problem, I'd follow diahs instructions, and post answers here.

[sansanpartida]

1- post photo please on the whole back of tv.

2- did you hear sound?

I can hear sound ✅

3- did you see image on screen with torch light ?

I can see an image using a flashlight ✅

4 please listed all voltage on the socket of PSU which it go to MB by name specially FB pin

Here is a close-up of the connector to avoid ambiguity, since I don't have an FB pin between the PSU and the motherboard. The FB pin is between the PSU and the DRIVER board:


Measurements of the 20-pin connector (on the left side of the PSU board)

Pin	Name	Measured
1	GND	0V
2	GND	0V
3	GND	0V
4	GND	0V
5	VAMP	18,09V
6	GND	0V
7	VAMP	18,09V
8	N.C
9	GND	0V
10	N.C
11	A13V	12,91V
12	N.C
13	A13V	12,91V
14	N.C
15	A13V	12,91V
16	P_ON	3,318V
17	A13V	12,91V
18	GND	0V
19	GND	0V
20	GND	0V

Measurement of the 18-pin connector (on the top of the PSU board)

Pin	Name	Measured
1	GND	0V
2	GND	0V
3	Vdrv	84,00V
4	Vdrv	84,00V
5	Vdrv	84,00V
6	Vdrv	84,00V
7	Vdrv	84,00V
8	Vdrv	84,00V
9	Vdrv	84,00V
10	Vdrv	84,00V
11	GND	0V
12	GND	0V
13	B13V	12,88V
14	FAULT	0V
15	FB	2,494V
16	GND	0V
17	GND	0V
18	GND	0V

[Diah]

the socket between PSU and LED driver boards are rail supply
we are interesting on the socket between LED driver boards and MB this is the key control of signal

[sansanpartida]

the socket between PSU and LED driver boards are rail supply
we are interesting on the socket between LED driver boards and MB this is the key control of signal

Sorry about that. 'Socket of PSU' was too ambiguous for me 😓. I'll quickly do the measurements again.

[sansanpartida]

Set the tester to max voltage/amps. It has a safety on it. Set it to 330v or max.

I have a thread here that diagnosed a 75 like this model in America version, but as you can see the led driver is for 75/82 version. The led driver was bad in that TV but it gives good reference to a specific chip you can check that is supposed to have 3 volts. Maybe it's a lead to two bad led driver boards you have.

Look at post 56 picture that shows 3v on the multimeter and test those 5 points voltages with everything plugged in as normal and the TV on. Look at the picture and there is 5 copper pads 1-5 what are those voltages?

https://www.badcaps.net/forum/troubl...samsungs/page3

This may be the wrong direction but it'd take 30 seconds to check if it's potentially a same fault. There should be 3v, I originally had 1.4v before I got one that wasn't broken.

Interesting problem, I'd follow diahs instructions, and post answers here.

Ok I've quickly took the measurement..

Here is the picture again to make sure I took the measurement exactly where you told me to (I also took some test points that were just next to it for good measure):

PIN1	1,367V
PIN2	1,192V
PIN3	1,127V
PIN4	0V
PIN5	0V

And here are the test points

TP1	1,252V
TP2	1,417V
TP3	1,481V
TP4	1,531V