[SOLVED] Repair of Samsung 55" TV - backlight problem

I have an HU board here in front of me that I grabbed the standby IC out of for another job, and I have the LED voltage marked as 325/375V on the BD9101 ferrite jumper (with respect to cold ground. Use the mounting screws for ground). I have seen a bunch of TVs that use a series/parallel arrangement, but I can't recall Samsung ever doing that.

I'm not a betting man, but put a gun to my head and I'm going with bad LEDs, bad connections on the interconnects (or both), but inside the display for sure.

Keep a meter on it all the time. If it has a min/max recording feature, connect a DVD player and leave it on the title screen so the TV doesn't dynamically adjust the back light to scene content or time out and shut off. Check the max voltage every once in a while. If your baseline drifts more than a tenth or so with no change of picture content, it indicates a failing LED (they do intermitt both towards open and leaky) or a bad interconnect between the A/B sections of the LED strips or where the LED strips plug into the main feed strip.

Sometimes we take the display out and they come to life by themselves. It happens. One thing to do is put a meter across every connection and see if voltage appears across any connection (should be zero volts). Any voltage across an interconnect means a bad connection. If there is any voltage drifting even with the LEDs appearing to be running properly, we know we can find it.

Sometimes we have to hard wire all the interconnects - if just one is bad, another will follow eventually. It's not intuitive but poor connections *don't* show much if any change in brightness even when tapped on as the LED drive controller is extremely fast and compensates immediately for any change in load impedance. Only when the connection is bad enough to cause the controller IC to detect either an overvoltage or undercurrent condition does the LED array shut down.

If it does not power up, you may try heating the capacitors one at a time. I have seen that work on equipment that comes on after the set is powered up for a while if the capacitor is bad.

Intermittents are a PITA. I'll spare you my guess of the day. The way I used to troubleshoot intermittents is heating with a hot air gun and cooling with freeze spray in a can. Whatever connection is intermittent, is going to move when it gets hot or cold, either making the connections, or as in this case, breaking the connection. Don't use to much cooling or heating. Dropping the temperature below the dew point will condense water all over the board. Heating the PCB too much will melt plastic parts. If you do condense water on the PCB, remove power from the TV, apply a little heat, and wait until the water evaporates before trying again.

You missed a golden opportunity when you discovered that:

The LED's are connected in series. That means you should see a voltage equal to 267V divided by the number of strips, across each strip.

Assuming they're all in series, that would be: 267 / 14 = 19V Walk your DVM across each strip and see which one has the full 267V across it. That's the defective (open) LED strip.

Never mind. It looks like you will need to disassemble the LCD panel in order to obtain access to the LED connections. Once everything is disassembled, you might as well replace all the LED strips instead of trying to find the one with an intermittent connection.

I just watched the LED replacement video at:

I can't deduce which LED strip is bad, but my guess(tm) is it's one of the two pin connectors on the LED strips. See video starting at: My free advice is to put the TV back together and hope it continues to function normally. This doesn't appear to be a paying repair, so you might be able to convince the owner to do it this way which minimizes the risk of breaking the panel. After all, the previous repair lasted

3 years. If it fails in the future, order the parts and follow the video. I couldn't (and wouldn't) do that to a paying customer who expects a repair and a warranty on the labor.

BTW, you were right about your suspicion that the problem wasn't the main board. The repair shop probably had the intermittent back lighting magically fix itself, and just added the board replacement to the invoice to make it look like they actually did something useful.

As you said later, I can't get to them without taking everything apart. But I'm confused about what the LED circuit is:

First, the ShopJimmy LED package has 7 strips of 8 LEDs, plus 7 strips of 5 LEDs. Is this an all-Samsung LED package so you might not need to use all of them, or are there 91 LEDs in my TV? If not 91, how many?

Second, there appear to be three chains of LEDs in the TV, connected in series, but the interconnects between the three chains are on the power supply board via the connector:

Voltage vs Chassis Ground

Connector Pin Black Mixed Light

1+ (also BD9101) 240 283 284

1-, 2+ 153 168 182

2-, 3+ 96 90 113

3- (also J858) 42 15 45

I got these readings by connecting my computer via HDMI. "Black" means a black screen. Light is an editor that's almost all whaite space, and Mixed is a video of Glenn Gould playing the piano. When in Black, the LEDs are dark - no light visible coming through the chassis. So the main board clearly has some way of modifying the backlight brightness.

But while Black looks like the LEDs are off, there obviously must be some current getting through.

So how many LEDs are in each of the three circuits? And do these numbers tell me anything about what might be wrong? The screen looks completely normal on the front, so I assume the readings above are normal.

I've decided that I'm not going to replace the LED strips unless I can prove that's the problem. I don't want to go through all that, and spend the money, only to end up with it still not working. I think proof would be high voltage applied to the LED string, but no light. And I can't get that unless it goes back to not working. Short of that, I can check the electrolytics on the power board, and replace any that look bad. And maybe try the coolant thing. However, the owner confirmed that once the backlight comes on, it never goes off on its own. It continues to work until they turn off the TV. So I would have to apply coolant while it's off, and hope it won't turn on.

I have not checked to see if there's any PWM going on in the LED supply. But I'm not sure it matters.

____________________ And once it is repaired, do not operate the set with the backlight set any higher than 12(I believe Samsung backlight scale is 1-20). Use a higher Brightness(black level) setting to see detail in darker parts of the image

Ok I think I've figured it out. One strip of LEDs across the panel consists of one 8-LED section plus one 5-LED section. So 7 strips times 13 LEDs is 91 LEDs. The voltages show there are one three-strip (39-LED) and two two-strip (26-LED) lines that are individually wired back to the power board, but are simply jumpered together there. This confirms what you guys said - in effect all the LEDs are in series, so any problem with one LED, or one connection, and they all go out.

I think this is the most relevant video on LED replacements. It's not exactly the same model, but very similar:

It doesn't look too bad, except I don't have the suction cups. But as of this morning, ShopJimmy is out of stock on these, with no indication of a restock date. I can get them on Amazon for $100, or I can get knockoffs on Ebay for as little as $40. I don't know if the Ebay Chinese ones are any good. And I don't know if the Amazons are different from the Chinese ones on Ebay.

I did try disconnecting the main board, and the LEDs did indeed light up fully, with about 281V across the LEDs. But unfortunately, it's still working, so there's still no way to tell where the problem is. It could be the main board (unlikely, but still possible), the power board, or the LEDs. And I just can't narrow it down unless it goes into failure mode again.

So I'm going to continue testing it for a couple days, but if it doesn't fail again, I'll give it back to the owner. After all, with all the messing around I did, I may have fixed whatever was wrong, and it may go another five years.

I really appreciate everyone's help with this. I just wish we had reached more definitive result.

Don't run it long with the main disconnected when testing - it will run the LEDs at higher current than full backlight adjustment will even drive them. Good for a quick test but don't leave it running that way.

Put the TV on a static input (DVD in pause) and watch that voltage across the LED array. If it drifts more than a couple of tenths either way, either an LED is starting to break down or there's a resistive contact in the interconnects. Sometimes one or both will settle down for a while before acting up again.

There is another possibility however. Most Samsung LEDs are built with a zener inside each die. Ordinarily, the zener does nothing but if the LED fails open, the entire voltage of the array will appear across the zener causing it to conduct and (hopefully) short hard effectively jumping out the bad LED. If it does, the problem will fix itself until another wayward LED crops up or if an interconnect gets resistive.

If I connect my computer to the TV via HDMI, and display a competely white (all FFs) BMP image full screen, would I be able to detect an LED that's not lighting up? Would there be a dark area on the screen, or is the failure of one LED not enough to cause a detectable darker area on the screen? When I try this, I don't see anything unusual. The display is a little darker in the corners, but I think that's normal.

Probably, but Samsung does a good job with their lens design on their back lit models. Think about it - there is a fair amount of dead space between LEDs as they're laid out and generally you don't see the individual LED illumination even on a plain white display. There will be *some* unevenness if an LED goes out, but it's generally not dramatic, but your all white bit map pattern might show it. Try different brightness settings when you're looking at it. I've also seen some older Samsungs with half the LEDs shorted and the customers didn't complain at all about the picture until one opened and shut the array down.

One of my employees brought in a Samsung 55" UN55JU7100 that has a shorted primary side of the switching supply. Hard short at the output of the first bridge rectifier. Noticed a few small value caps that are slightly bulging - is there anything else to check/replace on this series other than the shorted MOSFets and suspect caps?

I'm going to suggest he add a small fan to the backside of the TV to get longer life...

Thanks!

John :-#)#

No, you'll be good to go. Just change the shorted MosFets and the caps, and it should fly. If you're very unlucky, the gate driver IC might have failed but usually the fuse will blow quick enough. If the customer changed or jumped the fuse, the gate driver IC will most likely fail on AC plug in.

You don't need a fan, but they way to extend the life of these is to lower the back light setting. Any TV I repair automatically gets a small mod to reduce the back light current regardless of where the customer sets the adjustment in the picture menu. Most LED TVs use a fixed LED voltage on the high side of the array and a MosFet on the low side which controls the current. I raise the value of the MosFet Source resistor (senses current) to fool the feedback.