Flickering non-LED LCD monitors opinion - Page 1
Forum section: electronics / general

author: kalel
author_rank: Frequent Contributor
timestamp: December 25, 2017, 12:00:15 pm
content: I have noticed visible flickering on mostly gray backgrounds (on white it is difficult to see) with 2 old LCD monitors. They are both old, but seemed to flicker mostly in one room some years ago.One is not used anymore, and the other recently "died" (would restart when turning on a couple of times, then finally stopped turning on at all), and after a repair (changing the capacitors on the PSU) it seems to be working well again, but it seems to now flicker in every room. It did have aluminum tape on all sides holding the part with both boards (power supply and small board with VGA connector), but the aluminum tape was cut to get to the board. I did add some copper tape, but that will not be as conductive as they used a much larger (and more of) aluminum tape.Previously, the flickering did not happen if the monitor was connected to a laptop (with the ungrounded 2 pin supply type), both on battery or when plugged in, but did happen on desktop (tried with more than one, different cables, etc). I haven't rechecked the laptop situation after repair.The flickering does not happen on modern LED backlit monitors, only the old CCFL (or whatever the proper name is). It reminds a bit of CRT flickering, it is very noticable on gray backgrounds, but not always the same intensity. It does seem to go away on a reduced resolution, increased refresh mode rate.One electrolytic capacitor I did not change was the large mains rated one (the noticeably largest one in circuit). I don't know if it has any connection with the flickering.Any ideas on what might be the cause of the issue? I'm not sure if it's just the age of the monitors causing this issue. And they are not worth very much at all, so I'm more curious on what the problem might be than anything else.

author: SeanB
author_rank: Super Contributor
timestamp: December 26, 2017, 09:08:24 am
content: Flickering is often an old CCFL lamp slowly failing, and the arc inside becoming unstable and threading inside the glass tube. Sadly not repairable other than a new tube or a LED retrofit. the end of the tube will be well blackened and will show as a slightly duller light from the corner of the display.

author: kalel
author_rank: Frequent Contributor
timestamp: December 26, 2017, 12:03:49 pm
content: Quote from: SeanB on December 26, 2017, 09:08:24 amThanks, that might be the right explanation for the issue, but I have to verify on the laptop again. If it doesn't have flickering on a laptop (and it didn't some time ago when it still did flicker on desktop), then it might be some kind of other issue? It happens on 60 Hz, not on 75 Hz (but also at 75, the resolution is small).Obviously, if it's the tube itself, it should happen on all type of computers, but I don't know if an old tube could be more susceptible tonoiseand the laptop being a less noisy source for whatever reason (both with battery and it's power pack with a 2 prong non-grounded plug). If that's possible, that could be an explanation.I haven't actually looked at the tube so I don't know how it looks, I heard about high voltages for the tube drive, and only worked on the power supply PCB module, which is something that seemed the most likely broken part (the capacitors on the board aged) considering the old age of the monitor. Replacing those did fix the issues with start up, which it now does "instantaneously", but the flickering part is definitely unchanged.

author: kalel
author_rank: Frequent Contributor
timestamp: January 05, 2018, 05:53:30 am
content: I have had a chance to try on the laptop again (ungrounded plug) and it does not flicker at all, but does on desktop (not just a specific one).The only obvious difference is the grounding (of the computers, the monitor is always connected with a grounded plug).I will likely try to do an LED conversion as a learning experience, since LED backlight monitors do not flicker, but I am still curious to any guess on what might be causing the issue exactly (if there is any). I do not have any useful noise checking equipment. The flickering is not extreme, it is most visible on gray-ish backgrounds and is not something that would be properly visible on a recording.