LCD monitor: burn marks in backlight connection

[b700029]

Monitor: ProView PL926Wbi

Background: I've had this LCD monitor for three years. Earlier today I used it at a friend's house without problem.

Problem: After bringing it home and turning it on, I saw my desktop but the colors were all shifted. The screen went black after a few seconds. A very small amount of smoke was witnessed.

Without further ado, I started the disassembly.

Inspection:

All the capacitors look fine.


The source of the smoke became evident when I saw the burn marks around one pair of backlight connections.



There is another pair of these connections that look absolutely fine.

Hypothesis:

I'm guessing an electrical connection was broken and the high voltage current jumped the gap, releasing a lot of heat and charring the plastic.

So at this point it seems as though the best solution is to replace the connectors and plugs. Am I correct?

The big question here is whether this is the cause or just a symptom.

Could something have caused the connection to develop such a high resistance that the current preferred to simply jump through the plastic?

Or perhaps a wire came loose during the trip back home?

Background knowledge:

I'm handy with a soldering gun but this is my first attempt at fixing an LCD. Any advice is appreciated.

[PlainBill]

Re: LCD monitor: burn marks in backlight connection

Alternate hypothesis: The inverter is a constant current AC source. If a CCFL is not connected to each output the voltage rises until either the protection circuit shuts it down or an alternate path is found. A common problem with the backlights is poor solder joints at the point the wires are soldered to the ends of the CCFLs. One wire broke, and the arcing was the result.

Solution: Remove this pair of CCFLs from the LCD panel, inspect for loose wires or a broken CCFL, repair the CCFL, replace the plugs and sockets.

PlainBill

[jsog]

Re: LCD monitor: burn marks in backlight connection

Because the pair of connectors are involved, seems like it would be something they had in common. An inverter transformer or a pair of CCFLs, maybe. The plastic in the connectors is rated for the normal high voltage, so it was either much higher than normal or maybe too much current, not voltage.

Since it happened after moving the monitor, bad solder joints (often on transformers) or broken CCFLs arcing could be the problem.

[alexanna]

Re: LCD monitor: burn marks in backlight connection

I agree with the two above posts.
I had some connectors that were burnt just like yours. I thought I would get by easy just replacing the connectors. Well after I replaced them and powered on the monitor there was crackling and sizzling.
It really sounded like it was coming from the transformer, but it turned out to be the CCFLs.
You need to open the panel and inspect the CCFLs.

[b700029]

Re: LCD monitor: burn marks in backlight connection

Thanks for all the great information!

I will investigate further and report back with my findings.

[b700029]

Re: LCD monitor: burn marks in backlight connection

Ok, tore all the way through the LCD and backlight assembly. Very fascinating!

All four CCFL's look a little black towards their ends. However, the pair of CCFL's with the damaged power connector were at least twice as dark as the other pair.

Do these look like they could be the cause of the problem?

edit: Oops, sorry. Inline image moved to attachment.

[PlainBill]

Re: LCD monitor: burn marks in backlight connection

Not directly. This looks like normal aging. At end-of-life a CCFL starts drawing MORE current. The problem was due to a CCFL drawing too little (or no) current. I'd suspect a broken lead or bad solder joint on one of the CCFLs is responsible, or possibly a broken CCFL.

Please don't post pictures inline. Use 'Manage Attachments' (below the text entry area).

PlainBill

[jsog]

Re: LCD monitor: burn marks in backlight connection

The picture isn't high enough resolution to tell, but the bottom two look like they're cracked.

[smason]

Re: LCD monitor: burn marks in backlight connection

Agree with Bill, but because it happened suddenly, BOTH connectors are charred, and the monitor was moved, I'd suspect physical damage to the 2 CCFL bulbs involved.
I suppose both connectors could have been jarred slightly loose, or wiring damage on both, but pretty unlikely given the circumstances.

[PlainBill]

Re: LCD monitor: burn marks in backlight connection

Agree with Bill, but because it happened suddenly, BOTH connectors are charred, and the monitor was moved, I'd suspect physical damage to the 2 CCFL bulbs involved.
I suppose both connectors could have been jarred slightly loose, or wiring damage on both, but pretty unlikely given the circumstances.

It doesn't have to be damage to both CCFLs. There are several different designs for sharing the output of one transformer between two CCFLs, some of them could result overvoltage on both connectors if one CCFL was damaged. Of course, without a picture of the entire power supply / inverter, this is pure speculation on my part.

PlainBill

[b700029]

Re: LCD monitor: burn marks in backlight connection

I will get a picture of the power supply and inverter.

In the meantime, here are some closer shots of the suspicious CCFL.

Good eye jsog, it does look quite like a crack.

Also, there is a white/gray-ish residue along the bottom of the shiny metal piece (can be seen in photos A and B).

[b700029]

Re: LCD monitor: burn marks in backlight connection

Attached are a few pictures of (what I presume to be) the power supply board. Also pictured is the one other board found within the monitor.

[b700029]

Re: LCD monitor: burn marks in backlight connection

Bump! Any ideas?

I'm guessing we will need to replace both CCFL's (the possibly-cracked one and the one next to it)? I'll start searching for possible replacements...

[PlainBill]

Re: LCD monitor: burn marks in backlight connection

Sorry, I missed the earlier update. Looking at the back of the power supply / inverter, each end of the transformer secondary is tied to one of the two CCFLs. The return lines from the CCFLs go to ground. So, yes a single broken wire or CCFL COULD cause this damage.

PlainBill