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#1 |
New Member
Join Date: May 2009
Posts: 2
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![]() 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 |
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#2 |
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#3 | |
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Also, there could be bad solder joints (check transformer solder joints) or dead/shorted transistors in the unit. |
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#4 | |
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And welcome to the forums. ![]()
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#5 |
New Member
Join Date: May 2009
Posts: 2
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![]() oki ill take a pictures from inside and uploade it in to this forum as soon
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#6 |
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Join Date: Sep 2008
City & State: Near Seattle, WA
Posts: 95
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![]() 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 (http://www.dianyuan.com/bbs/u/58/783461192202365.pdf). 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? |
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#7 | |
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There is also the possibility that that one or more of the CCFL tubes is starting to fail. |
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#8 |
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Join Date: Sep 2008
City & State: Near Seattle, WA
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![]() 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.
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#9 |
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#10 | |
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Join Date: Feb 2009
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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
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For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored. Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic. |
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#11 |
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![]() 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 |
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#12 |
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Join Date: Sep 2008
City & State: Near Seattle, WA
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![]() 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.
Hopefully this makes some sense based on what's in the service manual on page 47. I took the measurements you requested:
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. Last edited by stevenps; 09-23-2010 at 01:45 AM.. Reason: Add result of checking U501 and U502 |
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#13 | |
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PlainBill |
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#14 | |
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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. |
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#15 | |
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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 |
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#16 |
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![]() 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.
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#17 | |
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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 Last edited by PlainBill; 09-23-2010 at 07:42 PM.. |
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#18 | |
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I'm attaching pictures of the scope screen for each, which provides a little extra information. 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. |
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#19 | |
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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. |
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#20 | |
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Join Date: Sep 2008
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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. |
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