Samsung UN48J5000: Backlight Strip Burn

[budm]

Re: Samsung UN48J5000: Backlight Strip Burn

The MOSFET is PWM so the Vdrop is the Average Vdrops. LED get full current if the MOSFET is fully on 100% duty cycle, but since it is PWM then it will be the average current. PWM is about duty cycle, we are pulsing the current through the LED. I guess you never replace the burnt MOSFET due to the someone decided to bypass a bunch of bad LEDs on the strips to save money.

https://cdn.badcaps-static.com/pdfs/...dafa76136a.pdf

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

You are reading 0.78V vF in forward bias of the Zener, any Vf of the Zener will be the same as Vf of the didoe, to read Zener Voltage you will have to connect the Zener in reverse bias to variable power supply with current limiter to read the Zener Voltage.

https://www.badcaps.net/forum/showth...ht=LG+47LN5790

I'll have to give this a further THINK.

I saw that thread. That's some serious burning damage! Fire hazard category.

Seems to be some confusion on just which LED replacement strips are appropriate. First I searched on "2015 SVS48 FCOM FHD DOE A RIGHT REV1.2 150605 LM41-00120Q_LM4100149A" which is what's in this set, which is rather long. Didn't see many hits. I found this string on a label stuck to the LED strip: "P9JYC41608C4Y06337297A". No hits on that. Found the panel sticker : CYJJ048BGEV1H. The set is a UN48J5000AFXZA Ver: ED02.

Saw a reference to BN96-37296A & BN96-37297A. Searched the metal back of the set for that #. No joy. Why isn't that # on the backlight strips? Where would/should I find this bn96- number?

A few hits corresponding to BN9637296A, show strips that upon further magnification don't show 2015 SVS48 FCOM FHD DOE A RIGHT REV1.2 150605 LM41-00120Q_LM4100149A printed on them. Rather it's something else.

Makes it difficult to order the correct part....

Also, where would I find what chassis this set is built on? Might help in finding a SM with a similar PS.

Please advise.

[budm]

Re: Samsung UN48J5000: Backlight Strip Burn

You have to match the LED strips to the model number of the LCD panel.
LCD panel may have more than one revisions of the same model also.
Service manual will not likely to have info about the LCD panel or LED strips since same model TV may use the LCD panels from more than one suppliers.

[nomoresonys]

Re: Samsung UN48J5000: Backlight Strip Burn

You are reading 0.78V vF in forward bias of the Zener, any Vf of the Zener will be the same as Vf of the didoe, to read Zener Voltage you will have to connect the Zener in reverse bias to variable power supply with current limiter to read the Zener Voltage.

"LG was mentioned often. Very alarming no protections were in place to prevent such damage long before it got to that point. Electric arcs burn very hot! Very illuminating and educational to say the least given my very recent introduction to what's going on 'behind the screen' in these modern sets." This situation is command in LG and Samsung that I know of due to bad design, I am surprised that no house burns down yet.

https://www.badcaps.net/forum/showth...ht=LG+47LN5790

Wonder which models have this design flaw and if they corrected it in later models.

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

I was curious whether the whole strip of 8 LEDs (both left and right sections) were at fault or whether the large current draw was due to either the left or right. So I lowered the bench supply to 11Vdc and measured current draw through half-strips made up of 4 LEDs. Turns out the one I repaired drew 450mA with 11V while its left half-strip only drew 70mA.

Next I experimented with just how much R to add to bring the repaired strip into line with the rest: About 44 Ohms brings it 65mA. Close enough. I was hoping that so tamed, the TV PS would light all the backstrips continuously.

No. Such. Luck....

A partially burned LED (that I'd forgotten about) on the strip above (also on the right side) went bzzzzzt and proceeded to burn through all of the copper on one side, resulting in an open ckt. So I soldered in a jumper, installed the 2nd damaged strip and plugged in the TV.

Nothing.

I tested each of the remaining 3 LEDs and they're OK. However, I'm unable to turn them on by applying 11V from the bench supply to one end like I can with the others. Meaning, this strip is now OPEN even though the remaining 3 LEDs still light. Further, I'm still puzzled by the mysterious little "blocks" with polarity marks. One here said they're a 5V Zener. I don't know. I'm also puzzled at how these strips are wired up. I took a photo, converted to pdf, then drew in the traces. It's odd how they're wired.

Seeking assistance....

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

Shortly after clicking SUBMIT, I looked at my photo and it dawned on me that the burned LED in a series ckt means OPEN! Geeze....

Also noticed that my 2 22R's in series were way over-powered as they were now burned. I replaced them with a chain of 10 + 15 + 14 Ohm R's rated at 10W. Across the 2nd burned out LED, I used 3 10R's in series rated at 5W each. Applied power. Top two strips of 16 LEDs lit brightly. Bottom two strips nothing. Current draw = 1.5A! Quickly pulled the plug to prevent cooking any R's. I found a 15R 25W Dale power R which brought the current down to 1.34A. Then found a 500R 5W I added to make the total 554R. Current draw now 250mA.

So the PS & LED driver is putting out plenty of current w/o cutting out. I'll have to add some temp test wires to quickly check V across the LEDs. Lighting up 16 LED's at 3V ea requires 48Vdc. Without a schematic to guide me through the sea of SMD IC's, and having no experience with one of these sets, I'm a bit lost.

[budm]

Re: Samsung UN48J5000: Backlight Strip Burn

Your TV has all 32 LED's connected in series (the series connections of the LED strings are done at the LED connector on the power supply board, if you look at the bottom side of the board you can easily see how it's done, so 32 LED's x 3.2V = 102.4V + V drops on the PWM controlled MOSFET = 130V.
The main key is to keep the current flowing through the LED's as low as possible to reduce the heat and extend the life of the LED.

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

Update:
Frustrated at why only the upper two LED BLs lit, I removed the PS connector to R3 (which contained the repaired strip mentioned earlier) and shorted the ends, completely removing R3 from power. Upon power on, rows 1, 2 & 4 completely lit! For the first time! I meas. the voltage drop across a good LED @ 3.08Vdc. The series current = 720mA.

Next I hooked back up the PS connector to row 3, removed the damaged right-hand section I attempted to repair and inserted a DVM across the end connectors of the left hand strip to meas the loop current. Upon power, I now had 3-1/2 strips lit! Loop current = 720mA. V across the LED backlight string = 106V.

Plugged row 3 right-hand strip back into place with a 500 Ohm R across the end where I cut the burned section out. V across strip now = 190Vdc. Rows 3 & 4 not lit. I shorted row 3 strips again and the V dropped to 94V (now illuminating 24 LEDs = 4V ea.)

Previously I'd called about 5 local TV-repair shops looking for 2 pieces of the right hand strip. Got lucky with one and picked them up today. I checked both for correct current before installation (with 11 Volts on the bench supply, one drew 48mA, the other: 45). I then plugged these both in, applied power and now all 4 rows lit up brightly (32 LEDs in all)! What a joy to see that given all the time I've spent on this my first 'modern' LCD TV with an LED backlight to fix. V across panel = 98Vdc.

Eagerly, I reassembled the entire TV. Meas. 90.7V @ Pin 1 of the PS BL connector and 188.9V on the other end (pin 9??). Looks good. I pwrd'd down, connected the spkrs, power SW, main brd, and LDC panel. Applied power.....

Now I have a screen with vertical white and some color bands!!! Power SW doesn't work either. Rats......

So evidently this free-to-me-TV has two layers of problems: First was the damaged LED BL strip that wouldn't even allow the set to stay on and Second, appears to be another, separate problem with either the main or timing-controller brd or a connector between them.

Well, I've come this far, learned a lot (with no schematic or SM), might as well plow onward. So what am I facing now?

TIA.....

[vinceroger69]

Re: Samsung UN48J5000: Backlight Strip Burn

Can you post a picture of what you seeing on the lcd screen now thanks

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

Can you post a picture of what you seeing on the lcd screen now thanks

Will do.
I used a loupe to look at the power SW/IR recv'r board. There is a tiny chip that partially melted. Might be TD1? Wasn't an R or C or D. Explains why it doesn't work.

BTW, it is possible to install an LCD panel wrong side out? Would the two panel connectors still match up?

[freakaftr8]

Re: Samsung UN48J5000: Backlight Strip Burn

I would imagine if the LCD panel were installed inside out you would have the SMD components facing the inside of the TV and not the outside of the 2 boards attached to the LCD panel. Also you would still be able to clip in the cable connectors but they will not be making contact so you would probably not have a picture at all.

But easy way to tell would be to look at the LCD panel as you're facing it. One side is extremely glossy in the other side has a matte finish. If the extremely glossy very clean side is facing you then the ax you could have installed backwards

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

Here are the screen photos:

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

Here are two more screen photos after disconnecting first one TCON to driver connector, then hooking it back up and disconnecting the other.

Please advise as I'm not sure what this test indicates. TIA.

[mmartell]

Re: Samsung UN48J5000: Backlight Strip Burn

Try reseating those connectors. Concentrate on one side at a time.

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

Try reseating those connectors. Concentrate on one side at a time.

See above. Been there. Done that.

[petehall347]

Re: Samsung UN48J5000: Backlight Strip Burn

if the LCD panel were installed inside out surely the connectors would be wrong ways up ?

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

if the LCD panel were installed inside out surely the connectors would be wrong ways up ?

Think about it turned left to right rather than top to bottom.

No problem anyway as I installed it right. Very shiny side goes inside, dull side out. Initially wasn't sure if this set had a glass mirror face or not.

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

I carefully cleaned all 6 sides of the ribbon cables between the main & t-con brds. Found nothing wrong. Also meas a few of the power rails. A bit tricky as they're very small. The PS conn. pins are hidden too close to a few AECs so access is restricted. I also carefully pushed on each LCD panel ribbon, gently flexing it. No joy.

When VAC is initially applied, there is ~ 5s delay before the backlights illuminate. At the same time, I see the white vertical stripes shown in the photos. It takes about 10s more for the colored vertical stripes to appear. Even when the set has warmed up a bit. The main t-con ASIC quickly feels warm but never too hot to touch. Pushing on the board made no difference. Plenty of very small test points to look at on the scope but I don't know what waveform to expect.

The above was done with no power SW/IR recv'r plugged in. I made a note of a burned chip in it above. I heard a BZZZT when it was connected, power was on, and I was moving the set around while it was standing vertically on the bottom edge. The TV probably trapped the unattached end between it and the carpet and shorted something. That bites....

Looking like this set is a tough one to learn on. The climb has been steep with no documentation nor experience with these. The defective 4 LED half-strip that did continue to light but drew way too much current was an interesting lesson. Current draw test was necessary to discover that.

Now I'm in deep digital terrain uncertain if the fault lies in the panel, panel conn., t-con or main boards....or more than one. So far I'm only out $20. This set was given to me in its current state. Not interested in spending $50 - $85 replacing t-con & main boards.

[Sleddriver]

Re: Samsung UN48J5000: Backlight Strip Burn

New information:
Plugged in the power SW/IR receiver module. Upon applying power, I see the red LED blink 7 times. Still looking for a Samsung TV blinking LED guide. Please advise if you have one. TIA....

Found post while searching from Tom66: "6 blinks means nothing. It is the normal Samsung power up routine. Every Samsung TV does the same. Anywhere from 4 to 10 blinks seems normal."

If true, I won't spend any more time searching.

[petehall347]

Re: Samsung UN48J5000: Backlight Strip Burn

i might be wrong but i noticed less blinks before power up means better health of the set sort of thing . like everything stabilises quicker for less blinks.