inverter problem in LG 1934S LCD flatron

[jay1]

Re: inverter problem in LG 1934S LCD flatron

thanks lexwalker.

cold/dry solder joints

last time I inspect everything looked pretty steady. i'll look again. but does a "dry solder joints" explains the display off with heat up?

SMD capacitors in the inverter

well capacitors on the inverter board are really tiny. are they replaceable (by a non prof person like me)?

[lexwalker]

Re: inverter problem in LG 1934S LCD flatron

I have the same monitor. here is my problem:

when i turn on the monitor in the morning, it works for 15/20 minutes then display goes black. the power LED stays on (blue). If i push the power off and turn on again (right way) i see the display for a second, then it goes back black. If i wait few minutes then turn on, then displays says for few minutes then goes black.


I opend up and inspected the capacitors visually. they all looks great. I didn't find any other damage or loose connection (visually) either.


then i thought may be its overheating. so i open the back and placed a table fan on full speed. it worked for hours! after half an hour it flickered a little, but did not go black. after about 3 hour it finally go black (with the fan on). in another word with the fan on it works longer the the problem is there.

so what is the problem? or what can i do to isolate the problem?

Try to look for cold/dry solder joints. There's also the possibility that some of the SMD capacitors in the inverter feedback and protection section is failing.

[jay1]

Re: inverter problem in LG 1934S LCD flatron

I have the same monitor. here is my problem:

when i turn on the monitor in the morning, it works for 15/20 minutes then display goes black. the power LED stays on (blue). If i push the power off and turn on again (right way) i see the display for a second, then it goes back black. If i wait few minutes then turn on, then displays says for few minutes then goes black.


I opend up and inspected the capacitors visually. they all looks great. I didn't find any other damage or loose connection (visually) either.


then i thought may be its overheating. so i open the back and placed a table fan on full speed. it worked for hours! after half an hour it flickered a little, but did not go black. after about 3 hour it finally go black (with the fan on). in another word with the fan on it works longer the the problem is there.

so what is the problem? or what can i do to isolate the problem?

thanks

[Lumberjack777]

Re: inverter problem in LG 1934S LCD flatron

But I had learned sometimes ago that you can replace lcd flourescent lamps with a led kit with lamps. I bought it and changed with the led lights. It worked. Finally my lcd monitor turned to be a led monitor. Thanks a lot.
VK

Nice job!

[vakar]

Re: inverter problem in LG 1934S LCD flatron

Thank you very much all contributers, especially PlainBill, alexanna and stevenps and others. I had an Lg w2253s monitör not working. The problem was after I turned on the monitör it stayed on about 1/2 seconds and it got completely black, after about 20 or 30 seconds it turned on and stayed for another 1/2 seconds and then it got black screen again. I could see nothing. But the stand up light was still blinking.
After I read this article about lg monitors I decided to dissassamble the monitor and change the cold-cathode fluorescent lamp ( CCFL). First I checked the power board very carefully. There was no problem with the power board and inverter. While I was dissambling lcd part it I realized that one of the wires welded to the one side of the CCFL was broken. Probably it would work again if I soldered the wire to the place where it detached . But I had learned sometimes ago that you can replace lcd flourescent lamps with a led kit with lamps. I bought it and changed with the led lights. It worked. Finally my lcd monitor turned to be a led monitor. Thanks a lot.
VK

[selldoor]

Re: inverter problem in LG 1934S LCD flatron

Hi - welcome to the forum - having read your introduction I doubt there is much I can help you with - fortunately there are much more experienced members and hopefully you will join as a contributor.
As you will have seen there is a schematic for this at elektrotanya.com and retiredcaps guide to 2 seconds to black is good - it has been updated and now starts at #19 .
If you get stuck many will help - if you post pictures please post pictures of each board, front and back and close up of connectors, (max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

Examples of what is needed
https://www.badcaps.net/forum/attach...1&d=1290283049

https://www.badcaps.net/forum/attach...7&d=1280167246

[4z4rb]

Re: inverter problem in LG 1934S LCD flatron

After reading all 4 pages I will see about my monitor
it has the same 2 seconds problem. before it died on me it used to flicker once or twice on start and work ok it did it 3 or 4 times and zapp
only on reseting the AC it will start and show for two seconds and then .....
thanks

[nikoslemnos]

Re: inverter problem in LG 1934S LCD flatron

The output voltages of the inverter are in the order of 1KV at 40 Khz. That is beyond the normal capabilities of a DMM, and trying to measure it can kil a DMM.

PlainBill

Thanks PlainBill.. my concern also.... I will try to replace the two power mosfets (AP9971GD IC's) and see if that fixes things...

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

hi all I am only using a dmm.... I need some help in measuring an LG flatron 1710 inverter voltages..... From the schematic the voltages are on the 1KV range.... Did anyone measure them? I want to trouble shoot my inverter...... so far I have found that the U501, and U502 are shorthed.... Help please...

The output voltages of the inverter are in the order of 1KV at 40 Khz. That is beyond the normal capabilities of a DMM, and trying to measure it can kil a DMM.

PlainBill

[nikoslemnos]

Re: inverter problem in LG 1934S LCD flatron

hi all I am only using a dmm.... I need some help in measuring an LG flatron 1710 inverter voltages..... From the schematic the voltages are on the 1KV range.... Did anyone measure them? I want to trouble shoot my inverter...... so far I have found that the U501, and U502 are shorthed.... Help please...

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

Thanks.

Ok. I have disassembled the monitor. It was a bit tricky to open it up even after looking at the service manual. So, the board seems to be without any visible damage.
I have already looked through the troubleshooting guide from the "jetadm123" post. I noticed then, that the 2nd pin of IC501 has not 5Vdc but 6.6Vdc. Is this normal? If not then what should I do next?
If it is normal then I faced another problem. I checked the pin10 of IC501 for high level voltage? it is 4.2Vdc. The thing is that I am not sure what "high level" actually means

While the pictures aren't great, they are good enough to confirm the caps LOOK good; of course appearances can be deceiving. They probably aren't the problem because of the time lapse between initially turning the monitor on and the backlights failing.

I'm afraid the next most likely suspect is the CCFLs, but let's try an easy test first. T501 and T502 are the inverter transformers. Pins 7 and 8 are the secondaries on each one. Measure the resistance between pins 7 and 8 of T501 and between pins 7 and 8 of T502. They should be about 1K, and must match to within 3%. Report the numbers.

I'll look up the information on the voltages at IC501. That may well be a clue as to what is going wrong.

I just reviewed the schematic; that clears it up slightly. Pin 2 is the Power pin of IC501, pin 10 is Enable. In this context 'high' means 2.2 volts or higher. The problem is the troubleshooting guide covers NO backlights, you have a variation of 'two seconds to black', so we know most of the test points (Power, Enable, are good).

One additional simple test. Does the screen have a pinkish tint when it is first turned on, particularly if it's been sitting off for more than an hour? This tint may last for only a few seconds.

PlainBill

[younio]

Re: inverter problem in LG 1934S LCD flatron

Take the %#@#%*%#@ monitor apart!!! See if there are any obvious problems. If there are not, take good pictures of the top and bottom of the power supply and attach them to your next post using 'Manage Attachments' - below the text entry area.

There are a number of possible causes, none of them are going to get fixed without taking the monitor apart.

PlainBill

Thanks.

Ok. I have disassembled the monitor. It was a bit tricky to open it up even after looking at the service manual. So, the board seems to be without any visible damage.
I have already looked through the troubleshooting guide from the "jetadm123" post. I noticed then, that the 2nd pin of IC501 has not 5Vdc but 6.6Vdc. Is this normal? If not then what should I do next?
If it is normal then I faced another problem. I checked the pin10 of IC501 for high level voltage? it is 4.2Vdc. The thing is that I am not sure what "high level" actually means

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

Hi,

I have the similar problem with the monitor mentioned it the first post of this thread LG Flatron W1934S.

Few days ago it suddenly went black while the blue light still lit indicating that it is turned on and connected to power and video source. I tested it with different cable and different machine, different OS, but the result was the same. So it is definitely a monitor problem.
In detail:
After turning the monitor on it goes fine for about 15-20 min. and then goes black. It usually goes black when bright colors are displayed. Also I noticed that when I set brightness (in the monitor settings) to the lowest value, it will work then for 1 to 3 hours. But after that it still goes black with blue light at the bottom still shining. Then if I turn it on again it displays for a few seconds flashes (bright, low bright and again bright) and goes black again. If I wait about 20 mins I can use it again for 1 to 3 hours at lowest brightness.

I think it could be a component that heats up too much and then shuts off. I haven't tried to take it apart yet.

So I am waiting for any help anyone can offer.
Thank you in advance

Take the %#@#%*%#@ monitor apart!!! See if there are any obvious problems. If there are not, take good pictures of the top and bottom of the power supply and attach them to your next post using 'Manage Attachments' - below the text entry area.

There are a number of possible causes, none of them are going to get fixed without taking the monitor apart.

PlainBill

[younio]

Re: inverter problem in LG 1934S LCD flatron

Hi,

I have the similar problem with the monitor mentioned it the first post of this thread LG Flatron W1934S.

Few days ago it suddenly went black while the blue light still lit indicating that it is turned on and connected to power and video source. I tested it with different cable and different machine, different OS, but the result was the same. So it is definitely a monitor problem.
In detail:
After turning the monitor on it goes fine for about 15-20 min. and then goes black. It usually goes black when bright colors are displayed. Also I noticed that when I set brightness (in the monitor settings) to the lowest value, it will work then for 1 to 3 hours. But after that it still goes black with blue light at the bottom still shining. Then if I turn it on again it displays for a few seconds flashes (bright, low bright and again bright) and goes black again. If I wait about 20 mins I can use it again for 1 to 3 hours at lowest brightness.

I think it could be a component that heats up too much and then shuts off. I haven't tried to take it apart yet.

So I am waiting for any help anyone can offer.
Thank you in advance

[jetadm123]

Re: inverter problem in LG 1934S LCD flatron

Thanks guys for pointing me to all that useful stuff! I think I'll make much better progress when I get going this time!

For reference and any further questions I ask, some photos and info:

Also the ICs, IC501 is an OZ9938GN
U501 and U502 are both a P6006HV

I have the photos in a much larger resolution but I though 800px wide would be enough, let me know if it's not

Thanks again all, great replies!!

If you haven't done so, try measuring the resistance across the inverter transformer secondaries (marked 7 and 8 on the circuit board) for transformers T501? and T502? Should be no more more than 3% difference in your readings.

[woody_294]

Re: inverter problem in LG 1934S LCD flatron

Thanks guys for pointing me to all that useful stuff! I think I'll make much better progress when I get going this time!

For reference and any further questions I ask, some photos and info:

Also the ICs, IC501 is an OZ9938GN
U501 and U502 are both a P6006HV

I have the photos in a much larger resolution but I though 800px wide would be enough, let me know if it's not

Thanks again all, great replies!!

[retiredcaps]

Re: inverter problem in LG 1934S LCD flatron

2 seconds to black guide.

Start with post #13 and #14 at

https://www.badcaps.net/forum/showthread.php?t=10419

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

What EGuevarae said. Also, retiredcaps has written extensively on troubleshooting '2 seconds to black' problems.

PlainBill

[jetadm123]

Re: inverter problem in LG 1934S LCD flatron

Hi guys, much searching and some testing and I'm a bit stuck. I have this monitor and the backlight goes out after a 2 seconds.

Here's what I've done so far:

I think I eliminated a bad CCFL by unplugging one at a time and turning it on. Same problem.

I've followed this flow chart in the service diagram and it all seems to check out.



I'm not sure if my results are any good though as I've not desoldered anything from the board, and I'm getting a 1.7ish volts reverse voltage on a couple of the diodes and transistor junctions.

Also when it said about instantaneous pulse I just stuck the dmm on dc and there was definitely some "pulsey" potential going on in there.

Am I going about this all the wrong way? Not done any electronics for quite a while and most of that was telecomms based.

p.s. I'm using a Fluke 25 I can get a 'scope from work but I'd rather not lug it home and set it up on the dining table if I can help it!

As an afterthought, I'm testing this with the CCFLs and the panel NC, is that going to put a spanner in my works? And there is no green light! I have a blue/orange one on the bezel but I thought there may be one on the PCB somewhere. Should there be?

Errr... I'm poking holes in my testing quite a bit here, I measured the inverter transformers just for resistance, they were the same and not a large number.

Welcome to the forum! There's a few things that need to be cleared up. First, like EGuevarae says, disconnecting one CCFL at a time only tells the CCFL controller chip that there's an open circuit and it will kill power to all the CCFL's. Ideally, you'd want to check the CCFL's by substituting a known good CCFL one at a time. With power off, disconnect one CCFL and connect the good one. Power up monitor. If it goes off then power down and try the next tube. Do this for all 4 tubes. If you get all 4 tubes to light, then you've found the bad tube.

Next thing, good flowchart. However, since we don't know what U501, U502, IC501, etc is, it makes it difficult to follow what's going on. This is why we request clear focused photos of the entire power board top and bottom.

Your problem can be caused by:

bad CCFL(s)
bad CCFL wiring
bad inverter transformer
bad CCFL controller or sense circuitry

[EGuevarae]

Re: inverter problem in LG 1934S LCD flatron

I think I eliminated a bad CCFL by unplugging one at a time and turning it on. Same problem.

The inverter will shut down if one lamp is not connected even if all the lamps work, as the inverter will "sense" there is a problem.

Post pictures of your boards ...