Question Samsung 48" Power Supply and LED Issues.

[HMark]

Samsung 48" PS and LED issues
Hi: I'm new to the forum but not to electronics. I'm a retired RF engineer with over 50 years in broadband telecom and video delivery. I took in a 48" Samsung TV (recent model) to see if I could repair it for a friend. No picture or sound. The red power LED light blinked 6 times when attempting to power up manually. According to Samsung, this indicated a switch model power supply problem. Upon checking the voltages on the SMPS board, all appeared to be normal except for the voltage fed to the LED backlighting. Given the number of LED's in series the voltage should be around 85vdc but was well over 200vdc! The LED string was open (tested using a variable DC power supply), so I 'assumed' the PS board went bad and burned open one or several LED's. I ordered new LED's plus a replacement SMPS board from shopjimmy.com. I've replaced the LED's (3 of the eight strips had at least one open LED) and have the new PS installed, but am hesitating to connect the LED string, as the voltage on the new board also measures over several hundred volts dc, and am afraid it might be defective and I'll burn out the new LEDs. Meanwhile, I still get an error code and the TV still won't turn on. It now blinks 5 times vs the original 6, but both codes indicate a PS problem. My questions are: (1) should the TV turn on with the LED's disconnected or can it sense they are not working? - and (2) is the high voltage for the LED strip feed normal (is this a current limiting circuit that will reduce the voltage and current to the proper values once I connect them)? Any suggestions or thoughts are appreciated.

 

[HMark]

I'm posting an update to my own post. I'm now reasonably sure that the switch mode power supply (SMPS), when working properly, provides current limiting to power the LED strips. Just to be sure, however, I've calculated that the entire string (at minimum LED firing voltage) should require 85.2 volts and draw .026 amperes. Using Ohms law, I can work from there to determine at minimum LED firing voltage the LED string has 3.3 k ohms (equivalent) resistance and would dissipate 2.2 watts. I've ordered a wire-wound 3.3 k ohm resistor with a 5 watt rating. I can then substitute the resistor for the LED string and fire everything up while monitoring the voltage and current at the resistor. If everything measures as it should, I'll cross my fingers and plug in the LED string. Also, the SMPS does indeed monitor the LED circuit voltage and current, and will shutdown the SMPS if parameters are not within a proper range, so that I think explains why the TV still won't turn on. If I had more experience with these modern TV's I wouldn't be so cautious. I'll post once more when I'm finished with this thing, one way or the other.

 

[BEAUFORD_SAVAGE]

When a backlight LED opens up, the voltage does rise some. You most likely need to replace the LED backlight strips. Never just replace one, as this is a big delicate job, and the whole TV needs to come apart to even
test the LED strips. It's not a job you want to do again soon after replacing one strip, plus they lose intensity over time, and might be a little unevenly lit if all aren't done. I am reasonably certain you will find an open. I have recently had a lot of bad backlight strips as the problem, and so know first-hand. You must be SUPER cautious not to break the very thin LCD screen.
Talk to me if you want before getting too far in.

 

[HMark]

Hi, thanks for your reply and suggestions. I haven't repaired a television since the mid 1970's and they have obviously changed a great deal since then. I'm attempting to repair a 48" Samsung for a friend, recently widowed, who can't afford a new one. I do some repair work now that I'm retired, but I stick mostly with RF test equipment; spectrum analyzers in particular.

Let me try to summarize what I've done thus far.

1. Initial troubleshooting with a LED tester indicated one or several of the LED strings (4 rows of 8 LED's) were open.
2. The voltage on the SMPS (switch mode power supply) feeding the LED string was at approx. 230 VDC, much higher than the 130 max VDC stamped on the board.
3. I have removed the old LEDs; and have installed and tested new LEDs. My LED tester shows the new entire series string just turning on at 85 VDC at around 85 ma.
4. I ordered a replacement power supply board as I suspected the old SMPS might be bad; however the replacement board shows the same approx. voltage (approx. 230 VDC) as the original WITH THE NEW LED STRING STILL NOT CONNECTED.
5. Testing and examination of the old LED's showed multiple LED's burned out, including one location where 4 bulbs in a row had arced over, indicating some sort of voltage surge?
6. I'm at the point where the next logical step would be to connect the new LED string to see if the TV is functional, but have this doubt that I'm missing something and I'll burn up the new LED's. That would mean another $50 on LED's, not to mention disassembling the LCD assembly again.

My question is: "is there some way to simulate the LED string 'load' on the SMPS to make it think they are connected, so that I can confirm the supply is working properly before actually plugging the new LED string in?" Or am I just being too cautious?

Thanks in advance for any advice you can offer.

 

[HMark]

I had a typo in my response just now. The new LED string just fires (minimum brightness) at 85 VDC, but at 26 milliamperes, not 85 ma.

 

[BEAUFORD_SAVAGE]

I,m glad you clarified as it changes my response completely. FYI, I am an Electrical Engineer, so you need not spell out SMPS for me, we can talk techie all day.
I was thinking that because these power supplies are more current than voltage limiting that that may be the reason for the abnormally high voltage, but that just didn't seem right from my experience.
You must have a bad LED power supply. For a test load, I would use large carbon resistors, as the wire-wounds that would be handier might add too much inductance, leading you astray on the problem. And the LED's are being strobed, another reason to go with a non inductive load.
For power (Yes you know this) each led is at about 3V * 50mA (Lets be conservative on the power draw for the SMPS) So .15W * your number of LED's) . As obviously the number of LED's vary from TV to TV. You know you voltage, 85Volts? Now make your load to fit. Then get out your O-Scope, as I'm betting you don't have a clean voltage source. Caps are by far the highest failure part in a SMPS, so look at those electrolytics first. Next the driver for the switcher(s). They are usually used in 2 or more banks for the sake of better picture clarity. So you may need to determine the number of loads you need. The last few TV's I've replaced LED's on, had 2 banks. The main board determines how the LED's strobe, often with a 5V strobe signal. It is still a single power supply that drives the banks. You may want to consider buying a new power supply board, as they usually are pretty low cost, or if you don't find a relatively easy problem to repair.

Nice of you to do this for a friend BTW.

 

[BEAUFORD_SAVAGE]

If you do replace SMPS caps, make sure they are Low ESR caps, or they will last 5 seconds to 5 days, I recommend Panasonic FM series caps, which you can get from Digikey. ( in the .0XX of an ohm range)
For a P.S. board replacement, Shop jimmy has earned a great reputation, and will help you make sure you get the right version board. Their prices are good as well.
Beats taking your chances on EBay in my opinion.

Additional thought, find a bright image, and pause it, and you see switching noise on top of the voltage under load, the problem is probably the big LED output filter caps. Compare to the voltage to the input of the switcher circuit, if the output caps are bad, it will still be pretty noisy, but better % -wise than the output. Without load, you should see a more stark difference on the output vs input.
Question: does your meter read True RMS?

 

[HMark]

Thanks for your responses. I've been under the weather (cold) since last Friday and haven't worked on the Toshiba flat-screen again until this morning.

Yes, one of my meters measures true RMS. I haven't measured switching noise on the unloaded output of the power supply yet, but I should do that next. I have a fairly good 200 MHz scope and a differential probe for making accurate switching noise measurements.

Your comment about carbon vs wire-wound resistors got me to thinking more about the apparent impedance of a LED, where the voltage vs current relationship is non-linear. I took some new measures of the new LED strings this morning. One set of readings was at minimum voltage (just enough to light the bulbs) and a 2nd set of readings at close to maximum brightness. There's quite a spread between them when considering the required carbon resistor to simulate the entire load and the wattage rating needed.

Minimum voltage to each LED yields around .072 watts per LED and an estimated total load resistance of 3,175 ohms at 2.3 watts.
Maximum voltage (as high as I was willing to take the LED brightness) to each LED yields approx .473 watts per LED and an estimated total load resistance of 531 ohms at 15.24 watts!

I've ordered a 5w carbon resistor assortment and will try several different load resistor combinations when it arrives. Meanwhile I'll use my scope to measure switching noise.

I have the growing feeling that the power supply is probably working fine and I'm just being too cautious in trying the new LED's. Don't really want to see them go up in smoke.

Thanks again!

 

[BEAUFORD_SAVAGE]

HMARK,
Your welcome, and it's nice to talk with someone who knows their stuff for a change of pace!
I think you you are just playing it safe, a prudent thing to do, but if you just take a relatively well thought
out low intensity range of the LED power, and the power supply comes down to a approximate 3V per LED level,
I think it would be safe to connect your LED's and enjoy your repaired TV.
Hey, I get accused all the time of over-thinking things, so as far as I'm concerned, better well thought out than
being a Bozo.
Hope you get better fast.
-Beauford
(online name or I'd change it)

 

[HMark]

Thought I should type a final post to close out this thread. To recap, I took in a 48" Samsung with no picture or sound for a friend. After attempting a hard reset with the external power switch, the LED light would blink 6 times indicating a switch mode power supply (SMPS) problem.

After some initial troubleshooting I determined the LED string (one series string with 32 LED's) was open in at least 2 sections (each row of 8 LED's looped back to the back plug at the SMPS, then on to the next row). I ordered all new LED sections, then carefully (I've never done this before) disassembled the front LCD assembly , replaced the LED's and reassembled (after testing them to make sure they worked). When testing the SMPS I became more and more convinced that it was defective as well; at least in the LED current-limiting circuitry. So I ordered a replacement SMPS and installed it. The TV is now up and running!

As a final comment, there are ways to 'simulate' the LED strings using a calculated value value carbon resistor (5W minimum); however I'm convinced no resistive load can ever accurately' mimic' the extremely non-linear current to voltage relationship found in a LED string. The best you can hope for is to determine the SMPS is providing some level of current limiting at something close to the desired voltage.

My special thank you to Beauford Savage who provided lots of spot-on advice and encouragement!