LG 50" PK540 Plasma Clicking, not turning on.

[andocommando]

I have acquired a broken 50" LG plasma. Looks to be some sort of Cyber Monday or Black Friday TV special from some google searching. I will post pics and a video of what it is doing: not powering on and just clicking, im getting a red light, and about 3 tries to fire up and then it seems to give up. The video should be up in 10-15 minutes. None of the CAPS look bad on a visual inspection, Ive unplugged both the Y and Z and tried to power it up but to no avail. Im new to fixing TVs. Just fixed my first one (40" Samsung LCD last night, thanks to badcaps.net!) I have some basic soldering and electronic skills but breaking things down in Dumb-dumb terms helps me learn best. Thanks for the help!

Video : http://s14.photobucket.com/albums/a3...t=IMG_1968.mp4

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

The video seemed to upload upside down but here is what the TV is doing
http://s14.photobucket.com/albums/a3...t=IMG_1968.mp4

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Thanks for the pictures and video.

Have you got a digital multimeter?

If you turn all the lights out in the room, can you see any flickering on the screen while it attempts to turn on? It might be a brief flash, or a slightly grey screen.

I repaired an LG 42PC1D with very similar symptoms but it had an entirely different power supply. In my case it was a fault with the AC detect, and it was easy to fix.

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

I will borrow one from my work and bring it home tonight. Ill see if I get anything from the screen tonight also and will report back, Thanks tom!

[ben7]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Could also be a shorted LDO regulator on the processing board.

-Ben

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Thanks for the pictures and video.

Have you got a digital multimeter?

If you turn all the lights out in the room, can you see any flickering on the screen while it attempts to turn on? It might be a brief flash, or a slightly grey screen.

I repaired an LG 42PC1D with very similar symptoms but it had an entirely different power supply. In my case it was a fault with the AC detect, and it was easy to fix.

Yep the screen flickers white for a fraction of a second while it tries to boot up. tries it 3 times then nothing

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Yep the screen flickers white for a fraction of a second while it tries to boot up. tries it 3 times then nothing

This confirms the Y-sustain is most likely okay, and it also suggests the Z-sustain is functioning. It means the plasma panel is okay (i.e. not cracked.) It suggests a problem with the power supply starting up, perhaps the AC detect or perhaps the main board.

Can you post a closer pic of this area?

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

My camera sucks

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Never mind about a photo then - what does the legend near the connector say?

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

here we go! Pic attached

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

[double post, my error.]

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Do you think it will be possible to probe the AC_DET pin? You may need a piece of wire. Attach the wire to the red probe on your multimeter, and stick the wire on the AC_DET pin. Place the black probe in an exposed screw hole (e.g., for the wall mount.) Set your meter to the 20V DC range and move the probe to the volts/ohms jack. Then plug the TV in. You should measure either 3V or 5V. If it drops to zero during the clicking, you have a very common problem with LG plasmas which is easily fixed .

In addition, use the ohms range to measure the fuses on the board. Do this with power off and leave the TV for about a minute to discharge the caps. Measure the resistance from one end to another on the 200 ohm range. You should measure close to zero ohms. Don't confuse the open circuit "1" on right hand side with "1.0" ohms. I expect these will be okay as the screen is flickering but it doesn't hurt to be thorough.

Post a picture of your multimeter if anything is confusing so we can indicate which modes to set the meter to.

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

You sir are an awesome help, thanks for dumbing it down for me. Ill do what you recommend when i get home tonight. Paintball is my specialty so if you ever need any help with that hobby, it is my expertise

I will report back later!

Thanks

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

I will be going to London tomorrow, so I may not be able to post - but there are many more experienced members here who can help you.

My real world hobby is model aircraft. I did once think about combining paintball and RC flying...

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Do you think it will be possible to probe the AC_DET pin? You may need a piece of wire. Attach the wire to the red probe on your multimeter, and stick the wire on the AC_DET pin. Place the black probe in an exposed screw hole (e.g., for the wall mount.) Set your meter to the 20V DC range and move the probe to the volts/ohms jack. Then plug the TV in. You should measure either 3V or 5V. If it drops to zero during the clicking, you have a very common problem with LG plasmas which is easily fixed .

In addition, use the ohms range to measure the fuses on the board. Do this with power off and leave the TV for about a minute to discharge the caps. Measure the resistance from one end to another on the 200 ohm range. You should measure close to zero ohms. Don't confuse the open circuit "1" on right hand side with "1.0" ohms. I expect these will be okay as the screen is flickering but it doesn't hurt to be thorough.

Post a picture of your multimeter if anything is confusing so we can indicate which modes to set the meter to.

Bingo! I measured 4.7 Volts before the click, and when the click occurred it dropped to 0V. What Do i need to do to fix it?

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Bingo! I measured 4.7 Volts before the click, and when the click occurred it dropped to 0V. What Do i need to do to fix it?

Check the fuses first.

Since the AC detect is dropping out we need to locate the AC detect optocoupler(s). These may be on the bottom of the board. Can you remove the power supply board and take a clear and focused picture of the bottom of it? (the best you can) I've only worked on an LG 42PC1D and LG 42PX5D, I've never seen that power supply, so the optocouplers will have to be located by deduction. On my power supply, they were the only optocouplers sending a signal from the primary side to the secondary side (most send from secondary to primary) - so they were clearly reversed and it was easy to find them. The optocouplers almost always bridge across the primary and secondary sides, across the boundary line. (You'll note that only a few components are allowed to do this, for reasons of safety.)

An alternative: Can you acquire a 1k ohm resistor? It might be possible to pull up the AC detect line. This depends on the main board having control over the power supply (which it might have, as it has an RL_ON line, and on the 42PC1D I worked on, the main board *did* control most power supply functions.) It also depends on 1k being sufficient - you may not be able to override the power supplies' signal so it *may* mean you need to cut a wire and splice in the resistor. I'm in favour of the AC detect optocouplers though.

Now, the disadvantage with disabling AC detect is that the speakers won't be muted if there is a power failure which could cause a loud "pop" or "click" when they turn off if the amplifier glitches. However, the pop causes no harm to the amplifier or speakers. It just sounds weird, so manufacturers add the AC detect to mute the speakers before power off. This won't happen if switching to standby or unplugging during standby. Oh, and the TV will last much longer if power fails. On my 42PC1D, it lasted half a second with no power on a normal TV picture. Not sure if that is any use as a UPS!

[andocommando]

Re: LG 50" PK540 Plasma Clicking, not turning on.

I checked the fuses and got little to no resistance on them all. Is the power supply board the one we've been working with? And we want a picture of the backside correct (green side?

We can also go the other route with the resistor as I have access to those, just whichever you think is better is what we will do

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

I checked the fuses and got little to no resistance on them all. Is the power supply board the one we've been working with? And we want a picture of the backside correct (green side?

We can also go the other route with the resistor as I have access to those, just whichever you think is better is what we will do

Fuses are good then.

Yep, that's the power supply board - converts the incoming AC to the sustain, standby and run voltages. A picture of the solder (usually green) side is what is needed.

I think you should try the resistor first, as that is the easiest fix to implement, but if that doesn't work you can fiddle with the optocouplers.

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

Thinking about it in more detail, from my plasma I remember it has a 1k resistor in series so maybe using a 220 ohm will be better, but try experimenting with a range of values if you can get hold of them.

[tom66]

Re: LG 50" PK540 Plasma Clicking, not turning on.

One thing I forgot to ask you to check is the standby voltages - check if they are still good during clicking.