inverter problem in LG 1934S LCD flatron

[woody_294]

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.

[retiredcaps]

Re: inverter problem in LG 1934S LCD flatron

but monitor still wount turn on. Can you give me some ideia what else can I check?

1) With power off and lcd unplugged, check your fuses with a multimeter. A good fuse should measure less than 1.0 ohms.

2) Is the green power led on?

Report back and we can suggest more test points. Also, please state your experience in using a multimeter. For newbies, we can give explicit instructions. If you have a Fluke 87-5, then I will assume you are an expert.

[ljubomir]

Re: inverter problem in LG 1934S LCD flatron

Hello people. I got one w1934s yesterday from a friend of mine, and he said to me that monitor just woun't turn on. I got it allready in peaces. I saw that the capacitor c506 is all black and around him too. in specification I've found that it is 10pF 3KV. I got one of those and have changed, but monitor still wount turn on. Can you give me some ideia what else can I check?
thnx

[helaleldeeb]

Re: inverter problem in LG 1934S LCD flatron

thank you verry much for all this informations

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

Congratulations on the successful repair.

Thanks To close, I did the same check on the second of these monitors that I had, and replaced the same CCFL (the one with the blue/black wires on the top of the monitor). Same problem on the inside - return wire was loose, and there were scorch marks on the rubber end cap.

Thanks again for all the great assistance!

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

I got my new CCFLs last night and replaced the one defective-looking lamp from the top pair (it had dark ends, and the return wire was loose). I'm not sure if it's possible to get away with not completely taking the reflector assembly apart, but I couldn't figure out how to do it. Either way, I soldered the new lamp in place, plugged it in, and it worked perfectly!

After doing the replacement, I was curious if I could measure current draw and voltage drop of the bad lamp using my case mod inverter. I know the measurement won't necessarily mean anything for math in a different circuit, but I thought it might be useful to spot a difference between lamps. I wasn't brave enough to hook my multimeter to the return end of the CCFL to measure voltage, but I did measure current into the inverter on the bad lamp and one of the new ones. The inverter used 473mA on both lamps after sitting for a few minutes, but the inverter used 437mA immediately when plugged in with the new lamp attached, and 455mA with the bad lamp attached. Looks like I might be able to use this trick in the future to pick out a bad CCFL if current through the inverter is different for one lamp in a monitor than the others.

I had one more question after finishing up - what do you do with old CCFLs? Can I treat my bad one like any other fluorescent bulb and take it to my local transfer station for recycling, or is there something special I should do with it since it contains mercury?

Thanks for all the help, everyone! I really appreciate it!

Congratulations on the successful repair.

Thanks for the information on the current draw from the case mod inverter with the bad CCFL. Altogether, a lot of useful information.

Standard florescent lights contain mercury, just like CCFLs (Cold Cathode Florescent Light). Take them to the transfer station.

PlainBill

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

If your realy wanting to stock your tool box look at http://ccflwarehouse.com/
They have a good assorment of ccfl pig tails,pin connectors-terminal pins and I think the kapton tape is important. It's the yellow tape.
Al.

Thanks for the tip; I may end up doing that if I start to see a number of these monitors with similar problems. I ended up paying $32ish (incl. shipping) for 4 CCFLs on eBay from an auction that specifically mentioned the model of my monitor, which works out to about $8/lamp (seems like a competitive price, and shipped from the US). Since I only ended up needing one, I have 3 spares now. I re-used the original yellow tape, but it would probably be a good idea to get some more if I end up doing this more often. Same is true of the metal foil tape (that stuff refuses to stick again after being peeled up).

I got my new CCFLs last night and replaced the one defective-looking lamp from the top pair (it had dark ends, and the return wire was loose). I'm not sure if it's possible to get away with not completely taking the reflector assembly apart, but I couldn't figure out how to do it. Either way, I soldered the new lamp in place, plugged it in, and it worked perfectly!

After doing the replacement, I was curious if I could measure current draw and voltage drop of the bad lamp using my case mod inverter. I know the measurement won't necessarily mean anything for math in a different circuit, but I thought it might be useful to spot a difference between lamps. I wasn't brave enough to hook my multimeter to the return end of the CCFL to measure voltage, but I did measure current into the inverter on the bad lamp and one of the new ones. The inverter used 473mA on both lamps after sitting for a few minutes, but the inverter used 437mA immediately when plugged in with the new lamp attached, and 455mA with the bad lamp attached. Looks like I might be able to use this trick in the future to pick out a bad CCFL if current through the inverter is different for one lamp in a monitor than the others.

I had one more question after finishing up - what do you do with old CCFLs? Can I treat my bad one like any other fluorescent bulb and take it to my local transfer station for recycling, or is there something special I should do with it since it contains mercury?

Thanks for all the help, everyone! I really appreciate it!

[alexanna]

Re: inverter problem in LG 1934S LCD flatron

If your realy wanting to stock your tool box look at http://ccflwarehouse.com/
They have a good assorment of ccfl pig tails,pin connectors-terminal pins and I think the kapton tape is important. It's the yellow tape.
Al.

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

What I am about to say is going to open the can of worms but here goes.
When you order replacement ccfls you're asked the length and the diameter. Is going from a 17” to a 22” cclf going to change the current flow enough to cause shut down of the ccfl circuit,I do not know.
I can tell you what I have had success with though .Its has been using 17” bulbs for testing monitors up to 22” wide.
I use a panel with the LCD removed that has good 17” bulbs in it with the proper connectors to plug in to the suspect inverter. I have been able to identify bad ccfls in a number LG 22w-Samsung 19” to 22”w and HP 19” wide monitors, if a bad ccfl is suspected I do open up the panel and replace/repair the bad ccfl.
I may have been extremely lucky but it has worked for me.
I wonder if a case study may be in order. I will continue to test this way and if I run into a particular monitor that this does not work on I will post a note about it. Maybe other members could help. It sure would save a lot of time on diagnostic.
Al.

A 'case mod kit' is one method. If you hook up the CCFL from the kit to the inverter in a problem monitor (an adapter would be necessary) and the 'two seconds to black' problem goes away, the problem is most likely the original CCFL. If the inverter from a case mod kit fails to light a monitors CCFL, the CCFL is bad or there is a wiring problem. If the monitor's inverter fails to light a ccfl from a case mod kit, the inverter is most likely bad.

I use 'probably' and 'most likely' because the monitor inverter closely checks the voltage and current through the CCFL. A CCFL with different voltage or current characteristics could cause shutdown.

PlainBill

Sounds like it might be worth my time to order a couple of extras next time I buy and just keep them around. I usually end up fixing Samsungs with bad capacitors in the power supply but that are otherwise fine, so I don't have very many spare parts hanging around. If I attach wires to a couple of bare bulbs, I could probably use these small jumper wires I have (the kind designed to fit a pin that's about the size of a component leg) to plug them in to the inverter instead of the usual tan plastic connector.

I bought a case mod kit a while ago for this purpose, but I wasn't sure if the results I got would be reliable after reading some other posts. It sounds like I can probably use spare CCFLs to test questionable inverters, and the case mod kit's inverter to test questionable CCFLs. After doing all this troubleshooting, it might be worthwhile for me to compile a list of things to check and some suggestions for how to test them so next time I have a problem I can come with a bunch of information ready to go.

I'm waiting on my new CCFLs now, so I'll post back after I get the replacement installed. Thanks for all the help you've given me so far!

[alexanna]

Re: inverter problem in LG 1934S LCD flatron

Maybe you will be lucky and just find a wire you will need to re solder.
I have had very good luck just testing with known good bulbs, and if the 2sec to black goes away with the test bulbs it's time to go inside the panel.
You will probably be able to see the failed bulb,If you cannot,Plug them in after there out of the pannel
Al.

What I am about to say is going to open the can of worms but here goes.
When you order replacement ccfls you're asked the length and the diameter. Is going from a 17” to a 22” cclf going to change the current flow enough to cause shut down of the ccfl circuit,I do not know.
I can tell you what I have had success with though .Its has been using 17” bulbs for testing monitors up to 22” wide.
I use a panel with the LCD removed that has good 17” bulbs in it with the proper connectors to plug in to the suspect inverter. I have been able to identify bad ccfls in a number LG 22w-Samsung 19” to 22”w and HP 19” wide monitors, if a bad ccfl is suspected I do open up the panel and replace/repair the bad ccfl.
I may have been extremely lucky but it has worked for me.
I wonder if a case study may be in order. I will continue to test this way and if I run into a particular monitor that this does not work on I will post a note about it. Maybe other members could help. It sure would save a lot of time on diagnostic.
Al.

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

I took the whole thing apart (which was not particularly easy) and found that one of the tubes is a bit more black on the HV end than the others. Its return wire also has signs of being hot as there is what appears to be a scorch mark on the rubber cap, and the wire was loose. I bought some new CCFLs on eBay, and I'll update again when I get them installed.

When you say "test bulbs", do you have a spare set with the connector and such on them? I had to completely disassemble the backlight (unstuck the wires, removed the connectors, cut off the heat shrink tubing, pulled the rubber caps off, and I will have to unsolder the bad tube and solder a new one in and then put it all back together), and it seems like a lot of work to test one lamp.

Also, I didn't see an answer so I'll ask again: Is a case mod cold cathode kit sufficient for most testing of inverters (using the kit's tube) and CCFLs (using the kit's inverter)?

A 'case mod kit' is one method. If you hook up the CCFL from the kit to the inverter in a problem monitor (an adapter would be necessary) and the 'two seconds to black' problem goes away, the problem is most likely the original CCFL. If the inverter from a case mod kit fails to light a monitors CCFL, the CCFL is bad or there is a wiring problem. If the monitor's inverter fails to light a ccfl from a case mod kit, the inverter is most likely bad.

I use 'probably' and 'most likely' because the monitor inverter closely checks the voltage and current through the CCFL. A CCFL with different voltage or current characteristics could cause shutdown.

PlainBill

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

Maybe you will be lucky and just find a wire you will need to re solder.
I have had very good luck just testing with known good bulbs, and if the 2sec to black goes away with the test bulbs it's time to go inside the panel.
You will probably be able to see the failed bulb,If you cannot,Plug them in after there out of the pannel
Al.

I took the whole thing apart (which was not particularly easy) and found that one of the tubes is a bit more black on the HV end than the others. Its return wire also has signs of being hot as there is what appears to be a scorch mark on the rubber cap, and the wire was loose. I bought some new CCFLs on eBay, and I'll update again when I get them installed.

When you say "test bulbs", do you have a spare set with the connector and such on them? I had to completely disassemble the backlight (unstuck the wires, removed the connectors, cut off the heat shrink tubing, pulled the rubber caps off, and I will have to unsolder the bad tube and solder a new one in and then put it all back together), and it seems like a lot of work to test one lamp.

Also, I didn't see an answer so I'll ask again: Is a case mod cold cathode kit sufficient for most testing of inverters (using the kit's tube) and CCFLs (using the kit's inverter)?

[alexanna]

Re: inverter problem in LG 1934S LCD flatron

Maybe you will be lucky and just find a wire you will need to re solder.
I have had very good luck just testing with known good bulbs, and if the 2sec to black goes away with the test bulbs it’s time to go inside the panel.
You will probably be able to see the failed bulb,If you cannot,Plug them in after there out of the pannel
Al.

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

Yeah, go ahead and swap them. I see three possible results - It now works the same, it works better, or it works worse. Each of those will tell us something.

It would have been nice if it turned out to be a bad solder joint on a SMD cap, but it's beginning to look like a CCFL. At least it isn't an unobtainable transformer.

PlainBill

Swapping transformers didn't change anything, and shorting the end of R506 to ground still kept the backlight on.

After all that diagnosing and measuring, it looks like one of the simpler explanations was the right one. I tried alexanna's suggestion of using only the lower CCFLs from both panels, and the backlight stayed on! Now to find a source for 19" wide CCFLs...

While thinking about this, I was wondering if there's a better way to test CCFLs to see if they're any good. I have a case mod kit that has a CCFL and an inverter that is probably good enough to diagnose a completely failed CCFL (if it's safe to use one of these on LCD CCFLs), but other threads I've read suggest that maybe these aren't suitable for this kind of testing. Is there a better way to diagnose bad CCFLs than replacement? If not, I'd better order a couple of extras for testing in the future. Also, how do I determine which of the 2 upper CCFLs is the bad one? Is this going to be trial and error also?

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

I think I read the suggestion a little too fast. I haven't tried using the bottom CCFLs on both monitors, but I need to reattach the transformers to the board first and I was waiting to hear if PlainBill thinks that swapping them would be a good idea before I solder them back on.

I also gave my second monitor (all parts, including the IP board) another try this morning and much to my surprise, it seemed to work fine. I had swapped the CCFLs though as I had inverted the board to gain access to the components, so I could very well have at least one CCFL in less-than-perfect shape.

Yeah, go ahead and swap them. I see three possible results - It now works the same, it works better, or it works worse. Each of those will tell us something.

It would have been nice if it turned out to be a bad solder joint on a SMD cap, but it's beginning to look like a CCFL. At least it isn't an unobtainable transformer.

PlainBill

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

I'm already doing that. To get access to the back of the board while powered on, I had to flip the back of the monitor over.

I think I read the suggestion a little too fast. I haven't tried using the bottom CCFLs on both monitors, but I need to reattach the transformers to the board first and I was waiting to hear if PlainBill thinks that swapping them would be a good idea before I solder them back on.

I also gave my second monitor (all parts, including the IP board) another try this morning and much to my surprise, it seemed to work fine. I had swapped the CCFLs though as I had inverted the board to gain access to the components, so I could very well have at least one CCFL in less-than-perfect shape.

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

Great idea!!!

PlainBill

I'm already doing that. To get access to the back of the board while powered on, I had to flip the back of the monitor over.

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

By chance is it the upper bulbs/connectors that you're having problems with?
I have had quite a few widescreen monitors that have had problems with the upper bulbs giving a problem, usually a return wire just barely making contact, I guess it's because of the heat rising inside of the panel. Could you try plugging in the bottom bulbs of your two panels to your inverter and see if it makes a difference?

Great idea!!!

PlainBill

[alexanna]

Re: inverter problem in LG 1934S LCD flatron

By chance is it the upper bulbs/connectors that you're having problems with?
I have had quite a few widescreen monitors that have had problems with the upper bulbs giving a problem, usually a return wire just barely making contact, I guess it's because of the heat rising inside of the panel. Could you try plugging in the bottom bulbs of your two panels to your inverter and see if it makes a difference?

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

Desolder the transformer. The ring tester I have uses an analog meter so it's possible to say something like 2.5 rings. With the LEDs it's a little more difficult.

Do label them so you know where they came from.

PlainBill

Swapping panels and backlights didn't change anything. Shorting the end of R506 to ground allowed the backlight to stay on again. I'm running the monitor with the panel disconnected, if that matters; the (LVDS?) ribbon cable won't reach.

I desoldered both transformers and tested them with a multimeter and the ring tester. They both measured the same on the ring tester, with the high voltage side of both returning 3/8 rings and the two windings on the low voltage side of both returning 5/8 rings. There is little resistance (< 0.6 ohm) between pins 1-4 and 3-6 on both transformers. T501 measured 621 ohm between pins 7-8, and T502 measured 642 ohm. All other pin combinations read open.

When I put the transformers back on, I can swap them to see if the problem follows a transformer instead of the remaining parts (assuming that I don't have two bad CCFLs in the same position on both monitors). Should I do that?