LG 26LG3000 white screen problem

[manoftheyear]

The screen for this is CMO V260B1-L11, problem is bright white screen only, otherwise working ok. T-con is I believe integrated with the lcd driver board (?) that's bonded to the screen itself. Fuse FP1 bad, gives infinite readings, when cable connected to the T-Con/driver board I get no voltage readings, if I disconnect the cable I get the 12v etc... so I guess mainboard is ok. I think this indicates a short on the T-Con? Uses littelfuse 1206 1,5A which I don't have yet.

Where to start the troubleshooting? There were some ceramic caps thet were next to each other and showed 0 ohm rating in-circuit, but when desoldered couple of those (CP3 and CP4) they measured fine. That's why those ones might look bit different from others.

Some pictures attached...

[tom66]

Re: LG 26LG3000 white screen problem

The fuse can sometimes blow without apparent cause, due to thermal stress.

If you can get hold of a 2A (or maybe 3A) automotive fuse, you can use that temporarily to test the unit. If the new fuse doesn't blow, you can find a more permanent replacement.

[momaka]

Re: LG 26LG3000 white screen problem

Where to start the troubleshooting? There were some ceramic caps thet were next to each other and showed 0 ohm rating in-circuit, but when desoldered couple of those (CP3 and CP4) they measured fine.

CP3 and CP4 seem to be on the main 5V rail that feeds the t-con. There are likely other ceramic caps with them in parallel. It takes one to be shorted to make it seem as if they are all shorted.

My usual approach is to find all parallel caps that show 0 Ohms and remove them until the rail they were on no longer shows shorted (0 Ohms).

Before removing anything, though, I would like you to check the following items first:
1) resistance between test point VDD and ground, VDA and ground, VGHP and ground, VGH and ground, and V25V and ground (all with the monitor turned off and unplugged, of course)
Note: if your multimeter is manual, make sure to use the lowest resistance scale (i.e. 200 Ohms).
2) resistance across diode DP1 (or is it DP2?) as well as resistance across DP3.

Post what results you get.

As tom66 said, the fuse can indeed sometimes blow for no apparent reason, but given the low Ohms reading you got across CP3 and CP4, there's likely a shorted cap somewhere on one of the main power rails of the t-con.

[budm]

Re: LG 26LG3000 white screen problem

I agree with momaka method of removing them until the shorts is gone, these MLCC failed short, does not take much to crack these MLCC. I repaired a T-CON board that has 6 MLCC in parallel, and guess what happen, it's the sixth one that I removed was the shorted one!
Be real careful if you are going the put the good one back in place, put it back one at a time and check to make sure there is no shorts before installing the next one.

@momaka, can you tell me what all these VDD, VDA, VGHP, VGH stand for? I see them printed on the T-CON all the time but never figure out what they are and what the typical values should be, thanks.

[retiredcaps]

Re: LG 26LG3000 white screen problem

A very similar case today of shorted ceramic caps

https://www.badcaps.net/forum/showthread.php?t=22269

[manoftheyear]

Re: LG 26LG3000 white screen problem

Before removing anything, though, I would like you to check the following items first:
1) resistance between test point VDD and ground, VDA and ground, VGHP and ground, VGH and ground, and V25V and ground (all with the monitor turned off and unplugged, of course)
Note: if your multimeter is manual, make sure to use the lowest resistance scale (i.e. 200 Ohms).
2) resistance across diode DP1 (or is it DP2?) as well as resistance across DP3.

1) All those test points gave infinite readings? Did I do everything right here?

2) DP1 (or DP2? strange marking) measured 140 ohm and infinite in opposite direction DP3 gave 470 ohm and 178 in opposite direction. Tested in-circuit. DP3 possibly bad? Other ceramic caps that show near zero reading are the CP30 and CP31 that are next to DP3. Also CP5 and CP6 give near zero ohm reading.

[momaka]

Re: LG 26LG3000 white screen problem

1) All those test points gave infinite readings? Did I do everything right here?

They *usually* do give some resistance, but it probably also depends a lot on the multimeter. When you tried to measure those points, did your multimeter flash some numbers on the screen for a second (as if the caps were getting charged)?

In any case, as long as they are not shorted or show low-Ohm readings, we can ignore them for now.

2) DP1 (or DP2? strange marking) measured 140 ohm and infinite in opposite direction DP3 gave 470 ohm and 178 in opposite direction. Tested in-circuit. DP3 possibly bad?

DP1 and DP3 seem fine. You can also try measuring them with the diode function of your multimeter but you'll probably get similar results - if you do, then they are fine.

Other ceramic caps that show near zero reading are the CP30 and CP31 that are next to DP3. Also CP5 and CP6 give near zero ohm reading.

All of those are probably in parallel, then. Looks like the 5V rail going to the t-con is shorted. A bit unusual, but anything is possible with ceramic caps.
I would suggest to remove those shorted caps one by one and see if any are shorted. Also, after removing each cap, check the resistance of the spot the cap was in. If the spot still shows 0 Ohms, then the short is still there.

Also, can you check the resistance of CP35 (or CP36 - either one is fine since they are in parallel) as well as CP7 (or CP8). CP7 and CP8 seem to be on the main boosted rail (more on that below).

@momaka, can you tell me what all these VDD, VDA, VGHP, VGH stand for? I see them printed on the T-CON all the time but never figure out what they are and what the typical values should be, thanks.

Well, every t-con has at least 5 major voltage rails. Those usually are:

1) The main power rail. Usually 5V. It comes from the power supply or the logic board and supplies power to everything on the whole t-con board. It's always protected with a fuse - FP1 for the case of this monitor. It doesn't have a standard name, so some t-cons label it as V5V, +5V, and possibly even VDA, VDD and various other names.

2) The main boosted power rail. Usually 9V to 20V (most often around 10-15V). Very easy to find it on the t-con since it's always near a large diode and a round inductor (diode DP1/DP2 and inductor LP2/LP4/LP5/LP6 for this monitor - yeah weird naming there ).
I'm not sure which exact components on the t-con use it, but I know it is used for generating the 2 secondary boosted voltage rails for the transistors in the TFT screen. This rail also goes by different names, depending on who made the t-con. Usual names I have seen are AVDD, VDD, and VBOOST. For the monitor in this thread, I think VDA is the test point for this rail, although I'm not quite sure.

3) 4) The secondary boosted voltage rails. These are derived from 2) above. They are used for driving the transistors in the TFT ON or OFF - i.e. to drive each pixel.
One is positive and further boosted above 2) (the main boosted rail), so voltages from 15 to 35V are normal for this rail. Goes by the names of VGH, VGG, VPOS and VOFF (or VON?).
The other one is actually negative. Typical voltages are from -5 to -10V. Goes by the names of VGL, VEE, VNEG, and VON (or VOFF?).

5) The supply rail for the TFT driver IC. Typically 3.3V, 2.5V, or 1.8V depending on what the IC requires (sometimes there's actually two of these rails). Derived directly from the main (5V) rail via a linear regulator. In the case of this monitor, this rail is generated by UP5. Seems like test points V33V and VDD correspond to this rail. Hence like the other rails above, this one doesn't have a standard name either.

[retiredcaps]

Re: LG 26LG3000 white screen problem

Well, every t-con has at least 5 major voltage rails. Those usually are:

Bookmarked!

[manoftheyear]

Re: LG 26LG3000 white screen problem

momaka, the test points you wanted me to test did show resistance, pretty high ones. All the ceramic caps you mentioned tested fine out of circuit. With ohm meter and the capacitance seemed to be 10 or 11 uF in all of them. CP35 and CP36 measured about 100 ohms in-circuit while the CP7 and CP8 were about 80 ohms in-circuit, but as said out of the circuit all were fine.

What wasn't a mainboard problem I believe now is, I slipped a probe while measuring the voltages from the connector to t-con, shorted accidentally two pins on that. Now the 12v VCC voltage is only 5,5V. A voltage regulator U7910 that is BA17809FP-E2 is getting the same low 5,5v voltage. If anyone has a good guess or can trace on what might gone wrong I'll be more than glad to hear. Uploaded service manual if it's of any help.

[budm]

Re: LG 26LG3000 white screen problem

So are you getting 5V at the T-CON fuse instead of 12V like it was before?

[manoftheyear]

Re: LG 26LG3000 white screen problem

I used to be getting 12V from the mainboard connector that goes to T-con, if the cable was connected between those boards I did not even get the 12V, it went to 0V. Now the 12V I used to get dropped to 5,5v after the accidental shorting. Or what do you mean?

[momaka]

Re: LG 26LG3000 white screen problem

All the ceramic caps you mentioned tested fine out of circuit.

So all of those caps that showed shorted before in circuit (in your first post) are not shorted in circuit anymore?

[manoftheyear]

Re: LG 26LG3000 white screen problem

So all of those caps that showed shorted before in circuit (in your first post) are not shorted in circuit anymore?

They still are, or at least show the same reading in-circuit as in the beginning. Nothing has changed. But when desoldered all the caps that are mentioned in this thread they all test ok with DMM using ohm and capacitance settings. Maybe there's some more caps with near zero reading in there? Will have to check again. Any more suggestions in the meantime?

I'll have to get the 12V rail (this is the t-con main rail?) repaired too as it's 5,5V at the moment. It's 5,5V for both the T-Con and the audio part of this TV while it should be 12V. Could the P-channel mosfet "AO4449 -7A/-30V" cause this if faulty? Anyone? I would suspect that as of now.

[tom66]

Re: LG 26LG3000 white screen problem

If that P-FET is burned, then yes, its resistance may be too high and it could cause the problem.

[momaka]

Re: LG 26LG3000 white screen problem

Maybe there's some more caps with near zero reading in there?

More than likely. It's probably just one that's bringing the whole bunch down. I think you'll just have to check all of the ceramic caps on the t-con. Best approach here is to check the large ones first, then the smaller ones. Also, when you find shorted caps, don't remove them. First find all shorted caps and mark them, then start removing one by one. Those that you checked already you can leave.

Could the P-channel mosfet "AO4449 -7A/-30V" cause this if faulty?

If that MOSFET blew, it was probably as a result of the shorted caps, not the other way around. I've had to repair a 17" Dell monitor like that because the t-con fuse didn't blow. At least you are fortunate to know what the MOSFET is. Mine was some small SOT-23 device and I just couldn't read the part numbers (it was half-blown). Wired the t-con so that it was permanently ON by installing a jumper from Drain to Source on the blown MOSFET pads. Not the most proper fix, but it worked.

Even if that MOSFET you mention is blown, first try to find the shorted cap. When I fix a t-con board, I usually power it with an external power supply first to see how much current it's drawing and to have more control (i.e. I can turn it ON and OFF whenever I like without having to fiddle with the monitor controls and ON/OFF button). This also saves time by not having to put back the monitor together.

If you are sure your t-con uses 12V, then you can feed it a regulated 12V power supply. 1-2A should be enough. If it draws more than 1A, remove power quickly and check if any components on the t-con are getting very hot. And of course, DO NOT wire 12V to the t-con without a fuse or some other way to limit the current. Hooking up a very powerful 12V power supply without a fuse on the t-con can burn traces on it.

[budm]

Re: LG 26LG3000 white screen problem

That is why I asked if the voltage on the T-CON fuse was 12V or 5V? That fuse is the main fuse for the T-CON. You may have blown the switched transistor on the main board that supply the votage to the T-CON. Voltage to the T-CON is turn off when TV is off or in standby mode.

[manoftheyear]

Re: LG 26LG3000 white screen problem

A little update on this one and some light at the end of tunnel: I went through the caps again as adviced and now found the shorted cap, it was CP29 next to the IC TPS65161. It can be seen in pic 6 in the first post that I made. After removing this there's no shorts! Now I would need to know with what kind of cap should I replace it?

[tom66]

Re: LG 26LG3000 white screen problem

Can you post of a picture of the surrounding components? We can make a guess.

[manoftheyear]

Re: LG 26LG3000 white screen problem

There are pictures in the first post? Don't have a camera right now here with me.

[Th3_uN1Qu3]

Re: LG 26LG3000 white screen problem

Going by the typical application in the datasheet of the TPS65161, it should be 1uF.