Help me fix my LCD monitor

[xboomz]

Maker : Daewoo Lucoms
Model : SL700C
Made in 2003

My LCD broke down and I decided to do something on it myself. The problem is that I have no idea how to fix it. So I searched on the internet and luckily found this helpful forum. I believe there is somebody who knows well about this matter.

Symptom : 1. At the first stage, the monitor's power light LED kept changing from orange to green and then green to orange while my computer was in 'sleep mode'. It showed me a color adjustment screen(stripes of Red, Cyan and Green) when the LED turned green. Normally it used to stay in orange when it got no signal from computer although its power was on. And the problem started when I woke up my computer by shaking the mouse.

2. The power LED started to blink rapidly. The color was green. And I couldn't see anything on my monitor. Sometimes the LED stayed in green not blinking but I could see it slowly gets to blink dimmly again.

I thought there's a problem on capacitors, so I checked everyone of them after buying an analogue tester. From some caps I got no reaction on my tester.

Question :
1. I tried to order caps. One question I still don't have an answer to is whether the ceramic capacitor on the board is for AC or DC. I know the home electricity is AC but I'm not sure if all the parts on LCD board are for AC.

2. I can see the biggest black cap(refer to the picture below) is dome
d a little bit. But when I check it with my tester, the reaction seems to be alright. The needle goes up immediately and comes down to 0 degree. Is my method of test wrong?

3. Since I cannot be sure if it will revive after I change all the suspicous parts, I would like to consult somebody. Will there be anywhere I should consider checking also?

-----
Below except the last one are the pictures of the board I suspect to be broken. (CML-700C POWER is written on the front surface)
In the last picture is the board which has a power connector linked to power source.
(L523B*/L703C* DAEWOO LUCOMS Co.,LTD is written)

[PlainBill]

Re: Help me fix my LCD monitor

Maker : Daewoo Lucoms
Model : SL700C
Made in 2003

My LCD broke down and I decided to do something on it myself. The problem is that I have no idea how to fix it. So I searched on the internet and luckily found this helpful forum. I believe there is somebody who knows well about this matter.

Symptom : 1. At the first stage, the monitor's power light LED kept changing from orange to green and then green to orange while my computer was in 'sleep mode'. It showed me a color adjustment screen(stripes of Red, Cyan and Green) when the LED turned green. Normally it used to stay in orange when it got no signal from computer although its power was on. And the problem started when I woke up my computer by shaking the mouse.

2. The power LED started to blink rapidly. The color was green. And I couldn't see anything on my monitor. Sometimes the LED stayed in green not blinking but I could see it slowly gets to blink dimmly again.

I thought there's a problem on capacitors, so I checked everyone of them after buying an analogue tester. From some caps I got no reaction on my tester.

Question :
1. I tried to order caps. One question I still don't have an answer to is whether the ceramic capacitor on the board is for AC or DC. I know the home electricity is AC but I'm not sure if all the parts on LCD board are for AC.

2. I can see the biggest black cap(refer to the picture below) is dome
d a little bit. But when I check it with my tester, the reaction seems to be alright. The needle goes up immediately and comes down to 0 degree. Is my method of test wrong?

3. Since I cannot be sure if it will revive after I change all the suspicous parts, I would like to consult somebody. Will there be anywhere I should consider checking also?

-----
Below except the last one are the pictures of the board I suspect to be broken. (CML-700C POWER is written on the front surface)
In the last picture is the board which has a power connector linked to power source.
(L523B*/L703C* DAEWOO LUCOMS Co.,LTD is written)

Congratulations, you have provided us with excellent information and pictures. You are also wise enough to ask questions before making expensive mistakes. That's the good news.

Now for the 'not so good news'. You have made a few mistakes. The very large capacitor seldom fails, that 'domed' appearance is due to a plastic cap intended to prevent contacting the top of the capacitor, which could be a nearly line voltage. Also, the ceramic caps very rarely fail; when they do, it will be very obvious. Lastly, while your testing process will catch a totally failed cap, it doesn't detect the type of failures that could be causing the problems you are seeing.

Now for the bad news. The symptoms you describe are consistent with failures of the smaller electrolytic caps. Given the age of the monitor, it is very likely that is the cause. I have marked two in red that are suspicious. One because it's top has an unusual appearance, another because it is under the heat sink. Are any others under the two heat sinks? Placing caps there was a very bad idea, the extra heat ages them. The two caps circled in blue look normal, but are in the power supply filtering circuit and must be considered suspects. The three circled in green are known to cause problems when they fail, and they seldom give any external sign.

The really bad news - is it even possible to replace the cap under the heat sink? And are there any more?

What brands are the electrolytic caps on this board?

PlainBill

[xboomz]

Re: Help me fix my LCD monitor

I really appreciate, PlainBill. Thanks to your advice, I could learn more.

To answer your questions, the caps' brand is Sam Young. And there are total 5 caps under two heat sink, 3 in one and 2 in another.

So, then the ceramic cap doesn't have to be replaced even though it doesn't react to my tester? Is there other ways I can tell whether a certain cap(ceramic or electrolytic) is alive or not with an alalogue tester?

And there are chip caps on another board. I was thinking of replacing them also. Is it ok for me to do so?

[PlainBill]

Re: Help me fix my LCD monitor

I really appreciate, PlainBill. Thanks to your advice, I could learn more.

To answer your questions, the caps' brand is Sam Young. And there are total 5 caps under two heat sink, 3 in one and 2 in another.

So, then the ceramic cap doesn't have to be replaced even though it doesn't react to my tester? Is there other ways I can tell whether a certain cap(ceramic or electrolytic) is alive or not with an alalogue tester?

And there are chip caps on another board. I was thinking of replacing them also. Is it ok for me to do so?

I am of the opinion that it would be pointless. The best approach would be to replace all the electrolytics on the power supply board EXCEPT the very large one.

PlainBill

[momaka]

Re: Help me fix my LCD monitor

The very large capacitor seldom fails, that 'domed' appearance is due to a plastic cap intended to prevent contacting the top of the capacitor, which could be a nearly line voltage.

While true, it is always a good idea to check that capacitor by hand. Simply push the plastic sleeve on the top with your finger, and if it feels hard, doesn't go flat, or if feels like the capacitor is still domed underneath it, that capacitor could be bad. Some power supplies tend to get horrible regulation when the primary capacitor(s) go out and this is what could be causing the abnormal flickering.
If I remember correctly, other people have also had Sam Young capacitors fail, so probably a good idea to replace those as well.
Before you buy anything, though, check the fuses on the inverter board. They're usually tiny. I can't seem to locate them on the pictures, but they should marked on the board. Probably something like F1, F2, F3, etc. If those are open when measured with a DMM, there could be more problems with the monitor.

[Agent24]

Re: Help me fix my LCD monitor

I am of the opinion that it would be pointless. The best approach would be to replace all the electrolytics on the power supply board EXCEPT the very large one.

PlainBill

I think this is probably the best way to go, too.

[xboomz]

Re: Help me fix my LCD monitor

Thank you Plain Bill, momaka and Agent24. I'll do so and let you know the result soon again.

[Radio Fox]

Re: Help me fix my LCD monitor

...I thought there's a problem on capacitors, so I checked everyone of them after buying an analogue tester. From some caps I got no reaction on my tester...

...I can see the biggest black cap(refer to the picture below) is domed a little bit. But when I check it with my tester, the reaction seems to be alright. The needle goes up immediately and comes down to 0 degree. Is my method of test wrong?...

...So, then the ceramic cap doesn't have to be replaced even though it doesn't react to my tester? Is there other ways I can tell whether a certain cap(ceramic or electrolytic) is alive or not with an alalogue tester?...

Which analogue tester are you using to test your capacitors?

From the quotes above it sounds like you are using it on a resistance range to see if the needle kicks when connected to a capacitor. This is only a very basic indication that the capacitor is holding a charge, and will only work on capacitors above a certain value. The ceramic capacitor in the photo is 10nF which is probably too low to test with your meter.

Your method of testing will not indicate high ESR, or show if the capacitors are the correct value (although it will weed out leaky or short circuit capacitors).

[xboomz]

Re: Help me fix my LCD monitor

Thank you, Radio Fox. Right you are that I'm using it on a resistance range. Do you mean that by doing so I can't know if the cap is shorted or broken since caps with low capacity doesn't react to that kind of testing?

[PlainBill]

Re: Help me fix my LCD monitor

Thank you, Radio Fox. Right you are that I'm using it on a resistance range. Do you mean that by doing so I can't know if the cap is shorted or broken since caps with low capacity doesn't react to that kind of testing?

The test will detect shorted caps (resistance will remain a 0 ohms) and will find open electrolytic caps, particularly the larger value ones. It is unlikely you would be able to distinguish between say a 330µfd and a 1000µfd cap, however.

That isn't really a problem in this case. The symptoms point to defective electrolytic caps. The layout of the circuit board ensures the electrolytic caps will be thoroughly cooked. The brand of caps used isn't particularly good. Since you obviously don't have a capacitance or ESR meter handy, the next step is to replace ALL of the electrolytic caps on the power supply. The only problem is removing and replacing those caps under the heat sinks. Do you have enough room to do that?

PlainBill

[Agent24]

Re: Help me fix my LCD monitor

If not, you'll have to desolder and remove the heatsinks. Not impossible, but not the easiest to do either...

[Radio Fox]

Re: Help me fix my LCD monitor

...Your method of testing will not indicate high ESR, or show if the capacitors are the correct value (although it will weed out leaky or short circuit capacitors).

Sory to quote my own post, but I should have qualified the above to say that using that method to test capacitors "in-circuit" may give a false result for leaky or short-circuit components, as nearby components could give incorrect readings. The capacitors would have to be removed first.

Anyway, as already recommended by others, replacing all the electrolytic capacitors will, hopefuly, cure the problem.

[kc8adu]

Re: Help me fix my LCD monitor

that primary cap is suspect to say the least.

[xboomz]

Re: Help me fix my LCD monitor

The only problem is removing and replacing those caps under the heat sinks. Do you have enough room to do that?

I thought I could remove the sinks first and then touch the caps. I don't think working under the sinks will do.
[QUOTE=Radio Fox] Anyway, as already recommended by others, replacing all the electrolytic capacitors will, hopefuly, cure the problem.
[/QUOTE}
Sure, I see. Thanks for your kind explanation.

If not, you'll have to desolder and remove the heatsinks. Not impossible, but not the easiest to do either...

Yeah, maybe I should practice with another material before I succeed on the real trial.

[Agent24]

Re: Help me fix my LCD monitor

[QUOTE=xboomz]I thought I could remove the sinks first and then touch the caps. I don't think working under the sinks will do.

Anyway, as already recommended by others, replacing all the electrolytic capacitors will, hopefuly, cure the problem.
[/QUOTE}
Sure, I see. Thanks for your kind explanation.

Yeah, maybe I should practice with another material before I succeed on the real trial.

Yes you should, if you haven't done it before. Obviously if you take too long you can overheat something and cause even more problems.

If you have enough room to unscrew the silicon from the heatsink first, that will help greatly.

You'll need a higher power iron than usual too probably. I'm sure someone else can advise on that better though

[xboomz]

Re: Help me fix my LCD monitor

Now I've got parts. Way to go fixing.

[xboomz]

Re: Help me fix my LCD monitor

Oh my got. I burned the 'line filter.' Still I can smell the chemical smoke in my room.

Here is what went. I noticed that one solder ball(of a capacitor that I was going to change) extended to another(of a resistor) making it look like a solder strait. But I didn't care since it had been as it was, and I resoldered as it was. I was testing the monitor everytime I changed one cap, and I don't know if it was right after I resoldered the one linked to resistor, the fuse cut off. I laid a solder string instead of the broken fuse recklessly. I connected the power again only to see the' line filet' bombing. The plastic part was melted and foamed up.

I'm thinking of buying a new fuse and line filter. And after changing the circuit with those two, I'll repeat the process again.

What do you think about this error and future plan?
(It's not easy at all for a layman to fix a electronic stuff.)

[momaka]

Re: Help me fix my LCD monitor

I was testing the monitor everytime I changed one cap...

That's usually not a good idea. Always replace all capacitors at once.

... after I resoldered the one linked to resistor, the fuse cut off. I laid a solder string instead of the broken fuse recklessly. I connected the power again only to see the' line filet' bombing. The plastic part was melted and foamed up.

I'm thinking of buying a new fuse and line filter. And after changing the circuit with those two, I'll repeat the process again.

Now that the fuse has blown and the coil has burned, it's very likely that there's something else wrong. Find whatever is causing the short first, otherwise if you just replace the fuse and the coil, the fuse will very likely blow again. Start with checking components on the primary side, most importantly the bridge rectifier, switching transistor, and all diodes.

What do you think about this error and future plan?
(It's not easy at all for a layman to fix a electronic stuff.)

Don't worry, you're not doing bad. Just one suggestion, though: when in doubt, ask before trying and don't assume .

[xboomz]

Re: Help me fix my LCD monitor

Thank you for your comment, momaka. Yeah I should have stopped when there was a problem. I was imprudent.

So I borrowed two books from library yesterday. They are about basic electronics. I'm going to read them and grasp the principles of this SMPS.

[Agent24]

Re: Help me fix my LCD monitor

Here's a quick guide which might help you troubleshoot SMPS. I have found it quite useful myself

http://repairfaq.org/sam/smpsfaq.htm