LG 50PS30 Image Problem

[mmartell]

All voltages are in spec however the image is not right. There is a rainbow defect visible and also a splotchy blackness that goes away quickly after cold start.

I think this set is a sibling of the 50ps3000 which is known for cracked joints under the sus coils. I had both y and z sus's out and they look real clean and the joints look good.

The internal test patterns also look much better than program material but I'm not convinced yet that it just isn't performing better in the patterns.

I have a scope and can check waveforms but don't know where the test points are.

Ysus EBR55360701
Zsus EBR55360601

Any suggestions ?

[freakaftr8]

Re: LG 50PS30 Image Problem

I believe vscan test point is on lower buffer. Is that in spec too? Try heating y board up a bit with a hair dryer and see is problem persists. Looks like low VS or Vscan to me.

Sometimes I try turning up VS about 10 to 15v tok alleviate symptoms of an old panel.

[mmartell]

Re: LG 50PS30 Image Problem

Vsc as in pic ? In spec as defined by the panel, 140v.

I think I may have boosted Vs but I'd doubt I went that high for fear of blowing something up lol but since it's possible I will revisit.

[mmartell]

Re: LG 50PS30 Image Problem

I turned up Vs but the set just shut down at about 208v with not change in the pic. I backed off the pot to something below the proper setting and when I restarted the pic was immaculate. I watched it for about thirty seconds then it suddenly switched back to rainbows.

Either a dry joint somewhere, a failing piece of silicon or an intermittent cable connection I guess.

Going to start by reseating all the cabling as that's the simplest.

[freakaftr8]

Re: LG 50PS30 Image Problem

Ok either a voltage dropped or problems within the Y board. Yes vscan + on that end of resistor with red lead of dmm Chassis ground with neg lead. What do you get?

[mmartell]

Re: LG 50PS30 Image Problem

I measured across the resistor as indicated on the silkscreen. I get 140v, matching panel sticker.

[freakaftr8]

Re: LG 50PS30 Image Problem

Try heating up logic board a little.

[chrismurphy]

Re: LG 50PS30 Image Problem

look for bad solder joints on Zsus board and make sure the Ysus screws are tight

[mmartell]

Re: LG 50PS30 Image Problem

All screws are tight and I did try heat on various boards. Will try again.

[mmartell]

Re: LG 50PS30 Image Problem

Can anyone point out where to attach a scope for the y and z sus boards ?

[mmartell]

Re: LG 50PS30 Image Problem

delete... was wondering if anyone had the SM as elektrotanya didn't have it... but google did

[mmartell]

Re: LG 50PS30 Image Problem

Here's a screenie of my zsus wave. I used the "Auto" function to zone in on the wave. I'm only seeing about 155v p/p but according to the sm I should be seeing about 100v more than that. Am I doing something wrong with the scope or have I found a problem ?

[mmartell]

Re: LG 50PS30 Image Problem

Here's a shot of the ysus. Does this look as it should ?

[mmartell]

Re: LG 50PS30 Image Problem

Any help with these waveforms ?

[mmartell]

Re: LG 50PS30 Image Problem

Looking at this again... tried adjusting Set_up and Set_down. Set up has a noticeable impact on picture quality but Set up has zero impact.

Shouldn't I be seeing a change in screen quality when adjusting this pot ?

[tom66]

Re: LG 50PS30 Image Problem

not always, set down can sometimes do nothing, depends how close the panel is to the limits, you'll probably notice some difference on fine gradients where more maldischarge is visible. or on lower Vs or combinations with set up that causes maldischarge.

Your Y and Z waveforms look normal, what's important for the Z is having the Z-bias voltage period (~100V) in between the sustain periods (200Vp-p). Which you seem to have. For the Y, you should have a nominal -190V base for the waveform, with ramps back to zero, and sustain portions from 0V to 200V. That being said, your set up period looks absent or too short, so there could be a fault there. Missing or rapid set up will cause overdiffusion, where pixels maldischarge all over the panel.

You need to rule out a control board issue by checking the test patterns are ok.

[mmartell]

Re: LG 50PS30 Image Problem

The test patterns "look" normal. I say that because I have this feeling they're misleading due to the nature of test content vs regular program material. I know that normally this would not be an issue - maybe I'm fooling myself.

If the patterns are ok I guess I have a mainboard problem.

I think what I'll do is hook up the scope again to the ysus and adjust the pot and see if the waveform is changing.

Thanks for confirming the Z looks good.

[tom66]

Re: LG 50PS30 Image Problem

It's probably not a main board issue. But you could see what the gradient test pattern looks like when using EXT AUTO GEN.

[mmartell]

Re: LG 50PS30 Image Problem

That would be my last guess too but...

The gradient patterns are black and white and look good. May be a colour issue(?!) I just remembered I was going to force the pic black and white (or find some old material) and see how the pic looks.

It's a peculiar problem, at least to me.

[mmartell]

Re: LG 50PS30 Image Problem

That being said, your set up period looks absent or too short, so there could be a fault there.

BINGO.

Had some time to look at this again and found an open 7.7ohm power resistor near the setup pot. Replaced it with a 10 ohm resistor (didn't have the same resistor so thought better to go bigger instead of smaller) and set has been working normally for the last few hours.

Thanks for the help guys.