Wondering about a synchronous fault of two Dell Monitors - Page 1
Forum section: electronics / general

author: rolfstargate
author_rank: Newbie
timestamp: August 12, 2025, 09:51:05 am
content: Hello,I have a question about a weird phenomenon I experienced, and I would like toknow what may have caused it or if it was only a coincidence. I bought threeDell Ultrasharp U3417W monitors, all used and not in perfect condition. Two ofthem arrived with a small crack in the corner, maybe 2 cm long. I did not reallycare much. Using them for work, they are mostly for reading text and that workedstill fine.After about two weeks, the first monitor started to show flickeringwith horizontal lines over the whole screen. At the bottom, the lines are2mm thick, gradually getting thinner toward the top. When powered on, it takesabout 4 seconds for the lines to start appearing, flickering every 1 second. Oneday after the first monitor, the second monitor developed the same symptoms.The frequency is a little different, and after about a minute, the lines becomepermanent. The lines appear in the same way on all three color channels. Thethird, without a crack, still runs fine. All three screens were mounted in avertical orientation, but the one still working was rotated in the oppositedirection from the faulty ones. The fault stays the same, regardless if themonitors are connected or not.Now I am wondering if a crack in the display can lead to components down theline being damaged, or if it is more likely that the impact causing the crackdamaged the screens.Can the vertical mounting position cause such effects?(They can't be pivoted from the original stand; I have a custom mount.)Does the flickering give any clue as to what the fault is?I will return the screens, so I do not plan to try to repair them.I am mainly curious.Thanks in advance to anyone who can shed a better light on this.Best regards,Paul

author: tom66
author_rank: Super Contributor
timestamp: August 12, 2025, 12:33:58 pm
content: If the LCD panel is what is cracked then it will generally sever the ITO lines in the panel, which will cause pixels to go off or get stuck on if the crack leads them to get shorted to something else. I have seen LCD panels with minor impact damage that still mostly worked, maybe with only the surface layer cracked, but once this progresses through the glass layer it can lead to the electrodes being damaged. Sometimes pressing on this crack will cause it to grow gently and progress the damage through the glass layers. I suspect what may have happened in your case is over time the crack has progressed due to thermal cycling, movement or otherwise and caused damage to the rows/columns where the crack exists.I have seen one photo of a plasma panel with a similar fault, where the impact damage was sufficient to damage just a local area of cells, leading to dead rows and columns in that spot, but not causing enough damage to release the gas from the panel and prevent functionality.A photo might help confirm the fault.(source - many moons ago I repaired flat panel TVs)Edit: here is a similar Dell monitor with a micro crack -https://www.dell.com/community/en/conversations/monitors/u3821dw-cracked-lcd-screen-experience/6570d04b10a0e11f6b7106d7Maybe these panels are susceptible to forming cracks on the edge due to bad design - mounting of the panel internally for instance - or maybe this is a coincidence.

author: rolfstargate
author_rank: Newbie
timestamp: August 13, 2025, 01:08:05 pm
content: Thanks for the reply,I had the progressing cracks before, and this is what I found weird, because it does not look like this at all. There are no dead pixels. The stripes are light, working pixels, and they blink. So while they are present for a second, they then disappear again the next, and the image is totally fine. I added a photo. But I now believe all of this has a different reason that is more plausible to me. I have a cheap USB fan that I plugged into the first monitor that failed, and after it developed the fault, I removed the screen from my setup a day later and plugged that fan into the other monitor, which then developed the same fault. So that both of them were the cracked ones is sheer coincidence and luck (kinda) because I can return them.Could this theory be plausible?If it is possible, it would be interesting to check the fan. I do have an oscilloscope, but my knowledge about electrical engineering is very limited. How would a signal look if it is possible at all that something like this can cause this?

author: tom66
author_rank: Super Contributor
timestamp: August 13, 2025, 05:19:08 pm
content: No, I doubt the USB fan caused this issue.That kind of image tearing problem can be caused by a missing connection to the LCD panel, usually due to a tab bonding failure. It's usually unrepairable, though there are exceptions to this rule, they usually require very precise tools not available to the layman.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 14, 2025, 11:36:54 am
content: Quote from: rolfstargate on August 13, 2025, 01:08:05 pmOh yes, indeed, it absolutely could be. In fact I would say it is more than plausible, it is highly likely. You obviously know how to confirm it.Thanks for getting back to us - very interesting.

author: rolfstargate
author_rank: Newbie
timestamp: August 16, 2025, 06:35:21 pm
content: Sorry for the late reply. I am a bit short on time at the moment, but I am alsovery intrigued. I hope I can find some time tomorrow to take a reading.Guessing that there will be some huge voltage spikes if the theory holds up. Howwould I protect my oscilloscope? A voltage divider with two resistors? Somethinglike this: GND -- 5 ohms -- 500 ohms -- VNC, and measuring between them woulddivide the voltage by 100x? As mentioned, I don't really have a plan, only somedistant memories, and I would be very sad if the oscilloscope died. It's one ofthose tools that I rarely use, so it is hard to justify replacing, but at thesame time, if you need it to quench some curiosity, there is not really analternative.Thanks for your interest and patience.

author: rolfstargate
author_rank: Newbie
timestamp: August 17, 2025, 07:23:50 pm
content: Ok, I did it, and it wasn't as bad as I imagined. The voltage spikes go up to 9.20V, and the signal is really noisy. No idea if this is enough to damage a monitor.

author: tom66
author_rank: Super Contributor
timestamp: August 17, 2025, 10:20:53 pm
content: How did you measure that? If the 5V output from the monitor looks like that, it's pretty bad, but I suspect something else is going on here.

author: rolfstargate
author_rank: Newbie
timestamp: August 19, 2025, 04:26:03 pm
content: I took a picture. I measured over the IN+ and IN-. The USB cable was connectedto a Samsung USB power supply. When off, normal 5V was supplied to the fan; whenon, the signal changed to this. It was pretty late, and I just now noticed thatthere is a resistor in the upper left corner. It doesn't look factory and seemsto be inline with the motor. Pretty weird. I found the fan a while ago in a giftbox, so it seems the previous owner fumbled with it. I assume it was to make itslower. Maybe this is the reason for the bad noise?

author: wraper
author_rank: Unknown
timestamp: August 19, 2025, 04:39:23 pm
content: QuoteOf course it's more plausible that it's fan's fault that cracked LCDs have problems... only according to some crazy woman logic.

author: rolfstargate
author_rank: Newbie
timestamp: August 19, 2025, 05:36:33 pm
content: Very useful... but also why? Something like, "Looking at that signal, I highly doubt that this could cause the fault, because..." would be way more interesting or even that's stupidbecause...

author: tom66
author_rank: Super Contributor
timestamp: August 19, 2025, 07:23:21 pm
content: That voltage waveform is pretty bad, but I wonder if you're just seeing motor commutation noise, and that isn't actually the 5V output from the power supply.I doubt it was the cause of the problem. I'd expect worst case a fault on the 5V supply of the monitor would lead to it not powering on correctly, not a distorted image.

author: thm_w
author_rank: Super Contributor
timestamp: August 19, 2025, 11:49:10 pm
content: The fan has a diode on 5V coming in, and then another diode, presumably as part of a boost circuit to boost 5V to 12V for the fan.Its really odd to see 9V there on the input like that, it should be blocked by the diode. Maybe check its not blown?The added resistor is probably just to slow the fan down as you say.The USB supply is hopefully a genuine samsung, it looks like.

author: rolfstargate
author_rank: Newbie
timestamp: August 20, 2025, 06:48:59 pm
content: Thanks for everyone's answers. The signal is from the motor, if I turn the fanoff, I get a normal and clean 5V. The Samsung power supply is genuine. I feelthe overall notion is correct, and it's very unlikely that 10V can beresponsible for the fault. I will still ditch the fan. Thanks again for yourreplies.

author: u666sa
author_rank: Frequent Contributor
timestamp: August 20, 2025, 07:11:32 pm
content: Used and cracked here is a major sus. How much did you pay for your monitor? Because I got me a perfectly good monitor off aliexpress for $160, same size. It's not cracked, it's not used, it's bran new and perfect. That's what I recommend you do, get things brand new.