LG 50PS30 Image Problem

[freakaftr8]

Re: LG 50PS30 Image Problem

Nice find!

[tom66]

Re: LG 50PS30 Image Problem

Nice! Out of interest, what does the waveform look like now? Goes to show how useful a scope is for plasma repair!

[mmartell]

Re: LG 50PS30 Image Problem

Sorry tom had this sitting out where it shouldn't have been so when I got it repaired I zipped it up and made the wife happy again. At least temporarily

[mmartell]

Re: LG 50PS30 Image Problem

File this one under "NOT SO FAST !". The resistor has gone troppo again, or on it's way anyhow currently measuring in the mega ohms...

Would it be fair to suggest there is too much current flowing through the resistor ? I have no idea how this circuit works or should work. I'll post a shot of that portion of the board, maybe someone has some ideas where I should be looking.

[mmartell]

Re: LG 50PS30 Image Problem

There is a pair of diodes tied to the resistor, will have a look at them first.

[mmartell]

Re: LG 50PS30 Image Problem

Here's a shot of the area in question. Now I'm as good as colour blind so my wife tells me the resistor is vvgg and my daughter tells me it could be rrgg. Only one can be right, which do you think it is ?

If it is rrgg then it's 2.2ohm instead of 7.7ohm. Would putting an 11ohm resistor in place of a 2.2 ohm one be sure to cause it to go open ?

And the two parallel diodes to the left of the resistor test about 0.3v forward and open in reverse.

[freakaftr8]

Re: LG 50PS30 Image Problem

Thats rrgg, 2.2 ohm. Could be the reason, also I thing those are high wattage. what wattage did you use? I think the original is a 2 watt

[tom66]

Re: LG 50PS30 Image Problem

Yeah, definitely red-red-gold-gold. 7.7 ohm (violet-violet-gold-gold) would be a very unusual value. It needs to be the right value. A big resistor like that is dissipating quite some power. An incorrect value will increase power dissipation by 5x.

What you can try doing is measuring the voltage across the resistor but you must use a good multimeter as a cheap one will pick up switching noises from the Y-sustain board. If you measure more than a volt or two, then there's a fault somewhere drawing much more current than it should.

[mmartell]

Re: LG 50PS30 Image Problem

Ya I freak I had replaced it with the same 2 w rated resistor, just a different value thanks to my wife seeing violet over red - thanks for clearing up the value.

I had a look around the board again and touched up the top-side joints on two mosfets in the vicinity that looked iffy. Then because I hadn't yet read the latest posts here I bodged in a 10 ohm 5 watt resistor ! Running good again but will recheck the value to make sure even that one is not increasing.

Is there a problem running a higher wattage resistor, I mean is it meant to act as a fuse should something go wrong ?

Also, does that resistor set the base range for the set_up trim pot, meaning if I can still dial in a good pic would it be ok to leave it ?

Will read the voltage across it, tom, later this afternoon.

[tom66]

Re: LG 50PS30 Image Problem

The resistor is probably important to set up some part of the set up circuit. An increased value is likely to cause excess power dissipation and the resistor will eventually fail as a result.

[mmartell]

Re: LG 50PS30 Image Problem

0.288v across the resistor.

I ran it last night and this morning with an empty analog channel (snow) and everything appeared good. The set auto shuts off under "no input" condition but I just tried again with antenna and regular program content. As I was typing this response the resistor went from mildly warm to hot so I'll be swapping it for the correct value.

I guess you think the voltage across the resistor is ok but shouldn't I be concerned with current THROUGH it ?

[tom66]

Re: LG 50PS30 Image Problem

V=IR (ohms law)

So, I=V/R
(I=current, V=voltage, R=resistance)

This means that the voltage can be used to work out the current. The figures you have provided indicate 28mA which seems quite reasonable.

I would make sure to install a 2.2 ohm resistor in the place of the 10 ohm one. The resistor is probably used to reduce the dissipation in the transistor and may shape the set up wave slightly.

[mmartell]

Re: LG 50PS30 Image Problem

Excellent. (I knew ohm's law would be useful here but my theory is lacking...)

I've already harvested another resistor of the same rating and wattage. Will install tonight and update.

Wouldn't it be odd for just the resistor to fail ?

[tom66]

Re: LG 50PS30 Image Problem

I have seen power resistors fail normally without obvious cause, and given the set otherwise works I'm not really surprised you can't find anything else that's bad.

While you're fiddling check the waveform again, would be good to see if it's looking normal or if something else is wrong.

[mmartell]

Re: LG 50PS30 Image Problem

Changed to a 2.2 ohm resistor, the voltage across it has gone down to 0.067v and I guess that means the current across it is unchanged.

Will get the scope on it and get a pic up.

[tom66]

Re: LG 50PS30 Image Problem

The current is 30.5mA, which is slightly higher, as expected.

[mmartell]

Re: LG 50PS30 Image Problem

The resistor value is holding steady and the picture is still looking good. Here are the scope images I just took using the same scope values. I don't see any changes. Am I not capturing the wave properly ?

[tom66]

Re: LG 50PS30 Image Problem

You probably want to capture the end of the sustain pulse here, as this is a reset pulse...the other one may be something different, different plasma panels have different drive primitives.

[mmartell]

Re: LG 50PS30 Image Problem

Well the set has been zipped up again but there does seem to be a difference between the wave you indicate and the earlier one I posted before the repair. Not much in the way of resolution but they do look different. For future reference, which part of the wave is the set_up ?

[tom66]

Re: LG 50PS30 Image Problem

The ramp from 0V to Vs, or 0V to a higher voltage than Vs on older sets (often called Vset or Vsetup, around 300-400V) is the set-up ramp.

Plasma TVs past 2008-2009 or so use reduced-voltage Vset, keeping it below or equal to Vs reduces the cost of the Y-sustain driver circuit.