Iiyama LCD Screen Repair

[indyld]

I'd assumed that the darkening of the board was due to something off-board in its mounted position, as the SMD side area there doesn't look unusual and the area isn't filled with what looks like particularly high stress components in the size/shape of the patch. If something specific was having to dissipate an unusually large amount of heat due to a fault, it would probably be more obvious at the component.

 

 

Each sense track goes through resistors etc and presumably eventually feeds into what I assume is the control chip. It's all a bit dingy in the picture.

 

Now the feedback tracks are pointed out, shutdown can't be eliminated. So a high R on a winding can still scupper things.

Edited October 9, 2020 by indyld

[dmxlights]

Rob, I know what your saying about the board going dark. I also though it may have been something off board. It was when I seen the feedback track or the tracks that go from the other side of the tube that the heat is from the component(s) on the board.

I know I had a dell monitor that had a backlight problem much the same as OP but a different power supply board. The problem I had was a 0R resistor going high in value but it only happened when the monitor was powered up and when I changed it to a piece of wire as in a single strand of fine wire as a test it worked fine and I have put in another 0R resistor and the monitor is still working today. Where the resistor was the board had gone dark even when the resistor was on the opposite side of the board and it was a fairly large area of the board to.

 

I would look at the components around the darkest area of the board first

 

Just to add to the above..

If you look at the silk screen on the board in the area where the board has gone dark the white silk screen looks like it has gone brownish or yellowish from the heat.

 

Also JR309 dose not look to good in the pic and I know its not near the area where the board has gone dark.

 

Edited October 9, 2020 by dmxlights

[IRW]

Here's another, better picture of the darkend area- I thought I'd mentioned this bit in the OP, but it would seem I didn't!

http://www.irwdesign.com/br/t2250mts/IMG_0550.JPG

 

The mess around JR309 looks to me just like a bit of stray flux. R331 is the one that's pretty central to the darkened area. There is also absolutely nothing else in the vicinity of this board, so if something's heating up, it's something in this picture! The diodes look a bit dicky, but I'd dismissed that as pretty much all the diodes on both boards look similar. Popping a meter across them gives (various) readings only in one direction.

Edited October 9, 2020 by IRW

[indyld]

Had a quick look at a few datasheet schematics for some controller ICs, just for interest really, and to understand the backlight circuit a bit better. Would be interested to know the chip number.

[IRW]

Had a quick look at a few datasheet schematics for some controller ICs, just for interest really, and to understand the backlight circuit a bit better. Would be interested to know the chip number.

 

If you're referring to U302, it says:

UTC PJT3

TL494L

01

 

 

Had to get the magnifying glass out for that one. No idea what U301 and U303 say on them though- it's far too faint!

Edited October 9, 2020 by IRW

[indyld]

 

If you're referring to U302, it says:UTC PJT3TL494L01

Had to get the magnifying glass out for that one. No idea what U301 and U303 say on them though- it's far too faint!

 

 

Yes, U302. U301 and U303 are just MOSFETs, if you look closely you should see (kinda shows a bit in the picture) that they are basically 3-legged affairs split across 4, 3, and 1 pins. They are switching and creating the voltage to the primary.

 

Yet to find a UTC controller at all, with parts of that reference or without! Have found a Texas TL494 PWM controller but was expecting to see a dedicated ccfl control. (Edit: Ok, now have found the UTC version. Still basically just the same PWM control though. Cheap, I guess.)

Edited October 9, 2020 by indyld

[indyld]

On the brighter picture the discolouration of the tracks back to the control circuit is of interest, like all modern power circuits the whole system is dealing with a lot of 'stuff' often toward top end of limitation. The state of the faulty secondary winding could be just a breakdown in the transformer dealing with a lot of 'stuff', or could be that the winding turned into a fuse due to another fault across it. If this was the case, one might suppose that something connected between each end of the winding has gone short and caused the fuse fizz-crack which may not be obvious on the winding if buried in layers of wire and the trafo casing. Things that are usually across these parts of circuits, and prone to fail, are capacitors and diodes including zeners. Obviously this is all in that blackened area.

 

If investigation continued, I would definitely eliminate the possibility of a short in that area, to rule out this being the cause of the trafo failure before burning out another one. Focus could be on the two lines that come from the two tubes fed from D, I think they are the top two of the set of four as they run back. Each of these four goes into their own network of components that are identical up to a certain point. To look around here, I'd put a DMM into diode mode with the red probe on the secondary track still connected and poke good point / bad point / bad point along the lines starting with the 1k resistor (102) and go along looking for significant changes in voltage drop until one of the suspect lines was eliminated. You can do the same poke with other repeated components, such as each leg of the 3-legged switching diodes - one assumes there is one per line. This helps narrow stuff down before looking for individual components.

 

The only caveat here is that, as has come up already, the failure may not occur with only DMM DC voltages. For example, a diode may hold up in reverse or the breakdown voltage may not be exceeded by a few volts off a battery. If an SMD cap goes short, it's often quite visually obvious but other components can just turn into a wire. temporary or permanent, and not make a fuss. Particularly if this fuses something else.

 

In the OP, it wasn't mentioned if the brief period of 'working' was noticed as having a tube missing. I'm guessing that the fact that the supply starts but won't stay on is due to feedback from the faulty side of the tube output, but then the question is what caused that side to fail.

Edited October 10, 2020 by indyld

[Limo2005]

On 10/7/2020 at 8:59 PM, indyld said:

Sometimes a short in the back light strips can cause a protective shut down in the PSU.

I have the same monitor.  It was working well until I picked it up and moved it.  Now the orange power light does not illuminated. Could the back light strips be the cause and if so where should I start to look to resolve.  

Thanking you in advance

Mickey