lg rt-42px11 plasma, picture starts fine....then goes crazy!

[Byronboy]

Hi, I got my hands on this LG RT-42PX11 plasma T.V, and I would love to be able to fix it. It is just such a shame to see these things all being thrown into the landfill...
Anyways, Picture is fine for the first couple of mins, then goes to almost a solarized picture, then to a multicoloured moving random colours. all the while flashing.(see pics) Then it will come back to a good picture for a while. It also seems that giving it an old fashioned slap on the back will get it to come good as well. Sound stays fine the whole time.

I have read lots in here about ysus, zsus, and control board issues, but I haven't had anyone describe these symptoms.

If anyone could point me in the right direction, that would be great. I would love it if it was something simple like a bad cap or broken solder joint. But really, what are the chances of something being easy with this kind of stuff

Any input is greatly appreciated!

[PlainBill]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

1. Thanks for the pictures. They help a lot.

2. If you keep slapping a TV on the back, sooner or later it will hit back - in your wallet.

3. The second picture is pretty definitive - a Z-sustain problem. Coupled with the slap information - a bad solder joint, most likely on one of the power transistors or the power connectors.

PlainBill

[Byronboy]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Awesome, thanks PlainBill, now I have an idea where to look, and I might actually be able to fix it!

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

What luck that I should find a thread discussing the exact fault in the same model of device that I am currently trying to fix....

I have same model of TV which someone has asked if I can take a look at for them. Almost exactly the same symptoms but probably not quite as extreme. (see attached photos - they are not the best as they are a bit yellow but it gives you an idea)

Was the original fault in this thread ever found as I would be VERY interested if it was.

Background is that the TV was switched on one day, and a loud pop was heard coming from the back, and this is now how the TV behaves.

I am currently opening, cleaning and looking for obvious physcial issues like exploded caps.

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

I have checked over the whole chassis and inspected each board on both sides and all looks good physically. I cannot see any missing components, burn marks, heat effected PCB or obvious dry joints etc....

I have noticed that the large black capacitors are rounded on top - I guess they are not meant to be like this? (see attached photo) All other caps look fine.

Checked all the fuses and all is good except the fuse on the YSUS board has blown.

The question is could I still get a picture with the YSUS fuse blown? - I would have thought the whole display would not be operational? Would the faulty caps cause the YSUS to blow the fuse or is the rest of the board dead as well based on the symptoms?

[tom66]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

I would place a bet on the Y-sustain. A failed Z-sustain usually causes either a very dim and blurry picture, or potentially no picture. If you adjust the timings incorrectly on the Y-sustain board you'll get exactly the same effect. Try tweaking the Y_setup and Y_setdn pots on the Y board. Hitting the TV is moving the pots back into position temporarily, improving the picture. But thermal expansion moves them back again.

Whilst you do need an oscilloscope to get the timings perfect, often a bit of tweaking with a screwdriver is all that is necessary. Careful though; the sustain board deals with ~200V DC.

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Hitting the TV is moving the pots back into position temporarily, improving the picture. But thermal expansion moves them back again.

That was actually the last guy. My TV is the same model and exhibits the same symptoms but has the problem continuously.

Also I discovered the YSUS fuse is blown (tested with DVM) so I guess its not running at all.
Did an inspection and the largest power caps seem to be bulging on the top so they definately need replacing on both the ysus and zsus boards.
Question is has the buffers or anything else on the YSUS board been damaged.
I am a bit confused as I thought the whole TV would shut down and not display any pic if the YSUS fuse is blown or board umplugged etc...

[tom66]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Try replacing the Y-sus fuse; if it blows again, you probably have a bad Y-sus. Sometimes the fuses just blow after being used for a long time (thermal stress.)

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Thats an excellent suggestion which I will try.
I have to say I have never had a fuse blow like that for no reason but I guess you can get lucky.

[tom66]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Thats an excellent suggestion which I will try.
I have to say I have never had a fuse blow like that for no reason but I guess you can get lucky.

I had this happen on a Z-sustain which caused a very dim and slow to update picture. Replaced the fuse and it worked fine.

The board said use a 2.5A or 4A fuse. It had a 2.5A installed. So I installed a 4A one to reduce the chance of it blowing again. The most common fault on these is a part going short circuit so it wouldn't really matter if the fuse is 2.5A or 4A, it's still going to blow in a reasonable time.

Fuse failure is proportional to I^2 * t. For example, 1A for 10 seconds is the same as 2A for 2.5 seconds.

Those caps which appear to bulge aren't actually bulging unless you can't depress the plastic disc. They are probably fine.

In fact, I have a TV with the very same board in my shed. I remember in an attempt to fix the sparkles I adjusted the SET_UP/SET_DN. If I went too far the picture went crazy like you show in your pictures.

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Next question - does anyone know or can they confirm what the spec is of the fuses on the YSUS board.

It has T4.0AH 250v or T2.0AH 250v printed next to the fuse holder. Which based on visual characteristic implies a 4A ceramic fuse.

I guess something like the following would be suitable - can anyone confirm?

http://www.jaycar.com.au/productView.asp?ID=SF2108
http://www.jaycar.com.au/productView.asp?ID=SF2118

Are the fuses on the YSUS fast or slow blow?

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Those caps which appear to bulge aren't actually bulging unless you can depress the plastic disc. They are probably fine.

I looked at the caps and thought the same as you as all 4 are bulged by exactly the same amount. (whats the chance of that) The top of the cap is depressable reasonably easy to flat and they look a little bulged on the sides.

I am going to look and see if in my research I can see a pic of a YSUS (eg. on eBay to get an idea if they are all like this and hence nothing to worry about)

[tom66]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Next question - does anyone know or can they confirm what the spec is of the fuses on the YSUS board.

It has T4.0AH 250v or T2.0AH 250v printed next to the fuse holder. Which based on visual characteristic implies a 4A ceramic fuse.

I guess something like the following would be suitable - can anyone confirm?

http://www.jaycar.com.au/productView.asp?ID=SF2108
http://www.jaycar.com.au/productView.asp?ID=SF2118

Are the fuses on the YSUS fast or slow blow?

Use a time-delay (T) fuse, not a fast (F) fuse. That's what the "T4.0AH" means - 4A time-delay.

[tom66]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

I looked at the caps and thought the same as you as all 4 are bulged by exactly the same amount. (whats the chance of that) The top of the cap is depressable reasonably easy to flat and they look a little bulged on the sides.

I am going to look and see if in my research I can see a pic of a YSUS (eg. on eBay to get an idea if they are all like this and hence nothing to worry about)

It's a matter of heat causing the disc to expand - the cap itself is not bulged. With a pair of plyers you can pull off the plastic disc and you will see inside they are fine. Underneath there is the standard vent - if that bulges then you have a problem.

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

In fact, I have a TV with the very same board in my shed. I remember in an attempt to fix the sparkles I adjusted the SET_UP/SET_DN. If I went too far the picture went crazy like you show in your pictures.

Yeh the research I have done tells me that you can adjust the voltage to try and fix that but there is actually a firmware upgrade for the controller which changes the drive waveform and this fixes the problem permanently. The issue arrises as the actual plasma tube wears.

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Use a time-delay (T) fuse, not a fast (F) fuse. That's what the "T4.0AH" means - 4A time-delay.

Ok I will go with the slow blow fuse unless you think its not suitable...

http://www.jaycar.com.au/productView.asp?ID=SF2118

[tom66]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Yeh the research I have done tells me that you can adjust the voltage to try and fix that but there is actually a firmware upgrade for the controller which changes the drive waveform and this fixes the problem permanently. The issue arrises as the actual plasma tube wears.

No, that's refering to "sparklies", the random firing of pixels across the panel. The picture is usually still watchable but the sparklies are annoying. Your problem is unlikely to be firmware.

Slow blow should be fine.

You should not get any picture if the Y-sus fuse is blown; are you sure it is?

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

You should not get any picture if the Y-sus fuse is blown; are you sure it is?

I am with you 100% here - I cannot understand why its still working at all?

I checked all fuses on all boards both in circuit and out of circuit with a DVM and this ones totally open circuit. (the others are all fine) I checked it twice because I figured I must have made a mistake first time round.

But the original owner did say he switched it on one morning and heard a loud crack come from the TV and then the off picture as a result. I cannot see any other visible physical damage (I went over the boards carefully) so it is most logical its a fuse based on the report but it does not make operational sense....

Also my bad about the firmware - I have read so much info in the last two days I have got confused. I have no intention of playing in this area.

[tom66]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

I am with you 100% here - I cannot understand why its still working at all?

I checked all fuses on all boards both in circuit and out of circuit with a DVM and this ones totally open circuit. (the others are all fine) I checked it twice because I figured I must have made a mistake first time round.

But the original owner did say he switched it on one morning and heard a loud crack come from the TV and then the off picture as a result. I cannot see any other visible physical damage (I went over the boards carefully) so it is most logical its a fuse based on the report but it does not make operational sense....

Also my bad about the firmware - I have read so much info in the last two days I have got confused. I have no intention of playing in this area.

There is one simple check - put the suspect fuse back in the TV and turn it on. Then set your multimeter to DC 1000V or closest to that setting (>200V). Probe the voltage on both ends of the fuse; one lead on one end of the fuse and the other on the case of the TV. If you get radically different values (more than 1V I would say) then the fuse has probably failed.

[JasonC]

Re: lg rt-42px11 plasma, picture starts fine....then goes crazy!

Got a chance today to get some new fuses of suitable type. Installed and powered on TV. The fuse did not blow and the picture is the same.

So my theory is something is crook on one of the SUS boards (leaning towards the Y as this is where the fuse blew). When the responsible component died it took out the fuse but now that that component is dead the fuse will not blow again as this part is open circuit.

Is it possible to do a component level repair on this board?