Iiyama LCD Screen Repair

[IRW]

Hey folks, one for the electronics boffins!

I've got a small collection of Iiyama Prolite T2250MTS for my main desktop computer and to take 'on the road' with me to use with Nomad. A little while back, one of them went faulty, by way of it's backlight going off after a while, and recently a second one developed a similar fault. Now I've had them both apart, and replaced all the main capacitors on the PSU boards, which seems to have given me one working unit again, but I'm still having trouble with the second turning on briefly, then going off after a second or two. The status LED stays blue, so it knows there is still a signal there, as opposed to going orange when there is no signal.

I've tried swapping the LCD panels, and I know they're both good as they both work fine on the PSU board that is behaving, so it definitely seems to be something PSU related- Does anyone have any suggestions as to where to look after the 7 main capacitors? It looks like part of the board may have got a bit warm at some point (top left in the picture of the PCB from the top), but the other board has a similar mark, and nothing looks visually particularly untoward on the other side of the board.

 

Here are a few pics to help things along:http://www.irwdesign.com/br/t2250mts/IMG_1280.JPG

http://www.irwdesign.com/br/t2250mts/IMG_1281.JPG

http://www.irwdesign.com/br/t2250mts/IMG_1282b.jpg

http://www.irwdesign.com/br/t2250mts/IMG_1283b.jpg

 

[indyld]

Have you metered the low voltages on all the output side of the PSU? Usually, connected to any other main boards if there is one as some parts of a PSU can be controlled by a chip on another board. (usual caveat about >300VDC knocking around the place, at the mains side and also the inverter). This one looks quite simple, but worth checking to see if some of it isn't meant to kick into life based on something somewhere else.

 

When you say powers on for a short while, is it just that an indicator comes on and looks hopeful for some while? Or does the screen actually do its job?

 

After seeing what output voltages are working, I usually check the supply rails to any chips that seem important. The bonus of a working screen is being able to take comparative measurements, voltage, R, or even volt drop in diode test mode.

Edited October 7, 2020 by indyld

[IRW]

Have you metered the low voltages on all the output side of the PSU? Usually, connected to any other main boards if there is one as some parts of a PSU can be controlled by a chip on another board. (usual caveat about >300VDC knocking around the place, at the mains side and also the inverter). This one looks quite simple, but worth checking to see if some of it isn't meant to kick into life based on something somewhere else.

 

When you say powers on for a short while, is it just that an indicator comes on and looks hopeful for some while? Or does the screen actually do its job?

 

After seeing what output voltages are working, I usually check the supply rails to any chips that seem important. The bonus of a working screen is being able to take comparative measurements, voltage, R, or even volt drop in diode test mode.

 

I've not really metered anything, as I'm not entirely sure what to look for! Good point about comparing values to the other one, although I'm rather wary of poking about in the wrong place and releasing the magic smoke from the working on! There is an extra board with the control switches on, but for the purposes of testing, I've essentially had everything connected (doesn't turn on at all without the power button to press!)

 

When I say it powers on, it effectively does what it's supposed to for about a second or so, in that I can see my computer desktop. After that second, the display goes dark, but by using a torch, I can see that the LCD itself is still functional, so it's just definitely just the backlight that's going off (both top and bottom sets of tubes). The status LED on the board with the switches stays blue, which in this particular screen means it's got a valid signal, and should be showing something. (If the computer goes to standby, the LED goes orange, if you turn the monitor off with it's power button, the LED goes off.

Edited October 7, 2020 by IRW

[indyld]

Well, at least a backlight problem narrows it down quite a lot and away from more esoteric problems. From the description, you have ruled out every part of the screen assembly including the backlight strips themselves as being OK? Sometimes a short in the back light strips can cause a protective shut down in the PSU. But as I understand it, this is already discounted.

 

When it comes to comparative measuring, diode mode testing of the two unpowered boards, disconnected and from everything else and discharged, can point to a potential short or partial short. It may not reveal breakdowns that are happening under normal voltage, for example any of those blue X or Y class caps. However the latter should fail open and the former blows fuses. It seems more likely that the PSU is reacting to a problem. It may be a SMD sized problem. (C302 looks oddly dark, but it's probably the photo)

Edited October 7, 2020 by indyld

[pmiller056]

That looks like a mains supplied switch mode power supply combined with an inverter for a fluorescent lamp backlight - in this case 2 tubes. From your description, it is the backlight inverter that is at fault. The transformer for this inverter is the lump that has the 'QC OK' sticker on it in the last photo. The inverter produces a very high voltage to strike the lamps on no load, with a much lower voltage to run the lamps at a few mA. If it is easy to do so, have a look at the lamps - if the ends are blackened due to wear, they're probably end of life, in which case I'd consider scrapping the monitor. The lamps used are likely to be custom made for the manufacturer and replacements unobtainable. Another symptom of end of life backlight fluorescent lamps is they have a pinkish orange tint to the light instead of a brilliant bluish white.

Peter

 

[IRW]

Well, at least a backlight problem narrows it down quite a lot and away from more esoteric problems. From the description, you have ruled out every part of the screen assembly including the backlight strips themselves as being OK? Sometimes a short in the back light strips can cause a protective shut down in the PSU. But as I understand it, this is already discounted.

Correct

 

 

It may be a SMD sized problem. (C302 looks oddly dark, but it's probably the photo)

 

Very well observed. It does indeed look a little unwell in comparison to it's friends. Doing an in-situe resistance test across it results in the same value as it's working cousin though (over limit).

 

That looks like a mains supplied switch mode power supply combined with an inverter for a fluorescent lamp backlight - in this case 2 tubes. From your description, it is the backlight inverter that is at fault. The transformer for this inverter is the lump that has the 'QC OK' sticker on it in the last photo. The inverter produces a very high voltage to strike the lamps on no load, with a much lower voltage to run the lamps at a few mA. If it is easy to do so, have a look at the lamps - if the ends are blackened due to wear, they're probably end of life, in which case I'd consider scrapping the monitor. The lamps used are likely to be custom made for the manufacturer and replacements unobtainable. Another symptom of end of life backlight fluorescent lamps is they have a pinkish orange tint to the light instead of a brilliant bluish white.Peter

 

 

Already done, and no, it wasn't particularly easy to do! From all the common 'LCD monitor flashing fix' videos on youtube, checking the connections at the tube end was stage 2 after checking the capacitors!

 

 

IIRC, there is a slight pinkish tinge to one of the lamps (I can't remember if it was the top or bottom one without running it up again, but as I previously mentioned, both LCD/backlight assemblies of monitors 1 & 2 illuminate (and stay illuminated) from the power supply board from monitor 1, so even if they are on their way out, I'm not convinced that's the problem at hand. The screen that is currently the problematic would live in a bag most of the time anyway, so whatever life I can eke out of it is a bonus, especially given that I'd prefer not to have to replace it in the current climate!

DEVELOPMENT:Having poked around a little bit with the multimeter, when metering across what I assume to be the transformer coils, pairs A, B and C all meter out at 0 ohms, however pair D meters out at 564K on the faulty board. On the good board, all pairs meter out at 0. Follow up question here I suppose is is this likely to be a custom winding tailored to the board/backlight, or am I likely to find one somewhere? Throwing the code on the top of it (LCWJ19S) doesn't seem to bring anything sensible up on google.http://www.irwdesign.com/br/t2250mts/IMG_1282c.jpg

 

[indyld]

Disconnect C318 and look at anything else across D and re measure. 99 times out of 100, it's not the winding. Could be a dry joint AND a leaky cap, for example. If the winding has become a fuse, that would be really quite boring as getting hold of these things isn't easy.

 

The circuit probably won't miss C318 for testing, if found faulty.

Edited October 8, 2020 by indyld

[indyld]

Another quick thought, you could also try powering up with no load on the D secondary side to see if the PSU continues to power the other two backlights. That might help eliminate any other issue with the inverter circuit further upstream, but only if the circuit doesn't shut down.

 

A good old dab with a hot iron and some new solder might take those dull looking joints on the transformer right out of the equation. If confident that there is a good connection to each end of the winding, then things aren't looking great.

Edited October 8, 2020 by indyld

[IRW]

Disconnect C318 and look at anything else across D and re measure. 99 times out of 100, it's not the winding. Could be a dry joint AND a leaky cap, for example. If the winding has become a fuse, that would be really quite boring as getting hold of these things isn't easy.

 

The circuit probably won't miss C318 for testing, if found faulty.

 

Thanks for the help with this- I actually took the winding assembly off the board this morning, and measured it again, but still got the same results away from the board :/. Starting to look like this might be a bit of a lost cause I feel- still, at least I got one of the screens up and running again! I've found a pretty similar looking (but slightly different model) power board designed for a 23" Acer screen on ebay for a tenner, so I've got that on order now as a final thing to try! If that doesn't work, then I may just try and get creative with some LED tape...

[indyld]

I've found a pretty similar looking (but slightly different model) power board designed for a 23" Acer screen on ebay for a tenner, so I've got that on order now as a final thing to try! If that doesn't work, then I may just try and get creative with some LED tape...

 

These things tend to do the rounds between different makers so it's possible to get something if only you can work out what it is. The only issue I've had with certain parts is that you can get them OK, but only buy them by the 1000 on Aliexpress. A replacement transformer itself no doubt exists too, it's just finding it. On occasion with less common windings and forms, I have had to resort to finding the right spec transformer and drilling/jumpering it onto the PCB.

 

If nothing comes from it, at least you have a donor unit to keep your working one going. It may involve finding room in the case for 2 half-working transformers..... :)

Edited October 8, 2020 by indyld

[IRW]

I've found a pretty similar looking (but slightly different model) power board designed for a 23" Acer screen on ebay for a tenner, so I've got that on order now as a final thing to try! If that doesn't work, then I may just try and get creative with some LED tape...

 

These things tend to do the rounds between different makers so it's possible to get something if only you can work out what it is. The only issue I've had with certain parts is that you can get them OK, but only buy them by the 1000 on Aliexpress. A replacement transformer itself no doubt exists too, it's just finding it. On occasion with less common windings and forms, I have had to resort to finding the right spec transformer and drilling/jumpering it onto the PCB.

 

If nothing comes from it, at least you have a donor unit to keep your working one going. It may involve finding room in the case for 2 half-working transformers..... :)

 

Yup- I've actually got four of them- three tend to live permanently attached to the computer at home, and the 4th is a bit more nomadic (see what I did there?!), occasionally joined on jobs by one from the computer. I got them all second hand a couple of years ago now, and given I've now had a bit of gyp from two of them, I'm rather expecting the others to get upset at some point in the future, hence my preference for knowing how to fix them!

 

Out of interest, if, on this board I've ordered, the windings are slightly different, what would the outcome be? I assume they're made to correlate somehow to the size of the CCFL connected to them, so guess the tubes would either run dimly, too hot, or not at all?

 

Edited October 8, 2020 by IRW

[indyld]

Out of interest, if, on this board I've ordered, the windings are slightly different, what would the outcome be? I assume they're made to correlate somehow to the size of the CCFL connected to them, so guess the tubes would either run dimly, too hot, or not at all?

 

I'm not a 'boffin', just someone who repairs a lot of different things including at component level, but I find a lot of these products are all much of a muchness when it comes to basic stuff like backlights systems so there is a chance of some compatibility between manufacturers - within reason. It's not a space shuttle, after all.

 

However, it may be academic now but I'm still puzzled by the original fault. From the board, I can't see why a high R in one of the secondary windings of the inverter trafo would prevent the 'good' side outputting or shutdown the primary side of the PSU. Looking harder at the tracks and components, and now knowing the fact that it's powering CCFLs and not LEDs, we can see that the circuit is AC into the primary (C302 gives this away) and therefore AC out on both secondaries. The blue caps across the windings are thus presumably for startup that create the initial high voltage. There are a load of EMI caps from the original design that got turned into jumper wires at some point in the design process. And that's it - no obvious feedback to the earlier stages. A high R in one secondary would surely just result in not very much happening on that side, in my mind.

 

(Incidentally, it's likely that the ohms across C302 will be the same whatever its condition if not shorted or leaking very badly. If you look at the board, the measurement is going across the primary of the trafo and two MOSFETs plus perhaps other stuff too. In this case, once the cap is charged we are just looking at the rest of the circuit. Depending on its capacitance, we might see it charge on the meter during an ohms or volt drop test but not always.)

Edited October 9, 2020 by indyld

[dmxlights]

Rob,

 

Have looked at the pic of the power supply and I see there is a feedback from the output of the high voltage side of the transformers and it feeds back to the compolents near the top of the board where the gone dark on the other side.

 

Ian,

Have you tried power up the monitor without the transformer that you think is faulty. It may work with out it I know the screen may be dark at the top or the bottom but it may prove if its faulty.

 

[indyld]

Rob,

 

Have looked at the pic of the power supply and I see there is a feedback from the output of the high voltage side of the transformers and it feeds back to the compolents near the top of the board where the gone dark on the other side.

Aha. Now you point it out, I can see the tracks when zoomed right in. Age is catching up with me. Cheers.

[dmxlights]

I am just after spotting something else,

I don't think the transformer is the problem now and the problem is the components where the board has gone dark.

 

the outputs of the high voltage side of the transformer has one leg going to each tube at the top and its the same at the bottom. The other side of the tubes to the components where the board has gone dark. I would put my money that the fault is the components where the board has darkened abit.