Flicker LCD Backlight - Need help identifying a component - Page 1
Forum section: electronics / repair

author: leestrong
author_rank: Newbie
timestamp: June 20, 2021, 11:05:38 am
content: Greetings,This LG LCD monitor has been making buzzing noise for few months, recently the noise grew progressively louder, accompanied by flickering backlight in synchrony with the buzz.By listening I identified a few possible culprits (photo), although I can't tell exactly which one is buzzing, what do you think is most likely the source?In addition, I wasn't able to identify the component on the left, it says "7AW"(horizontally) and "27" (vertically), any ideas?

author: tooki
author_rank: Super Contributor
timestamp: June 20, 2021, 11:31:18 am
content: The components here that are capable of making acoustic noise are ceramic capacitors (piezoelectric effect) and inductors (magnetostriction).However, the fact that theydidn’toriginally make noise makes me think that theloadsthe ceramic caps and inductors are seeing have changed. This makes me think theroot causeis not a bad ceramic cap or inductor (neither of which are components that tend to fail), but rather something else, like a failed electrolytic capacitor (which have a great propensity to fail with age and heat). Do you have an oscilloscope? If the circuits here are all low-voltage, check the power rails to see if there’s excessive ripple on them.

author: Fraser
author_rank: Super Contributor
timestamp: June 20, 2021, 12:29:22 pm
content: First thing to check is the load on the power supply as a varying load can induce noise in a drive circuit. If it is an LED backlight, it is a string LED’s in series driven at quite a high voltage. You need to test the LED string for an intermittent connection or failing LED. Household LED lamps can fail in a similar manner…. first the power supply starts to ‘sing’ as one of the series wired LED elements begins to fail, then the failure progresses to a point that the LED string becomes intermittent and flickers or flashes. When inspecting the LED elements a small black dot is often visible at the point of intermittent failure.If the LCD panel uses two separate strings of LED’s it can sometimes be obvious that one string is flickering and the other is stable. This can suggest a failure in the flickering LED string, rather than the power supply.If you prove the LED backlight is working reliably, then you can look more closely at the behaviour of the power supply. Use a suitably isolated oscilloscope to look at the output and determine what is happening from the waveform produced.

author: leestrong
author_rank: Newbie
timestamp: June 20, 2021, 03:07:56 pm
content: Quote from: tooki on June 20, 2021, 11:31:18 amSadly I only have a multimeter. Been wanting to buy an Diligent Analog Discovery 2, but the price jacked up from $269 to $379 these few months. I placed an Amazon tracker on it, hope that price will come down soon...Since the capacitors are cheap, I think its worth a try to just buy some and hope that it would fix the problem.The circuit is low voltage, there is an external power brick, and the input to the monitor is 19V DC.

author: gamalot
author_rank: Super Contributor
timestamp: June 20, 2021, 03:20:37 pm
content: The transistor you cannot identify is one of the most commonly used - MMBT3904http://www.farnell.com/datasheets/454047.pdf

author: leestrong
author_rank: Newbie
timestamp: June 20, 2021, 03:21:13 pm
content: Well... very much appreciate the help fromtookiFraserandgamalotbut I am afraid that our little journey has come to an end when I accidentally cracked a tiny piece off the corner of the TFT glass backplate...such is life.

author: Fraser
author_rank: Super Contributor
timestamp: June 20, 2021, 04:12:50 pm
content: An LCD panel is not a vacuum device. You may find that the small piece cracked off carries no signal traces. Worth testing to see if the panel still works.Note that when I said the LED backlight is high voltage… I am talking about 30V to 70V which is high compared to the voltages on the logic board. The backlight power supply board takes the 19V external supply voltage and boosts it up to the required LED drive voltage. Note that the two output electrolytic capacitors are rated at 100V for this reason.Fraser

author: leestrong
author_rank: Newbie
timestamp: June 20, 2021, 05:14:58 pm
content: Quote from: Fraser on June 20, 2021, 04:12:50 pmThe panel would go dark and "reboot" every 10 seconds or so. In between reboots, content is displayed but with vertical and horizonal color lines flashes in and out.There is only one LED strip, and it seems fine. Every LED chip looks perfect, no discoloration and all lite up $ flickers at the same brightness level.

author: tooki
author_rank: Super Contributor
timestamp: June 20, 2021, 05:24:17 pm
content: Oh well. I’d save it for parts though. The diffuser films in LCDs are fascinating and useful for retrofitting backlights onto LCDs that didn’t originally have them.