Onn 70" Class JE695R3HB9L - No backlight

[fuxxy]

This unit will flash the backlights once on powerup. Still responds to remote commands, and plays audio.

I have a backlight tester.
Two backlight connections to PSU. I have them connected opposite of OEM in the picture.

CON301 - Red/White wire - tests 95.5v on my backlight tester.
CON302 - Red/Gray wire - tests 92.5v on my backlight tester.

Is a 3v difference between strips enough cause to tear down the panel? It doesnt trigger any red flags for me. The backlight flash is enough for me to knee-jerk blame bad backlight strips as a first guess assumption.

Voltages on PSU board

3 large main caps - 370v DC

GND
ADJ -
BK -
+5V - Not Connected
PSON -
+5VSB - Not Connected
NC - Not Connected
ND - Not Connected
GND
GND
GND
+12V - 11.98v
+12V - 11.98v




Edit: Model # is actually 100012588

[angelos]

Re: Onn 70" Class JE695R3HB9L - No backlight

A backlight tester is not the perfect tool in diagnosing faulty led's. Many times I had the same symptoms and it turned out as backlight failure. Remember that the LED tester does not test these led's under load (280mA - 300mA). The power supply has very sensitive circuitry that trips out the power supply under certain conditions like over current or overvoltage. The one time it was a bad solder connection under one of the led's causing this problem.
By the way, there is a reservoir cap for the led supply voltage on some designs that goes either bloated or high on ESR which can also cause the same problem.

The discrepancy of 3V tells me one of the LED's is shorted out. If there is one that has failed there could be more issues with them.

I once had a similar doubt where i had backlights tripping out on start up and my led tester was showing that they were ok so you know what i did??? I built up led strips externally and connected them on the led plug fed from the power supply and switched on to find out that the led's were lighting up just fine proving i had a bad backlight strip.
To do this you need to know what the arrangement of led strips is like to simulate it externally.
I keep records of all the tv's I work on with led strip number, led type and voltage and also configuration of strips. Sometimes you have 8 Led's on a strip but it is 4 series Led's in parallel with another 4 in series.
I have repaired many led strip faults and only once had a problem on th eelectronics causing led failure, mostly dry joints or a cap failure.

I hope this gives you some direction?

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

Building up external LCD strips in indeed an option.

I guess there's no need to use actual backlight LEDs when testing via this method? As long as the forward voltage is 3v, I suppose it shouldnt matter if 'regular' 3mm LEDs are used on , say, stripboard.

As you say, as long as they are wired identically to the original.

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

Dissasembled the panel. Attached backlight tester.

This panel arranges the backlight LEDs divided vertically - with the left half powered by one connector, and the right half powered by the other.

3 LEDs didn't light on one half. The half that is working normally shows 95.5v on my backlight tester.

Working on removing the white plastic to determine the PCB layout.

Edit Arranged thusly:

Each strip has 6 LEDs in series.
Each strip is connected to the next strip in series as well.

So 36 LEDs in series * 3.3V forward voltage (generic value found for white LEDs) = 118.8vDC max.
36 LEDs in series with a 2.65 forward voltage would be 95.5v.

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

I've identified about 20 individual LEDs in the backlight array. Only 3 failed to product any light at all (shorted). The others seemed to require more initial voltage to produce light (they were slow to turn on). All of these individual LEDS have been replaced on the existing strips.

The single backlight flash still remains. I"m also getting a red status LED giving a "2-blink" pattern - I'm assuming indicates some sort of error?

I'm still wondering if the PSU itself could be damaged.

I'm attempting to power the backlights with the LCD drivers and panel disconnected from the TCON. Maybe that could be the current problem, but I"ll have to jury-rig a way to connect without completely reassembling the panel.

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

Voltages on PSU board - powered ON

3 large main caps - 370v DC

GND
ADJ - 1.712v
BK - 3.19v
+5V - Not Connected
PSON -2.94v
+5VSB - Not Connected
NC - Not Connected
ND - Not Connected
GND
GND
GND
+12V - 11.98v
+12V - 11.98v

[angelos]

Re: Onn 70" Class JE695R3HB9L - No backlight

I Still think it is backlight related.

[angelos]

Re: Onn 70" Class JE695R3HB9L - No backlight

You must see my dilemma. When ordering brand new backlight strips from China these parts take around 2 to 6 months to arrive. My customers are not prepared to wait so long so now I manually repair them. On a 70" set like yours there should be about 100 leds.
That kind of job takes me 8 hours to do and results depend on the lens being properly aligned. I really hate having to do a manual fix but who wants to wait 6 months for parts to arrive? Those lazy leds you spoke of are a sign that the led strips need replacing. Sometimes the led current finds a carbon path to the underside where the aluminum back plate is situtated. When this happens the tv shuts down also. That's why the best fix is to inatall new strips in the tv, no comebacks.

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

Understood and agreed. I have already ordered a set if replacements, figured I'd attempt a compo e t-level repair in the meantime just to see if it worked.

I noticed In addition to a voltage marking 9. the PSU, there's also an amperage for the backlight circuit. Just as a test, and to solidify the hypothesis, I have a regulated power supply I can attach the strips to to measure their current draw at target voltage. Ive seen signs of carbon deposits on the metal behind the strip connections, it's probably as you describe.

I'll wait for the replacements to arrive and go from there.

[angelos]

Re: Onn 70" Class JE695R3HB9L - No backlight

Just another note here. If you would like the TV's back lights to last a long time then you need to reduce the LED current. With the TV's in circulation in my country I would notice they are being driven at around 80% to 100% of their maximum rating. For example one sets LED's get replaced by me, Once I installed the new ones and have closed the set up to the point that I am getting ready to fit the back cover I test LED current.
I put an ammeter in series with the plug feeding the back lights from the PSU and I measure current at around 280mA or 303mA. Now let me tell you some of those LED's are rated at 300mA. So reducing the current can be done in two ways,
1. Changing the resistance of the current sensing resistor by removing a resistor in parallel with others. These are like 4 SMD type resistors of values 2.2 ohm, 2.2ohm, 4.7ohm and 4.7 ohm. Remove one and notice your current drops by 80mA or so. I normally chop and change until I am around 180mA to 200mA and the brightness change is only slight, the lifetime is extended by a factor of multiples.
2. Apparently in service mode some TV's allow you to adjust the back light current but I have never encountered one like that.
3. Some TV's have a back light setting in the "Picture settings" menu. Change it there and educate your client.
If its slightly darker just increase contrast and brightness!!!

Hope you understand!

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

New strips have arrived!

Did some testing, mabye it's useful for others? Hooked up to my benchtop constant-current PSU.

With a single strip (6 LEDs in series) attached to the PSU. Judging from the markings on the power board 100-126v is supplied by the TV. With a generic forward voltage of 3V, I'll assume the two "arrays" of 36 LEDs are wired in parallel. 100V / 36 LEDs gives me a forward voltage of 2.7777 volts. 126V on the same 36 LEDs gives a forward voltage of 3.5V. With only 6 LEDs, I shouldn't send any higher than 21 volts for a single strip.
Clamp @ 100mA, the strip requires 16.38V
Clamp @ 150mA, 16.73V
Clamp @ 200mA, 17.18V

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

Finally got all the strips replaced. Accidentally damaged one of the strips while trying to slide it into the slot. Slipped and bent the strip about 90 degrees, breaking the copper traces on top of the substrate.

Fixed it by scratching away the enamel about an inch away from the crack on either side, then soldered a bare wire on each leg. added heatshrink on the wires, then soldered the other side.

FINAL RESULT: TV turns on! Yaya!

BUT WAIT THERES MORE!

The backlights no longer turn off. I'm able to turn off the TV, and the picture goes dark, but the backlights are still lit.

There's also a static type sound coming from the power supply that I've never heard before on any TV I've worked on.

Tested voltage output on the backlight connectors, with both backlights plugged in, measured 130v on each connector. Looked at the back side of the board earlier, the two backlight plugs are connected in series on the board. Measuring the output across the backlight plugs presents 257 volts.

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

Replaced all 10 LED strips again.

After plugging the set in, I get a single flash from the backlights when powering on, then nothing. Picture is displayed if I shine a light at an angle.

[Storyteller12]

Re: Onn 70" Class JE695R3HB9L - No backlight

Sorry to hear about the problems. Onn brand is Walmarts private label and honestly they are junk. I would not waste my time or money on it. Even if you get it working the picture quality on these sets are horrible.

[Rajnhl]

Re: Onn 70" Class JE695R3HB9L - No backlight

check mosfet switching led on off is short.

[fuxxy]

Re: Onn 70" Class JE695R3HB9L - No backlight

Found another surge of energy, and read up on some other threads.

currently following https://www.badcaps.net/forum/showth...=110193&page=3 , since it seems to detail a common issue on these power supplys.

R355 is acting like a fuse for the Auxiliary supply from the transformer, D107, EC103 If this resistor is open, the power supply will try and start then fail and try again.

R355 was not open it just shows as 20.1k when it should be 2.2ohm. R354 seems in spec. 4701 marking means 4.7k and it reads about 4.6k

The same is true for my board. R355 was actually two resistors stacked in parallel, both marked 2R20 = 2.2ohm. Removed them and each resistor read 12.2 ohms out of circuit. Tested again a few mins later, now reads 11.6 ohms.
R354 is a single resistor marked 4071. This one read 4.7K, which was exactly on spec.

I don't think this is the cause of my particular issue, but it does appear that it's suffering from the same issues plaguing the OP in the other thread.

[mecha1166]

Re: Onn 70" Class JE695R3HB9L - No backlight

Just repaired 2 of these myself. 10 uf 450 v capacitor on power board. It is right by the plugs that go to the backlights.

[EazyBone]

Re: Onn 70" Class JE695R3HB9L - No backlight

Just repaired 2 of these myself. 10 uf 450 v capacitor on power board. It is right by the plugs that go to the backlights.

This was the fix for me. I had 12v to main, tcon working, sound but no picture.

I had a flash of light from backlight and then it shut off. I manually used LED tester on each side while the TV was on half on each side and has picture. Each backlight seemed to be working.

The backlight was getting 380v and 240v which is obviously off from what It should be.

Noticed the cap next to LED was round very slightly.. took it out it tested at 0 capacitance. Replaced and is working.

Old thread but this was the answer for me.