LCD Monitor flicker - Page 1
Forum section: electronics / repair

author: cprobertson1
author_rank: Frequent Contributor
timestamp: February 27, 2015, 04:12:21 pm
content: Good Evening Folks!Having just fixed the microwave (burnt out turntable motor... and some cheese underneath it somehow? Still haven't figure out how all that flaky stuff got down there...) I've decided it's time to tackle my LCD monitor.Now, I know a lot less about LCD monitors than I do about microwaves... so I figured I'd check on here before yanking anything out of its innardsLets see... it's a... Flatron W2230S: probably about five years old now: and suffers from screen flicker - everything else is fine:colour, contrast,sharpness etc - it's just flickering - in fact it's not even that bad.Contraindications:-Wiggling the power jack causes the flickering to worsen: it also seems rather looseI suspect it's one or two things: some of the caps are probably gone - and in addition to that I reckon there's a cracked solder joint on the power jack.Which leaves me with a problem: the power jack I can fix... the capacitors I can replace, if I know which one's are blown - but therein lies the rub! Only one of the capacitors looks dodgy: and I've replaced it to no avail (I marked the trace it's on, will figure out the rest of it later - I've only really had a peek inside so far!)The problem is that I don't have an ESR meter: the only test equipment I have are a bench power supply, standard multimeter, and an analogue oscilloscope - is there a workaround for figuring out dodgy caps or would it be worth just replacing all of the caps on that particular trace (and probably the power filtering caps while I'm at it)?Any help would be appreciatedAs I said, I figure I'd better bother the experts before diving in! (though, to be fair, I usuallydojust dive in - it's just I don't want to risk wrecking this monitor as I can't afford a new one until April xD)--EDIT--Just took apart the Microwave's motor - turns out it wasn't actually burnt out: the rotor-magnet was split more or less in half (probably taken some strong knock at some point I reckon!)

author: Shock
author_rank: Super Contributor
timestamp: February 27, 2015, 11:40:54 pm
content: You can assume that if one cap has gone that the others in that same series/brand/location are also suspect. Visual indicators such as bulging, venting electrolyte from the top or bottom of the cap which can cause discoloration of the PCB and fade the printing on SMD caps. The operational temp and voltage can cause a cap to fail early such as caps near heat sinks.Obviously you can check capacitance with a meter that measures capacitance. A multimeter can perform a basic test on the ohm range. An ESR meter is the quickest way to see if the electrolyte is drying out as the resistance of the capacitor will increase dramatically. There is also the dissipation factor which is used to check leakage or "how well the capacitor is working". You can also make (or buy) an octopus component testing circuit for your oscilloscope, this will allow you to display voltage/current, you can then compare it to a working capacitor.You can measure ripple in a circuit with an oscilloscope which will show if the capacitor is doing it's intended job, you can add another capacitor in parallel and observe if the problem is corrected, and you can remove the capacitor and measure in a test circuit. You can measure between the signal/rail and ground to check resistance. In some cases temporarily removing a bad capacitor will fix the problem (where it's operation was marginal).While not a total test, an ESR meter is pretty useful and will save you a lot of time and find caps you may otherwise have had no idea were faulty or out of tolerance. The Chinese ebay ones start at about $10-$15 delivered which also test a variety of components and capacitance. The DER EE DE-5000 is about the best bang for buck at around $80 on ebay which is also multi function.You can also use the freeze and heat method, freezing capacitors raises it's ESR so faulty or borderline caps may reveal their symptom. Heating will cause the ESR to lower, so if you have a symptom that goes away after the circuit has been powered on for a while you can isolate which capacitors or other components are the cause. To do this you will need freeze spray or a can of compressed air turned upside down, to heat the circuit you can use a hot air gun to gently warm areas.Make sure you discharge the capacitors before measuring and keep in mind "in circuit" measurements can be misleading due to capacitors and other resistances in parallel, some measuring equipment may also turn on semiconductors and give misleading results.Measuring any live circuits can be dangerous, especially on SMPS and inverters on LCDs and Plasmas, also CRTs and Microwaves. Large capacitors and other circuits can storage large charges so voltages/currents are there even when the equipment is disconnected.

author: Shock
author_rank: Super Contributor
timestamp: February 27, 2015, 11:51:17 pm
content: To answer the question about, replacing or not replacing. If they are the same batch or have visual or tests indicating to do it, then yes.Getting a cheap ESR meter will save you time and at least confirm your suspicions. By the time you have done your second cap repair you probably will wish you had one, especially if you start replacing random capacitors with guesswork.

author: tautech
author_rank: Super Contributor
timestamp: February 28, 2015, 12:36:51 am
content: I'd also add that if cap values have drifted more than say 10% they are suspect IMO.If you have a scope, why don't you just check for ripple on each rail?More than a 50 mV

author: edavid
author_rank: Super Contributor
timestamp: February 28, 2015, 01:06:22 am
content: It could be the inverter or backlight, in which case you wouldn't need to change any caps...

author: cprobertson1
author_rank: Frequent Contributor
timestamp: February 28, 2015, 10:46:01 am
content: Aye, I considered the backlight as well - will need to see once I crack it open again. It was basically just a visual inspection I done earlier: need to find the time to do it though =/ Auch well.I'll let you know my findings!

author: cprobertson1
author_rank: Frequent Contributor
timestamp: March 14, 2015, 08:16:20 pm
content: Well! I fixed it!I don't knowhowI fixed it... but I did...I opened it up, took the chipset out, probed it with my oscilloscope from the bench power supply, couldn't find anything untoward, and put it back in assuming I was just going to give up and wait for the monitor to die completely...And when I plugged it back in, it works fine.There are two boards (actually, I found this rather neat!) - one was the video input and decoder - and the other was the power supply / LCD-driver - meaning the HD version of the monitor would require you to simply swap the video input/decoder chip with a different version.Anyway, they were joined with a simple 12-pin right-angle header - which I can only assume must have had a dicky contact which magically cleaned itself when I disconnected/reconnected it!Any other suggestions on my monitors miraculous recovery?

author: wraper
author_rank: Unknown
timestamp: March 14, 2015, 08:29:42 pm
content: Quote from: cprobertson1 on March 14, 2015, 08:16:20 pmThis is just how it is in 99% of the monitors. PSU always will be on separate PCB. One could expect more than 2 PCBs but not less.