LG Flatron M2794A TV won't turn on properly

[miyke]

Hi people,

So I have two of the same LCD TVs, LG Flatron M2794A. They were bought about 1 year apart and both of them have the exact same problem!

In both cases they were working 100% fine and then randomly stopped working. I turn the power on using the button on the TV and then the blue light turns on, the TV goes from completely black (like when it is turned off) then it lights up for about half a second and then goes back to complete black.

I took one apart to check the capacitors and every single one of them looked to be in perfect condition — none split open, none bubbled up on top. I really don't know what is wrong.

Any suggestions or advice? I would quite like to avoid getting it repaired by a professional.

Thanks!

[jman98]

I have a suggestion for you that while it won't fix this problem, whatever it is, it might help you to avoid this in the future. I live in a part of the US where unfortunately my electricity is not reliable. About once a month, I have a brown out. I used to be too cheap to use UPS devices on my PCs and I always had weird disk drive problems where every year or two my PCs would refuse to boot. All my data was completely accessible, but Windows would no longer boot. The disk drives were corrupted and they would go through CHKDSK all the time. I am sure that losing power was causing this. I've never seen this problem since I started using UPS devices. I paid good money for my LCD TV, so I bought a small UPS that I have the TV connected to. The whole point of the UPS for the TV is simply to provide stable power to the TV and to keep it from shutting down hard during brown outs. You might consider running your TVs through a UPS in the future.

[miyke]

You might consider running your TVs through a UPS in the future.

Both have been connected through UPS devices throughout my use with them. And my power supply is very constant — I don't get blow outs.

Thanks for the input, but I don't think this is the issue.

[turk690]

I turn the power on using the button on the TV and then the blue light turns on, the TV goes from completely black (like when it is turned off) then it lights up for about half a second and then goes back to complete black.

I have a similar TV/monitor, M237A. The power button on the side is fragile and gets stuck, simulating a constantly pressed situation. In this case, power cycles on then off again (you know it's on without signal input because indicator turns from blue to yellow after a few seconds, instead of turning off). I removed the power button and just used the remote. This button pushes an SPST switch, which itself can also get shorted or act flakily one way or the other; may have to be replaced.
Another problem (which ultimately happened to mine) is one or more of the 4 fluorescent lamps die. When the death is it shorts, the PSU, after performing POST decides to abort and turns off because a hot inverter can be a fire hazard. I replaced the backlight with an LED system. It's all now brighter and cooler.

[miyke]

The power button on the side is fragile and gets stuck

I'm afraid this is not my problem either.

[turk690]

It turns off quickly likely because there is an unexpected load (short) somewhere.
When you take it apart, turn it on, look at edges and see if fluorescent lamps turn on. If they don't, or if they do for a split second only (so you see "no video" message on screen), then one or more are defective (shorted or open) and need to be replaced. It's rare that all will be defective at the same time, so the trick in testing is to disconnect them one by one to see if monitor will come back to life. This model range uses 4 lamps, each driven by a separate inverter (note the four separate inverter transformers); the lamp connectors should be near each (in most cases they have a white and pink wire each). Cold-cathode fluorescent lamp (CCFL) systems that serve as backlights for LCD monitors are the Achilles' heel of the appliance. They consume the lion's share of power, are hot in operation, exceptionally fragile (lamps have been known to break when monitor is dropped or subjected to rough handling, or because the soldered ends become cold-jointed they just come right off), and generate about 1KV for each lamp. The heating and cooling cycles when powering on and off eventually create cold joints (points of high resistance) on the PCB itself, which heat them up even more and can actually be a fire hazard. In many instances, the areas on the PCB populated by the inverter components have blackened because of this.
Just because capacitors look fine doesn't necessarily mean they are. In general, I replace electrolytic capacitors in LCD monitor power supplies if they are 5 years old or more. Capacitors die when their electrolyte eventually dries, increasing the equivalent series resistance (ESR) first, then eventually short out, putting untold loads across the PSU and preventing it from powering on.
All told, nothing may be wrong, except that there may be cold-joints not just on the PSU or CCFL inverter circuit, but on other circuits. Solution could be as simple as resoldering them. Alas that would be for a pro to do now, no?

[miyke]

Thanks very much for the answer turk! I will have a look at the bulbs and all the other possibilities you mentioned and if that fails I will take it in to a pro.

Thank you!