Dell u2311h monitor gone bad, which capacitors do I replace?

I have 3 u2311h monitors in an Eyefinity setup. For aesthetic purposes, I do not want to replace the faulty one with a newer u2312hm. The problem with the faulty one is as follows:

For the past few weeks, when I turn it on, it flashes the screen (ps3, Windows 7, whatever is connected) for a few seconds then turns black. Turning it off and on a few times and if I'm lucky, it sticks and works properly. Recently however, it never stayed on and always went black. I have read online that it's a sign of faulty capacitors.

I have opened the monitor and there are two PCBs. One for the main power connector and one for the VGA, DVI, USB etc. I can't for the life of my identify any capacitors that look faulty based on "spot a faulty cap" guides on the internet. None of them look bloated :/

I am fairly proficient with a soldering iron so I am confident I can replace any parts of the board. Am I going along the right tracks here or is this problem nothing to do with the capacitors? Also if it is to do with the caps, which ones do I replace? The ones on the power board, or the other board? Or both?

Edit: I have replaced all the small caps but unfortunately the same problem was still there. I didn't replace the massive cap on the board as an oversight. I have just ordered a new replacement powerboard anyway so that should do the trick.

Ozzy

Posted 2014-03-16T16:30:13.820

Reputation: 247

1These sorts of symptoms (called "two seconds to black") generally indicate CFL issues – either in the lamps themselves or in the inverter. – ntoskrnl – 2014-03-16T17:39:14.593

1Whatever you end up doing Ozzy, don't forget to come back to let us know :D – cde – 2014-03-17T03:25:40.967

Don't forget that capacitors in modern switched-mode power supplies can be terrifying. Always make sure the voltage across the leads is safe. Discharging can be misleading too. – None – 2014-03-17T04:02:18.097

@cde, i will keep you lot updated :) – Ozzy – 2014-03-17T09:31:47.510

@ntoskrnl, ill have a look at that if the cap repair doesnt work – Ozzy – 2014-03-17T09:32:13.530

@idrumm, I have disconnected the monitor from the power supply for a few days now and by the time the caps arrive, the existing caps should be completely discharged. If not, how would you recommend I discharge the caps? – Ozzy – 2014-03-17T09:34:34.273

you can usually test with a voltmeter to make sure the capacitor is discharged. Modern SM supplies should hav bleeder resistors that slowly discharge the caps when the power is removed. If the caps still have a non-negligable voltage, use a 1000Ω/1watt resistor across the leads and watch the voltage drop. As long as you check the voltage is safe, and use common sense you should be fine. My comment was not meant to be alarmist, but SMPS can kill. http://www.repairfaq.org/sam/smpsfaq.htm#smpssaf

– None – 2014-03-17T20:05:32.190

Answers

It has been my experience that the problem symptom you have seen is related to bad capacitors in the power supply board. I've replaced such capacitors in almost a dozen different monitors that had the same symptom and ended up working like new after repair. There have also been a couple of monitors where capacitor replacement did not fix the faulty behavior of the unit.

Some hints to think about before you dig into things and rip everything apart.

Label all wire and cable connections before taking anything apart so you can easily get it back together again in the correct manner. There is usually a lag between opening the unit and when you are ready to snap it all back together again that could be even 10 to 12 days. It is easy to forget how it all connected.
Make up a detailed list of every capacitor on the board as you work including the location where each was soldered in.
Most monitor power supply boards are cheap cheap one sided boards. It is very easy to damage the board copper pads and lands when removing the old capacitors. Make sure to use careful techniques when unsoldering including use of solder wick. Make sure the leads are free before trying to pull out the capacitor or you risk ripping the pads right off the board.
Some capacitors may be mounted on their side and glued down to the board with a hard white compound. Best first trick with those is to slit open the outer plastic wrap on the capacitor with a razor knife so that the capacitor body can move around while you free the leads.
Measure all the capacitors accurately. Replacements come in many
sizes and often size a replacement that is slightly larger may not
fit correctly once the monitor chassis enclosure is put back
together. Measure the length of the body, its diameter and the lead spacing. Replacements are often specified in metric mm dimensions.
When looking for replacements make sure to observe the temperature rating and voltage ratings. For instance do not replace caps that were 105C rated with 85C caps. The life time of repair will be significantly reduced if you try to scrimp on the replacements.
Select replacements from respectable vendors such as Panasonic and Nichicon. Stay away from unrecognizable brands like the types you are removing.
Note that the capacitors in these switching power supplies generally need to be high quality types with very low ESR and good Ripple Current ratings. Replacing them with cheaper garden variety capacitors is inviting early failure after the repair; if they work at all.
When reinstalling the capacitors make sure to observe the polarity markings and get the minus and plus sides aligned correctly.
I have found www.mouser.com to be a good source for replacement capacitors.

Good luck with your repair.

Michael Karas

Posted 2014-03-16T16:30:13.820

Reputation: 426

Thank you for your detailed reply. I took a video of the disconnect process, luckily there are only a few cables that needed to be removed so it wont be difficult to put back. I will only remove the caps once they are ready to be removed. I have found some replacement boards for about £40 but just bought replacement caps from ebay for every capacitor i could find on the power board. better safe then sorry :) – Ozzy – 2014-03-16T17:10:50.447

The caps i went for were Panasonic Electrolytic Radial Capacitors and they are rated at 105c. hopefully these are good enough to do the job. I was unaware that capacitor polarity mattered so thank you for making me aware of that. Hopefully i will be able to resurrect this monitor :D – Ozzy – 2014-03-16T17:12:47.657

I have found it much easier to measure things by removing them ahead of time. Many times it can be near impossible to read the labels on the capacitors when they are still on the board. – Michael Karas – 2014-03-16T17:12:59.643

1Sounds like you are somewhat new to electronics things. Yes polarity is VERY important. So are capacitance and voltage ratings. Make sure that you purchase low ESR types. Some switchers will just not work with the more garden variety capacitors. BTW Panasonic has a broad range including both low ESR types and the garden variety types. – Michael Karas – 2014-03-16T17:16:26.250

@ozzy keep in mind, caps are easily and often counterfieted. Ebay panasonics might not be panasonics, nor the right size or capatitenc or any spec they say they are. – cde – 2014-03-16T17:22:01.080

This is the reason I suggested to to use a place like Mouser.com to get the replacements. Panasonic caps there will have come from Panasonic. – Michael Karas – 2014-03-16T17:24:45.643

Yea, im a programmer by trade and the only electronics I have dabbled in is basic DC circuits with car audio. i am a bit out of my depth here but can solder well enough and dont mind learning as I go along :)

@cde, like i said in my previous comment, i was a bit impatient.. hopefully the ebay powerseller i bought it off is genuine as he has no negative feedback. fingers crossed i suppose :) – Ozzy – 2014-03-16T17:40:53.880

@MichaelKaras, uif the ebay ones do not work out, i will either use that site or if its cheap enough, get a replacement powerboard – Ozzy – 2014-03-16T17:41:24.253

This does sound like a bad caps on the inverter section of the power board. Dell has used subtandard caps for the last decade. Prone to capacitor plague. But 90% of the time the caps are physically blown or leaking. Even without the physical signs, that it turns on for a few seconds screams bad caps. You can buy a new power board (or swap one from the other 2 working ones, for testing), or you can bite the bullet and replace all the large caps with some decent low esr, higher voltage, same capatitence parts. Typically only 6 to 10 caps needed.

cde

Posted 2014-03-16T16:30:13.820

Reputation: 1 665

Is there a reason for higher voltage caps? I just bought replacement panasonic caps at the exact ratings of the ones on the board. Hopefully they will do the job. – Ozzy – 2014-03-16T17:13:49.827

@ozzy because the caps are undersized. If the voltage on the line spikes to say 20v, a 16v cap wouldn't be happy. A 24v cap wouldn't care. Dell went cheaper to save a penny or two per board, at the expense of product life time. Its not mandatory, but the voltage rating is a max tolerance spec. Next time go slightly higher voltage as a precaution. The same rated ones will do. – cde – 2014-03-16T17:19:19.250

ahh that makes sense. unfortunately, i am quite impatient when it comes to trying to fix things so bought the ebay caps before I read your answer. I will bear that in mind if the other ones die tho. Can it be a much higher voltage rating? or does it not matter as long as its reasonably bigger? – Ozzy – 2014-03-16T17:37:06.597

@ozzy the voltage rating is a 'max' spec. So it just needs to b higher than the voltage you will see on the line. The trade off is size. The higher the voltage rating, the physically bigger they tend to be. It could be a 1000v cap if that's all you have, but it probably won't fit. So just size it the next voltage up (16 -> 20 -> 24, etc). – cde – 2014-03-16T17:47:19.890

The caps specced at a higher voltage and the same capacitance tend to be either larger or higher ESR so I wouldn't recommend it. – Gunnish – 2014-03-16T22:05:27.047

Last bad (valuable) LCD monitor I had- one of the 30" Samsung monitors on my CAD workstation- was a BGA solder joint that went bad. Irritating when they don't match cosmetically and in terms of control placement.

Yours does sound like it could be the power supply. Maybe look at the startup resistor as well. I think these repair questions are considered by the powers that be to be off-topic, but perhaps it's interesting as an indication of where designs are vulnerable.

Spehro Pefhany

Posted 2014-03-16T16:30:13.820

Reputation: 331

BGA and samsung go hand in hand. With dell, its bad caps, 10 to 1. – cde – 2014-03-16T16:45:03.717

Im glad it most probably is the caps. I would hate to try and identify poor solder joints as every time I repair one, I would identify it again :P – Ozzy – 2014-03-16T17:14:23.400

1@ozzy worse, bga is ball grid array, all the solder joints are under the chip. You need an xray machine to properly identify poor joints! – cde – 2014-03-16T17:31:34.967

.. lucky its a dell monitor then! ill just replace the caps, if that doesnt work, new powerboard for me – Ozzy – 2014-03-16T17:35:39.320