inverter problem in LG 1934S LCD flatron

[flexx]

hi , i got 19 inch LCD monitor from LG model 1934S , and after using it it seems that the monitor can operate for just 5 seconds and shut down and the blue light seems the monitors is runing

i looked deeply in the monitor , ican see my desktop in very dark way without backlight

i opened it and really changed all electrolyte caps with new , but it seems without any result

this is my monitor



and please give me away to fix it and fix the backlight inverter

[Bobdee]

Re: inverter problem in LG 1934S LCD flatron

hi , i got 19 inch LCD monitor from LG model 1934S , and after using it it seems that the monitor can operate for just 5 seconds and shut down and the blue light seems the monitors is runing

i looked deeply in the monitor , ican see my desktop in very dark way without backlight

i opened it and really changed all electrolyte caps with new , but it seems without any result

this is my monitor



and please give me away to fix it and fix the backlight inverter

I guess you are right on it being inverter, but there's no info on this Monitor in forum so pics will help thoses who help you, (inverter and ps board both sides if poss)

[Leopard]

Re: inverter problem in LG 1934S LCD flatron

i opened it and really changed all electrolyte caps with new , but it seems without any result

Pictures would be VERY helpful. Also, we need to know what capacitors you used as replacements, and what capacitors you took out of the monitor. Capacitor brand, series and voltage plus uF values are needed. These monitors need low ESR capacitors.

Also, there could be bad solder joints (check transformer solder joints) or dead/shorted transistors in the unit.

[EGuevarae]

Re: inverter problem in LG 1934S LCD flatron

Pictures would be VERY helpful. Also, we need to know what capacitors you used as replacements, and what capacitors you took out of the monitor. Capacitor brand, series and voltage plus uF values are needed. These monitors need low ESR capacitors.

Also, there could be bad solder joints (check transformer solder joints) or dead/shorted transistors in the unit.

I agree. We need original and replacement cap info (maybe you used incorrect caps as a replacement and the problem lies there. We could not tell without the info), pictures of the boards (yes, the unit is LG on the outside, but you'll be surprised how many "brands" are brands only on the outside...and how many internal variations of the same unit exist), and that if any question arise, you can provide part info (specially on transistors/ FETs, ICs).

And welcome to the forums.

[flexx]

Re: inverter problem in LG 1934S LCD flatron

oki ill take a pictures from inside and uploade it in to this forum as soon

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

I realize this thread is over a year old, but I have this same monitor and what sounds like a similar problem and I was hoping that I could get some help fixing it. The monitor is a LG Flatron W1934S and its backlight won't stay on. I replaced the caps with new ones (Panasonic FC and Nichicon HZ) but that didn't help. I have another monitor that still has original caps (Su'scon and Lelon) that behaves the same way.

After reading through some other posts, I tried measuring the voltage on the return lines from the CCFLs and I had readings all over the place. Two consistently read around 0.04v, but the other two are 0.2v and 0.7v (swapping tubes doesn't change this - the higher voltages stay with the connector). The inverter IC is a OZ9938GN (). The chip is acting like the OVP/OCP circuit is shutting things down as the voltage on pin 3 (TIMER) increases to 2.75v over about one second then the backlight turns off. What more can I do to troubleshoot and hopefully fix this monitor?

[jetadm123]

Re: inverter problem in LG 1934S LCD flatron

I realize this thread is over a year old, but I have this same monitor and what sounds like a similar problem and I was hoping that I could get some help fixing it. The monitor is a LG Flatron W1934S and its backlight won't stay on. I replaced the caps with new ones (Panasonic FC and Nichicon HZ) but that didn't help. I have another monitor that still has original caps (Su'scon and Lelon) that behaves the same way.

After reading through some other posts, I tried measuring the voltage on the return lines from the CCFLs and I had readings all over the place. Two consistently read around 0.04v, but the other two are 0.2v and 0.7v (swapping tubes doesn't change this - the higher voltages stay with the connector). The inverter IC is a OZ9938GN (). The chip is acting like the OVP/OCP circuit is shutting things down as the voltage on pin 3 (TIMER) increases to 2.75v over about one second then the backlight turns off. What more can I do to troubleshoot and hopefully fix this monitor?

Assuming that you have verified the power supply is supplying the proper 5V and 12V outputs, you might try and check to see if there are any surface mount fuses (on foil side of board) that might be blown. If there are no fuses (or if they check good), then with power off and monitor unplugged, check U501 and U502 for shorts. These are probably dual mosfets (can't read the part number) driven by the OZ9938. If one is bad, then replace them both.

There is also the possibility that that one or more of the CCFL tubes is starting to fail.

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

U501 and U502 are Niko-Sem P6006HV(dual N-channel FET). I'll check them tonight. I didn't see any surface-mount fuses, though I'd expect the inverter would not work at all if a fuse was blown. The only fuse I see (aside from the line fuse, F801) is FB501 which is provides 12v to the inverter, and it's fine. I believe the 12v output from the power supply is actually a little over 13v, which is hopefully within tolerance.

[retiredcaps]

Re: inverter problem in LG 1934S LCD flatron

The monitor is a LG Flatron W1934S and its backlight won't stay on.

1) Just to clarify, does the backlight initially come on and then go to black? If yes, how long does the backlight stay on?

2) Or does the backlight NOT come on at all?

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

U501 and U502 are Niko-Sem P6006HV(dual N-channel FET). I'll check them tonight. I didn't see any surface-mount fuses, though I'd expect the inverter would not work at all if a fuse was blown. The only fuse I see (aside from the line fuse, F801) is FB501 which is provides 12v to the inverter, and it's fine. I believe the 12v output from the power supply is actually a little over 13v, which is hopefully within tolerance.

You've done an excellent job of troubleshooting . You've established the problem lies with the inverter, not the CCFLs. You are also correct, it's not due to a problem with a fuse.

Breaking the problem down, there are two possibilities. The drive voltage to one set of CCFLs is wrong, or the circuitry sensing the voltage / current has a defect.

Your troubleshooting of the inverter controller was good, but you did not go far enough. If you look at figure 1, the protection logic control turns off the FET, allowing the cap on Timer (pin 3) to charge, shutting off the inverter. If you look at the application drawing (figure 2) in the OZ9938 datasheet you will see that the voltage at Pin 6 comes from the circuits feeding D2 and D4. C22 filters the pulses, R25 provides pull down. Unfortunately, while the picture of the bottom of the board is excellent, I can't quite trace the actual circuit. However, there is another approach. Note that D2 and D4 in the app note form an OR circuit. The voltage at pin 6 will exceed 3 volts only if one (or more) of the voltages at the anodes of D2 and D4 exceeds about 3.5 volts.

Referring to the picture of the bottom of the board, with the CCFLs operating, measure the voltage at the following points:
Left end of R506
Bottom end of C509
Left end of R525
Bottom end of C528? (blue cap, lower right hand corner of the picture).

If you are unable to measure the voltages reliably, instead ONE AT A TIME short each of these points to ground. The one which results in the backlights staying on is the culprit. Report the results, we'll carry on from there.

PlainBill

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

The service manual (including schematic) for a LG W1934S is available on www.elektrotanya.com.

This employs a rather unusual design. An inverter is a constant-current AC source with over-voltage protection. Normally the return currents on the four CCFLs are summed and used to to regulate the inverter output. Voltage sense is used to shut down the inverter if a CCFL goes open. The individual return currents are used to shut down the inverter if the current through a CCFL goes too high. (As a CCFL nears 'end of life' the current increases). Also, the four current sense resistors have three different values!

PlainBill

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

Thanks for the suggestions. The backlight comes on and goes off again twice after I push the power button. The first time the enable signal to the inverter IC goes low and the backlight turns off. A second later, the backlight turns on again for about half a second then turns off; the enable signal to the inverter IC stays on this time (this is where I think the problem is). Also I did find a surface-mount fuse near the connector to the display board (CN801). It's F802 (4A, 32v) and it's fine. I checked U501 and U502, and found that none of the pins are shorted.

Thanks PlainBill. I think I sound like I know more about this than I actually do. I went back and took some more readings and found that the voltage from the power supply is about 13.7v when the inverter is running, which is pretty close to the 14v mentioned in the service manual. I also re-measured the voltage at the CCFL returns, and found that two are higher than the other two.

• CN501 = 0.032v
• CN502 = 2.3v
• CN503 = 0.058v
• CN504 = 0.6v

Hopefully this makes some sense based on what's in the service manual on page 47.

I took the measurements you requested:

• Left end of R506 = 0.2v
• Bottom end of C509 = 0.45v
• Left end of R525 = 0.15v
• Bottom end of C528 = 0.47v

I also tried shorting each point to ground, and found that the only point that made the inverter stay on was the left end of R506. I'm attaching another picture of the whole bottom of the board. Lighting in my house is not particularly good for this, so I have to hold a flashlight or use the flash on my camera to get a reasonably clear picture.

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

Thanks for the suggestions. The backlight comes on and goes off again twice after I push the power button. The first time the enable signal to the inverter IC goes low and the backlight turns off. A second later, the backlight turns on again for about half a second then turns off; the enable signal to the inverter IC stays on this time (this is where I think the problem is). Also I did find a surface-mount fuse near the connector to the display board (CN801). It's F802 (4A, 32v) and it's fine. I checked U501 and U502, and found that none of the pins are shorted.

Thanks PlainBill. I think I sound like I know more about this than I actually do. I went back and took some more readings and found that the voltage from the power supply is about 13.7v when the inverter is running, which is pretty close to the 14v mentioned in the service manual. I also re-measured the voltage at the CCFL returns, and found that two are higher than the other two.

• CN501 = 0.032v
• CN502 = 2.3v
• CN503 = 0.058v
• CN504 = 0.6v

Hopefully this makes some sense based on what's in the service manual on page 47.

I took the measurements you requested:

• Left end of R506 = 0.2v
• Bottom end of C509 = 0.45v
• Left end of R525 = 0.15v
• Bottom end of C528 = 0.47v

I also tried shorting each point to ground, and found that the only point that made the inverter stay on was the left end of R506. I'm attaching another picture of the whole bottom of the board. Lighting in my house is not particularly good for this, so I have to hold a flashlight or use the flash on my camera to get a reasonably clear picture.

That would indicate that either R506, R510, or D502 is the problem. There is about 100Khz AC on the points I had you measure; an oscilloscope would be the best way to measure the voltage. You can check the resistance of R510 in circuit; lift one end of R506 to measure it. About the only wan to reliably test D502 is by replacement. The diode in question is called D502 on the board, D602 on the schematic.

PlainBill

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

That would indicate that either R506, R510, or D502 is the problem. There is about 100Khz AC on the points I had you measure; an oscilloscope would be the best way to measure the voltage. You can check the resistance of R510 in circuit; lift one end of R506 to measure it. About the only wan to reliably test D502 is by replacement. The diode in question is called D502 on the board, D602 on the schematic.

PlainBill

I re-measured the voltages using my oscilloscope:

• Left end of R506 = 706.5mv RMS, 2.24v pk-pk
• Bottom end of C509 = 454.3mv RMS, 1.92v pk-pk
• Left end of R525 = 589.2mv RMS, 1.96v pk-pk
• Bottom end of C528 = 478.3mv RMS, 2.32v pk-pk

I checked R506 in-circuit and it measures 2.96MΩ. For reference, R525 measures 3.005MΩ and the parts list says this should be 3MΩ 3KV.

I desoldered R510 and it measures 3.37KΩ. The parts list says it should be 3.4KΩ and it is labeled "3401" which I think means it's supposed to be a precision resistor. If that's the case, then this one is close to the bottom end of 1% tolerance.

I don't think I have appropriate tools to desolder D502 without damaging something. I used hot air from a butane torch to get R510 off, but the diode is a bit bigger and I don't want to risk melting anything. Looks like I'll have to order some Chip Quik (and probably just get another BAV99 for the replacement anyway).

Since I already have to order parts, I might as well replace the resistors as they should be pretty cheap. What type of resistor would be best for R506? It's tan colored and should be 1/2W. How about R510? Looks like I can get thin film, thick film, carbon film, wirewound, metal film... I have no idea which to choose.

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

I re-measured the voltages using my oscilloscope:

• Left end of R506 = 706.5mv RMS, 2.24v pk-pk
• Bottom end of C509 = 454.3mv RMS, 1.92v pk-pk
• Left end of R525 = 589.2mv RMS, 1.96v pk-pk
• Bottom end of C528 = 478.3mv RMS, 2.32v pk-pk

I checked R506 in-circuit and it measures 2.96MΩ. For reference, R525 measures 3.005MΩ and the parts list says this should be 3MΩ 3KV.

I desoldered R510 and it measures 3.37KΩ. The parts list says it should be 3.4KΩ and it is labeled "3401" which I think means it's supposed to be a precision resistor. If that's the case, then this one is close to the bottom end of 1% tolerance.

I don't think I have appropriate tools to desolder D502 without damaging something. I used hot air from a butane torch to get R510 off, but the diode is a bit bigger and I don't want to risk melting anything. Looks like I'll have to order some Chip Quik (and probably just get another BAV99 for the replacement anyway).

Since I already have to order parts, I might as well replace the resistors as they should be pretty cheap. What type of resistor would be best for R506? It's tan colored and should be 1/2W. How about R510? Looks like I can get thin film, thick film, carbon film, wirewound, metal film... I have no idea which to choose.

Note that R506/R510 form a voltage divider. If the resistance of R506 is higher than spec or of R510 is lower than spec, the voltage at the junction of pin 3 of D502 will be lower. I don't think it is necessary to replace either of the resistors. The peak voltage at pin 3 is below the normal threshold for triggering shutdown, well below if you consider the .5 - .7 volt drop across the diode.

I'm wondering if the voltage at the OVPT pin is lower than normal. Could you measure that? Maybe that will provide a clue.

PlainBill

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

I'm wondering if the voltage at the OVPT pin is lower than normal. Could you measure that? Maybe that will provide a clue.

I measured voltage on all pins of the inverter IC when I first started gathering data. I measured 0.24v on pin 7 (OVPT) when the power was off, and 0.71v when the power was on. If something doesn't add up, I can re-measure tonight.

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

I measured voltage on all pins of the inverter IC when I first started gathering data. I measured 0.24v on pin 7 (OVPT) when the power was off, and 0.71v when the power was on. If something doesn't add up, I can re-measure tonight.

DuH!!!! The light dawns!!! I was looking at this backwards. This is REALLY a screwy design!!!

Look at CN501 and CN502. The H pins are outputting sine waves 180° apart - ends of a transformer winding, right? These numbers are approximate, by the way. When CN501H is at +800V CN502H is at -800V. Since the CCFL draws about 6 ma, the voltage at OP1 should be about 5.6V P-P, centered around 0 volts. On the positive peaks D506 is reverse biased, but on the negative peaks D506 conducts and pulls the voltage at the anode of D506, D510, D501, D505 down.

Now looking at CN502, things are way different. When CN502H is at the negative peak D504 1-3 is forward biased, and there will be about -3.5V at CN502L. Because D504 3-2 is reverse biased, the voltage at OP2 will be being pulled toward 0V at a rate determined by the time constant of R535 and C532. On the positive half cycle D504 3-2 is forward biased, so C532 is being charged by R523 and being discharged by R535.

CN503 behaves pretty much like CN501. The negative peaks will pull OP2 down in proportion to the current flowing through the CCFL and R527.

CN504 is a whole 'nother matter. The negative peaks go through R520 and D507 1-3 to ground. The positive peaks go through R520 and D507 3-2 to the parallel network of C524 R528 and R533 and to the ISSEN input on the inverter controller. This provides the feedback to regulate the drive current.

Now, what does all of this mean? I'm not sure, but I have at least one more test for you to try. What are the RMS and P-P at the L end of CN501 - CN504. I'm assuming the readings you gave earlier were DC voltages.

As an aside, troubleshooting the 'two seconds to black' problems has always been difficult. The bulk of my electronics training was nearly 40 years ago, and I had a hard time grasping what was happening without a schematic. Even with a schematic, the guy I was helping usually had only a DMM. About all I could do is trace circuits and look for components with unusual readings. Now I've got a schematic, a well documented controller, and someone with both a DMM and a scope. And I'm STILL struggling!!! I'm beginning to wonder is Shakespeare was right. "The fault lies not in the stars, but in ourselves." Or something like that.

PlainBill

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

Now, what does all of this mean? I'm not sure, but I have at least one more test for you to try. What are the RMS and P-P at the L end of CN501 - CN504. I'm assuming the readings you gave earlier were DC voltages.

I re-measured the connectors with my scope tonight, and I don't quite believe the readings. Hopefully this is due to my less-than-stellar job of re-attaching R510, but everything except CN501 measures really high. I had 32mv, 2.3v, 58mv, and 600mv for the 4 connectors last night. Tonight (after removing and replacing R510):

• CN501: Pk-Pk = 2.280v, RMS = 687.0mv
• CN502: Pk-Pk = 21.80v, RMS = 7.960v
• CN503: Pk-Pk = 7.600v, RMS = 2.698v
• CN504: Pk-Pk = 10.48v, RMS = 3.878v

I'm attaching pictures of the scope screen for each, which provides a little extra information.

I'm beginning to wonder is Shakespeare was right. "The fault lies not in the stars, but in ourselves." Or something like that.

That could absolutely be true. Don't discount the possibility of a case of operator error somewhere in here - I haven't had any training. I do this stuff as a hobby and most of the monitors I fix (Samsung) just require capacitor replacement and they're fine. I've got two models giving me trouble right now (this one and a Samsung Syncmaster 940T), and I've been dragging my feet about getting them fixed because I had no idea how to go about troubleshooting them.

[PlainBill]

Re: inverter problem in LG 1934S LCD flatron

I re-measured the connectors with my scope tonight, and I don't quite believe the readings. Hopefully this is due to my less-than-stellar job of re-attaching R510, but everything except CN501 measures really high. I had 32mv, 2.3v, 58mv, and 600mv for the 4 connectors last night. Tonight (after removing and replacing R510):

• CN501: Pk-Pk = 2.280v, RMS = 687.0mv
• CN502: Pk-Pk = 21.80v, RMS = 7.960v
• CN503: Pk-Pk = 7.600v, RMS = 2.698v
• CN504: Pk-Pk = 10.48v, RMS = 3.878v

I'm attaching pictures of the scope screen for each, which provides a little extra information.

Yes, it does, and it provides plenty of questions, too. I hope you don't mind, but we are going back to the basics on this for a few minutes. The most significant thing I see is the frequency on CN502 is shown as 13Khz, it's about 45Khz on the other connectors. From what I think I know about inverters, 45Khz is possible, 13Khz is flat out wrong.

So to start at the beginning, what model Owon oscilloscope do you have? What kind of probe are you using, and where are you connecting the ground lead for the probe? And it appears you are using AC coupling, switch to DC coupling.

Next, I would like to see the traces at OP1, OP2, OP3. Probe the cathode ends of D506, D510, D501. Also, I'd like to see a trace for pin 11 of IC501. Clip the scope ground as close as reasonably possible to that area. Soldering a short piece of wire (resistor lead) to a ground point is a good idea.

Hmm, another problem with SMD technology. No ready source of short lengths of pre-tinned wire.

I had to go back to school to figure this out, but the impedance of C532 at 45Khz is 540 ohms. With the parallel resistance of R535 I'd expect the voltage at CN502 to be roughly double the voltage at CN501.

By the way, the purpose of the waveform at pin 11 of IC501 is to get a reference.

[stevenps]

Re: inverter problem in LG 1934S LCD flatron

So to start at the beginning, what model Owon oscilloscope do you have? What kind of probe are you using, and where are you connecting the ground lead for the probe? And it appears you are using AC coupling, switch to DC coupling.

The scope is an Owon PDS5022S (http://www.owon.com.cn/eng/pdsSeries.asp). The probe is a P6060 with both scope and probe set at x10. I just switched to DC coupling. I was attaching the ground clip to the case of the monitor, as the center screw hole is attached to cold ground and the board is screwed in to the case. I can try using another location, though it may be a little tricky.

I took new readings but I don't have time to post the values right now; I'll get that done later this afternoon. I moved my ground position to the negative leg of C501 and used DC coupling. I have two sets of measurements, one with and one without shorting the end of R506 to ground to keep the inverter running long enough to properly adjust the scope and let it get a good reading.