Samsung led monitor s27d390h backlight wont stay on, boost converter issue? - Page 1
Forum section: electronics / repair

author: vis321
author_rank: Contributor
timestamp: December 01, 2019, 10:45:31 pm
content: Hello,I got this 27in Samsung monitor.Issue with it is that the back light wont stay on, only comes on for about a second, common issue with PC monitors.I can't quite figure out where I should be looking for the problem I figured out it must be the LED boost converter because if I shine a light into the LCD I can see a dim picture, so the logic does work.I tried some measurement and for the most part, it seems fine, I think.I am not sure what the exact issue is, all 4 100uf electrolytic caps test ok.DC power plug is 14v I have tried another adapter and running straight off my bench supply as well.I'v uploaded some pics as well.https://drive.google.com/open?id=1bQCOwkBmb28ky3U-6DIwVyDRgjBw_36mvoltageKey_+3.3v 3.3 vEdid_5v_p 4.25 v ? kinda low?+3.3v_MAIN 3.38 v+5v_MAIN 4.98 v1.25v 1.24 vcn300 1.55 vcn701 x401 1.55 vc801 14.07 vvled 13.86v when backlight is off 50-60 v short burst when backlight is onic300,ic100 atmlh448ic401 cet433 unreadableic600 D9321 Drive IC?

author: TheMG
author_rank: Frequent Contributor
timestamp: December 02, 2019, 12:43:14 am
content: Most of the time a backlight that turns on briefly then off signifies a problem with the backlight itself. If the LED driver sees that the voltage and/or current is out of range (could be due to shorted or open LEDs), it will turn off.

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2019, 02:22:19 am
content: I've dealt with this on several TVs, at least one of them was also Samsung. In my case it was a shorted LED causing a section of the backlight to be lower voltage than it should be. In another case a bad LED had gone open and caused a similar symptom so that's where I'd start. LED backlights seem not nearly as reliable as they should be, I suspect they drive them very hard to get that bright eye-popping picture that stands out in store displays. Unfortunately they also are almost always made so that it's about as difficult as possible to get to the backlight despite the fact that it would be trivial to make the LED strip and heatsink easily removable.

author: Dacke
author_rank: Regular Contributor
timestamp: December 02, 2019, 03:40:37 am
content: Quote from: vis321 on December 01, 2019, 10:45:31 pmAs others have said this is a common symptom of a blown or shorted LED in the backlight or a bad connection there. Usually you can further confirm this by measuring the negative wire to the backlight with respect to chassis ground. If it doesn't jump a few volts when the backlight tries to start (typically 2V - 10V) and just sits there at around 0V, it's an open circuit and an LED is likely blown. If it does jump and the backlight actually turns on for a second or two before it shuts off, it could be a shorted LED or LEDs, though this is not definitive.

author: vis321
author_rank: Contributor
timestamp: December 04, 2019, 02:32:49 am
content: So this is what I uncovered:My bad for not including the model number of the monitor: s27d390hThanks for the suggestions and help.Even with no load (led strip completely unplugged it still jumps in voltage unstably, I think my volt meter was just over shooting the inductive spike? 50v seems to be a bit much now that i think of it or something is broken in the boost converter side)? so the boost converter is no good/stable, and I don't think its repairable (without alot more digging).I did though find a rather cheap led replacement looks exactly like the one in this monitor. Here is the aliexpress linkhttps://bit.ly/2Pe4RxnTurns out there is only a single led strip on the left side, I used my cheap bench supply and powered it up. I found that 1 or 2 of the banks was dead the other two seemed ok, I confirmed this since I have carefully taken apart the LCD assembly. (and yes I also ran a HDMI video signal from a laptop and it totally work and got a picture, as did the OSD buttons as well, sweet)I could see the discoloring of one the banks pcb, so its pretty clear now it how it failed.The monitor was picked up from the junk pile, it was gonna be thrown away.From the ad, looks like the led's are 3v each, not sure the current, any idea? The strip does have some square item at the end is it a resistor network?36 led's total, 4 banks, leaves 9 led's per bank, so it would probably be driven at 27v per bank. that's doable. So I might just order that leds strip its only 17$ CAD and try a cheap boost converter. I also haven't found the 14v power adapter for it. Might still be in the junk bin, ill have to go look again.So not sure if I will have to current limit (cc) the leds or will it be ok if I just run them without CC only in CV with the boost converter?I guess the question is also for 17 bucks is it worth to fiddle with it.

author: james_s
author_rank: Super Contributor
timestamp: December 04, 2019, 02:44:01 am
content: I would expect the voltage to shoot way up without the LED strip connected. These are constant-current sources so it will try to dial the voltage up until the current reaches the set point. An open circuit will never draw current, well not unless the voltage can go high enough to jump the gap but that's academic.

author: Dacke
author_rank: Regular Contributor
timestamp: December 08, 2019, 04:54:13 am
content: You should be able to test whether the power supply for the backlight works or not, check each LED with a multimeter on diode range. They should all be either in a series string or multiple parallel series strings. If only a couple are blown, just bypass them with a 1N4148 diode or similar, I've done it many times. You could even use jumpers and it should work just for testing, though if too many are blown using jumpers could push the supply out of it's tolerance range.Edit - Here's an example of using a zener diode in reverse bias to do this on an edge-lit backlight:

author: wraper
author_rank: Unknown
timestamp: December 08, 2019, 05:23:30 am
content: Quote from: vis321 on December 04, 2019, 02:32:49 amWhy not? It's quite decent monitor with PLS panel. Also it should be possible to find separate LED if you measure size.

author: vis321
author_rank: Contributor
timestamp: December 08, 2019, 04:53:15 pm
content: Yeah I ordered this led strip, it should be a direct replacement. worst comes to worst, I'll just power is from a separate boost converter.https://www.aliexpress.com/item/32880934139.html?spm=a2g0s.12269583.0.0.2dc26576LRoO1qThe strip consist of 36 led's total. 4 banks total, 9 leds per bank(parallel series). On the listing it says the led's are 3v forward voltage, so per bank its about 27v.Without the led strip connected, the output jump briefly, (too quick for my meter to measure). I might try connecting the new led strip to the board when it arrives. Hope it working and doesint kill the new strip when it arrives lol.

author: vis321
author_rank: Contributor
timestamp: December 08, 2019, 04:58:42 pm
content: Neat ok, So I just watched part of the video, So I guess part of the reason I am seeing the voltage boost and then cut off, because the boost converter is not seeing the proper load possibly. I hope this is the case. I'll have to confirm when the new strip comes in.

author: Dacke
author_rank: Regular Contributor
timestamp: December 08, 2019, 05:55:55 pm
content: Quote from: vis321 on December 08, 2019, 04:58:42 pmYes, the supply for these backlights will often cycle if they detect an issue. The purpose of the diode fix is too fool the protection circuit into seeing the correct voltage drop, usually anything that you can use that will provide between 3V and 8V drop will work. Much of the time, for just one failed LED, I don't replace the strip or the LED and just use the diode, as once the TV/monitor is reassembled there is too much diffusion in the backlight to notice it. You would need a very bright, solid backround color and would have to know where to look to even see it.

author: vis321
author_rank: Contributor
timestamp: December 08, 2019, 06:34:04 pm
content: Yeah thats a neat trick. sadly my backlight is too far gone. and I can even see the COB led discolored. Its not to hard to replace the strip.

author: vis321
author_rank: Contributor
timestamp: January 05, 2020, 10:51:25 pm
content: So update time.I got my led strip in the mail finally.I installed it and all, carefully reassembling the panel and probably getting the diffuser on backwards.And to my surprise! it powered right up! sweet! I guess that one section of the led strip was actually causing the issue not powering on.But its not all perfect, I tried swapping the power wires around a few times, but it seems its an issue with the LED strip I received where the 3rd bank (of the 4) from the bottom seems to be brighter then the rest. I played with all the menus setting and even measured the voltage, (around 53 volts per bank I think it was) and no change so I'll have to get in contact with the aliexpress seller for a replacement, ugh but anyways, NEAT it works!Thanks everyone for all the suggestions. Now I just need a working led strip and a power brick for it.(the original got lost someplace, I never got it)