LCD Desktop Monitor Fading to White, then Black

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So I tried various techniques to try reproducing the problem today, all to no avail. I tried heat. I displayed a solid white image--with the brightness & contrast at their max level--for several hours straight; nothing. I turned it off for ~4 hours, and when I turned it back on, the problem repeated itself. So clearly heat is not a trigger, not is prolonged use. Unfortunately, the "randomness" of the problem persists.

Here's more observations:

When the monitor is in it's "black" state, it's not as if the monitor was off--there is a very low level of light on the screen, so it's more of a dark gray than black. Is this the backlight?

With the monitor in it's "gray/black" state, I pointed a flashlight at the screen at all angles & at a close distance. I do not see any of the image.

I noticed that as the problem was occurring, and the screen was fading first to white, and then to gray/black, the image is not being updated. I moved the mouse over to that screen while the fading was occurring, but the mouse on that screen did not move. Does this suggest a defective video board?

I also tried adjusting the brightness & contrast, but I couldn't, because the on-screen menu does not show up.

I don't mean to be overly critical, but I think I see a problem. You remind me of something I had to deal with when training technicians in a past life. I referred to it as "Circle the wagons. The Indians are attacking". This was in reference to the tendency for some techs to march around the problem area, gain a little more insight, but burn huge amounts of time going literally in circles. The Indians never had a chance, after tiring their horses, by going in circles around the wagons. Both you and the attacking Indians would be far more effective with a direct assault on the problem. At some point, you will need to dive into the LCD monitor and get your hands dirty. No amount of additional insight or diagnostics are going to prevent this from happening. Even if you isolate the problem to a single capacitor and totally undestand the failure mechanism, you'll still need to crack the case and do the necessary parts replacement. Might as well get it over with now (before your horse gets tired).

Excellent points, that need to be repeated often.

They're related to the issue of whether you're trying to figure out exactly what's wrong, or simply getting the unit working again. Often (too often), you have to abandon the analysis, and "dig in".

My guess is that this problem is related to the power-supply voltages for the video driver circuits. Second guess would be a bad video driver. (I don't know whether it's a board or a single chip in this unit.)

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Yes, I understand your point.

I think I'm more apt to approach this cautiously for several reasons.

One is that I don't have an urgent need to address the problem. This is partly due to my other monitor working fine, and also because the monitor does not malfunction frequently or for extended periods of time (a couple hours of downtime over several weeks).

Another reason is that I'm not very knowledgeable or experienced on this type of problem, so I'm ramping up quite a bit.

Yet another reason is that I'm a big proponent of measure twice, cut once. From what I've read, opening the case isn't simple, and will likely lead to blemishes. So I'd prefer to minimize the number of times I have to open it, which can be accomplished by having some confidence in the problem & solution. I understand that I won't know or understand every detail. But I don't think knowledge will hurt, and I'm not in a rush.

So given this list, I find it reasonable to perform some extra diligence and learn a lot that I don't know, before digging in. I realize some people have suggested that the caps are a common problem w/ this monitor, but their problem doesn't seem to be the same as mine. And I'd prefer to avoid unnecessary de-soldering & soldering, given that I can actually make things worse. Given the symptoms of the problem and all that I've learned from this group and the web, I'm inclined to think the problem is the video board. But since I need to open the case to get the video board's part & rev numbers, I'll have a chance to inspect the caps before proceeding.

Thanks to everyone for your help, I really appreciate it. I'll post back w/ my progress.

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I opened the case and analyzed all the caps--none are bulging or leaking/oozing.

One small step... etc.

Look at the photos in:

Do you see any bulging or leaks? Nope. Not all caps fail with obvious physical evidence. That's where the ESR meter is used. You can test the capacitors without removing them from the circuit. Lacking that, just get some replacement caps, replace everything listed in the article, and it should be back to working.

As for a better understanding of what's happening, it's difficult enough to analyzer a circuit when all the parts are within normal design limits. However, if you want to see what happens when you insert a defective capacitor into the circuit, download a free copy of LTSpice (formerly known as SwitcherCAD),

and buy a collection of SPICE switcher models:

The author suggests you need the book to understand the models. I agree.

Get the program working with a proper switching power supply. It doesn't have to be exactly the same as the Samsung monitor. Then cram in the equivalent of a bad capacitor, which would be a normal capacitor, with the same capacitance value as on the label, but with an added series resistance equal to about 2-5 times maximum ESR.

A subtle hint is that with a switching power supply, the output voltage almost never goes up when a component fails. It either goes down, or massive amounts of high frequency ripple appears on the output. How the rest of the LCD monitor responds to this is a bit of a guess. Usually, it just shuts down, resulting in a black display.

Also, I forgot to caution you to *NOT* constantly power cycle the monitor in an attempt to revive the circuit. It's bad enough having the capacitors blow up. There's a chance that the associated switching transistors will not appreciate the added load (which is what's heating the capacitors) and blow up. For example, this loser of a Dell monitor:

usually takes out 6 xsistors, a diode, and anywhere between one and eight electrolytics. I once tried to fix one, and didn't catch one of the caps. Within a week, all 6 transistors were again blown.

Also, if you read the above instructions carefully, note that it mentions resoldering the xformer leads. I do this whether it needs it or not. What happens is that the xformers operate at about 40KHz(?). Some of this 40KHz energy is transfered to the xformer leads, resulting in cracked solder. It's very difficult to see without a microscope. It was a problem with Tin-Lead solder but is really a PITA with brittle ROHS Tin-Silver solder. The same thing can happen at audio frequencies:

Anyway, your next ordeal will be selecting the proper replacement capacitors from the Digikey or Mouser online order pages. It's very easy to pick the wrong part. Be careful, or ask for help. Also, buy some spares. My prediction is that your matching monitor will soon have the same problem.

Thanks. Is there a particular brand of caps which is considered the best? From my reading, I should stay away from CapXon (which is what the Samsung monitor uses).

Looking at my inventory, I've been using Panasonic-ECG, type A, Series NHG, with 105C temperature ratings. For example, the Digikey part number for 1000UF/16v is P5533-ND

If you want to avoid the problem entirely, search for Polymer Capacitors, which don't have any electrolyte. Fujitsu, UCC, and Oscon make those.

I just tried to fix a Samsung Syncmaster 192N monitor. I replaced 6 assorted CapXon electrolytics but the monitor doesn't work. My ESR meter is at home so I'll drag the pile home and report on the measured ESR values. A bit of Googling seems to indicate the CapXon are junk.

More:

Hmmm... Weird and not according to my guesswork, as usual.

Value Max ESR Measured ESR

Max ESR from this chart:

Out of 6 caps removed and tested, only one of them is somewhat bad. No wonder replacing the caps didn't fix the monitor. Something else is wrong. So much for the easy fix.

Hi, i'm new here and sorry 4 my poor english, first of all this long flame seems to me much out of site (? exagerate..?), too much word maybe for a very simple problem, i'm not a tech but from the first agree with talks of J.Liebermann, the strange thing i spect from somone that suspected the main 5v line that ususally supply the graphic/video chip, so is not (for me) a bad idea to test the 5v line on the psu connector to the v.board since you don't found any bulged/ dirt capacitor, and if you find the v changes according to the fades then go to order caps, if not try at least to measure on the regulator on v.board to see if exit 3,3v fixed or some like it. I don't agree to what said, it's a luck that you have 2 equal monitor so in the last if you don't find culprit you can swap boards and see

bye bye.

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sorry, i espected... i've made a jam.. the strange thing was: nobody has told to test 5v line..

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go to order caps, if not

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Yes, I've been considering swapping one of the boards between the monitors. This seems like the easiest way to narrow down the problem.

is also the last way, control first out voltages, if bad change caps

ame seems to me much

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I swapped the power boards in the monitors. The "good" monitor immediately exhibited the problem that the previously malfunctioning one did, while the "bad" monitor works fine (at least, so far). So I'm confident that the problem is from the power board, and given this thread and some web sites, I'll start w/ the caps. Now that I had both power boards to examine, I also noticed that the C110 and C111 caps were replaced by Samsung when I sent the monitor in for repair earlier this year (the last month of its warranty). I'll get the specs of all caps today, find corresponding parts on DigiKey, and post back here for confirmation.

I will research this topic on the web, but are the Polymer caps entirely superior to electrolyte ones? Are there drawbacks to polymer caps?

This article covers most everything:

Polymer caps solve just about every complaint I can think of with electrolytics. The major benefits are much longer lifetime, lower ESR, and better temp stability. The down sides are the higher cost and that they are available only in fairly low voltages.

If you can find them, and can afford the cost, do it.

you say the bad mon. with the good power board i think...

ok, 1-0 for me... :) (ah it's a soccer score sorry..)

from what you noticed? The shining solderings?

i think is not so vital to find the specs, only take uF and V and order a good brand of low esr type. (105c)

As above i'm not an expert at all but i think.. with a good brand like Sanyo, Rubycon, Panasonic etc, you can go quiet and live life in peace for at least 3-4 years intense-use, it's a good idea replace all area's caps, to be sure.. like i said it's safer and instructive to test the 5v line output for instability, or the 12v if 5v is stable.. but it is no so safe whitout precautions so it's up to you, if the boards stays screwed on the back and face outside is more safe.

good brand of low esr type. (105c)

nyo, Rubycon, Panasonic

se-use,

afer and instructive

but it is no so safe whitout

d face outside is more

Yes, I'm confident the the problem is w/ the power board, since that's the only thing that changed, and the problem followed it. The monitor w/ the power board from the non-malfunctioning monitor has been functioning properly for 24 hours.

There are three reasons that I suspect the C110 and C111 caps were replaced. The first reason is that all caps across both power boards except those two have a hand-drawn black mark (line on the radius) on top. Those two have slightly shinier solder. And there are small scratches on the underside of the PCB around those caps' pins.

As for testing the 5V or 12V lines, that's not easy because the boards are not screwed down. There is a metal plate which sandwiches these boards to the back of the panel, and which provides the mounting point.

a good brand of low esr type. (105c)

er

Sanyo, Rubycon, Panasonic

ense-use,

safer and instructive

. but it is no so safe whitout

and face outside is more

Here's my first stab at finding replacement caps. I was not able to find any polymer caps to replace these on Digikey or Mouser. I opted for Panasonic if it was available.

PCB Designation Farads (=B5F) Voltage (V) Temp (C) Length (mm) Potential Length (mm) Diameter (mm) Lead Spacing (Board) (mm) Mouser Replacement Mouser Replacement URL C105 150 450 105 41 45 20 7 661-EKMQ451VN151MP40

If this doesn't display well, I can upload the spreadsheet as a PDF somewhere.