Backlight cuts out on LG 55LD520 55" LCD

[SurrealMustard]

This one starts up and runs okay, but the backlight cuts off after between 2 and 10 seconds (almost like it fades out; it's not an abrupt thing). I see inverter boards, so this one is a CCFL-backlit model. The question is, what is the easiest way to find out which inverter (if any) is bad or if we have a backlights problem?

[SurrealMustard]

Anyone? Bueller... bueller...

Any ideas?

[ReeceyBurger123]

Re: Backlight cuts out on LG 55LD520 55" LCD

Easiest way is diagnose it using the forum to help with methods shown in 100's of other threads.

[SurrealMustard]

Fixed itself!

In an interesting turn of events, this thing somehow managed to fix itself. Between my angry glares and inspecting of the inverter board, I turned the set back on and was not able to get it to fail. I wanted to take some pictures to see if I could see dim spots that could reveal a bad backlight, but was surprised when it didn't turn off. Eventually, I cleaned it and then put it all back together and ran it for close to 20 hours. No more problems.

I guess only time will tell if it remains fixed. Fingers crossed!

[Nick's Tvs]

Re: Backlight cuts out on LG 55LD520 55" LCD

This could be due to a cracked solder joint on the power supply which could be making temporary contact due to moving the tv around.

[SurrealMustard]

It's back!

This could be due to a cracked solder joint on the power supply which could be making temporary contact due to moving the tv around.

Sure enough! It is back to having the same issue again. How confident are you in bad solder joints on the power supply versus the inverter board(s)?

[SurrealMustard]

Some Measurements from the power supply

I took another crack at it and got some measurements on the connection from the power supply to the master inverter. Here's what I came up with:

ERR - starts at around 3.5, drops to 1.3 after the thing finishes booting up
PDIM #1 - stays at 2.7
INV-ON - holds steady at 3.4
ADIM - 0 volts
24V - holds steady at around 24.8

Given that the power supply is giving consistent power to the inverter, is it reasonable to assume that the inverter is where the problem lies? Backlights don't exhibit intermittent failure, right?

Now, the question is finding which inverter is giving the problem.

[ReeceyBurger123]

Re: Backlight cuts out on LG 55LD520 55" LCD

What does the Bl on signal do when it fails ?

[SurrealMustard]

3.3

What does the Bl on signal do when it fails ?

I don't know if it has one. The only other point of any consequence that I could find was the PWR (labelled as "RL-ON" on the mainboard side), which holds steady at 3.3.

[budm]

Re: Backlight cuts out on LG 55LD520 55" LCD

RL-ON = Relay on, same as PS-ON or PWR ON.
It should have command pin for the Backlights ON/OFF. The cables are blocking the view of the connector on the power supply board that goes to the inverter board so we cannot see the pin labels.

[Alastair E]

Re: Backlight cuts out on LG 55LD520 55" LCD

BL-On = Inv-On

[SurrealMustard]

ah ha! (2)

RL-ON = Relay on, same as PS-ON or PWR ON.
It should have command pin for the Backlights ON/OFF. The cables are blocking the view of the connector on the power supply board that goes to the inverter board so we cannot see the pin labels.

BL-On = Inv-On

That is a good thing. If those two have the same meaning, then that means that the power board is giving the inverters everything they need to run (both the voltage and the "go-ahead"). With that in mind, it sounds like the power board is good, so the question becomes how to find out how to identify which inverter (or inverters) is (or are) causing the issue.

[SurrealMustard]

possible lead

I had a conversation with a parts supplier and he had a hot tip - he thought that a cold solder joint on the mainboard could be causing this problem and recommended a reflow. Has anybody else heard of a backlight cutout issue being caused by a cold solder joint on the mainboard?

[ReeceyBurger123]

Re: Backlight cuts out on LG 55LD520 55" LCD

No point in guessing the fault, we need you to diagnose and test it. Check what Bl on does when it fails and Bl error.

[SurrealMustard]

Diagnostics

No point in guessing the fault, we need you to diagnose and test it. Check what Bl on does when it fails and Bl error.

INV-ON holds steady at 3.4 and ERR starts at around 3.5, but drops to 1.3 after the set finishes booting up.

[SurrealMustard]

No news

I tried the dryer trick on this one also by individually heating each board for a few minutes with the dryer before powering it up, but had no luck or change of any kind.

I did get a better look at the problem though (which manifests almost instantly now, so there is no way to get a test pattern up). The interesting thing is that when it first turns on, it displays at full brightness for a split second before becoming really dim and then ultimately turning off. This might be part of the normal startup routine, (I can't remember for sure anymore) but if it's not, it may provide a valuable troubleshooting clue. Unfortunately, I'm just not very experienced when it comes to electronics repair, so it's tough for me to say.

[SurrealMustard]

Maybe he's on to something

I've been researching this a bit more and found a handy troubleshooting video that seems like it could be a good reference. The only problem is that the transformers are a little bit different. What points would need to be tested on this board to see which one (if any) has a higher voltage draw? Also, would the standard chassi ground be the reference point for testing this as well?

[Nick's Tvs]

Re: Backlight cuts out on LG 55LD520 55" LCD

Check the power supply board visually and look for cracked solder joints on the power supply. I still think you have an issue with a bad solder joint. Especially if the TV started miraculously working for a bit and then went back to the same issue. If you had a shorted diode or resistor or some sort of other issue like that, the tv wouldn't all of a sudden start working again.
Cracked solder joints occur due to heat stress. So look at anything on a heat sink/near a heat sink, transformers, or anything on high voltage lines. I have seen connectors get cracked solder joints, look at those as well.
If you keep turning the set on and off say about 10 times, do you see any difference?
Also to rule out your main board, bypass the power supply with BL-on and PS-on to 5 volt.

[SurrealMustard]

Power supply not the culprit

Check the power supply board visually and look for cracked solder joints on the power supply. I still think you have an issue with a bad solder joint. Especially if the TV started miraculously working for a bit and then went back to the same issue. If you had a shorted diode or resistor or some sort of other issue like that, the tv wouldn't all of a sudden start working again.
Cracked solder joints occur due to heat stress. So look at anything on a heat sink/near a heat sink, transformers, or anything on high voltage lines. I have seen connectors get cracked solder joints, look at those as well.
If you keep turning the set on and off say about 10 times, do you see any difference?
Also to rule out your main board, bypass the power supply with BL-on and PS-on to 5 volt.

Subbed out power supply board with one that is allegedly good; still no change. Looking down at it from that angle, the backlighting that does come on (only for less than a second now) appears to be uneven. I'm beginning to wonder if we've got a bad inverter board (let the hunt begin) or backlight (seems difficult to troubleshoot).

[SurrealMustard]

Bad luck; feel better!

Phew, we're just having the worst of luck with this thing. The first inverter board I ordered was the wrong one ("Rev:6" instead of "Rev:4", which are identical boards, but Rev6 has a slightly different connector that connects to the inverter right above it) and the second one was the correct one, but the seller sent me a "Rev:6" anyway, so I arranged for an exchange. Unfortunately, the replacement was also a "Rev:6", so now I'm in the process of getting that one exchanged for the correct one once again.

While we're waiting for that, does anybody have any lamp troubleshooting tips (just in case those are the issue) now that we've ruled out a power supply board?