First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

That is actually a pretty nice scope! Only limitation is its memory is I think 2.5k samples, but that's only useful if you want to look at several subframes. With most plasmas, all you need to see is if a particular waveform is there. Just make sure you use a 400V rated (or more) 10X probe with it. You do not want to blow up the input amplifier/attenuator section! Turn a dual channel scope into a single channel one . How much did you pay for it, if you don't mind me asking?

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I borrowed it from a friend at work. Unfortunately it is only rated for 300V max. I found an older (early 90s) meter/scope on Craigslist that I might buy for $100. According to the manual I found online its capable of inputs up to 400V and 1Ghz

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Using a 10X probe with a 400V rating makes the signal only 40V at the scope, so you can use the oscilloscope with it. My Rigol is 300V rated but can survive a 1000V transient, so brief overvoltage is usually fine. Just either use a 10X fixed probe or be very careful not to move the switch from 10X to 1X.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Where am I actually connecting the leads of this scope? What am I looking for? What am I setting the scope to?

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Can't see the pictures but you're usually looking for a test point marked "Waveform". Can you use Manage Attachments to upload higher res images in case it is named something else?

[Cosmo66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I would take a DMM real quick and probe around the y and z board looking for shorts. I've had 3 of these IPM failures and they all failed short to gnd.

You can also remove the y and z boards, the IPM pins stick through the back and should be labeled. If you measure a short from one of the voltages to ground you know its bad.

I wouldn't recommend trying to change the IPM unless you have a good heat gun, just replace the whole board

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I would take a DMM real quick and probe around the y and z board looking for shorts. I've had 3 of these IPM failures and they all failed short to gnd.

You can also remove the y and z boards, the IPM pins stick through the back and should be labeled. If you measure a short from one of the voltages to ground you know its bad.

I wouldn't recommend trying to change the IPM unless you have a good heat gun, just replace the whole board

Wouldn't there be blown fuses if something was shorted to ground? Both y and z sus boards get warm to the touch when the unit is powered on.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Can't see the pictures but you're usually looking for a test point marked "Waveform". Can you use Manage Attachments to upload higher res images in case it is named something else?

I'll have to look into the higher res images. Right now I can't even load pictures to the site. I've had to resort to using photobucket.

[Cosmo66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Wouldn't there be blown fuses if something was shorted to ground? Both y and z sus boards get warm to the touch when the unit is powered on.

Probably but not always

[Cosmo66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

The IPM's don't always fail short to ground. But as someone stated earlier, this is by far the most common cause of failure in the LG plasmas. The IPM's are garbage. When they fail, you get sound with no picture.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I found a point on the y buffer that says waveform. The problem is that I have next to no idea what I'm supposed to do. I know it makes me sound really ignorant. I do have experience with a scope. I took two years of part time EET classes at the local junior college but that was at least three years ago. What am I setting the scope to? Other than the waveform point, where do I touch for the other point? CH 1 to waveform, EXT TRIG to to ground? Seriously, I can do this, but not without some guidance.





Are any of you able to post pics to your profile? I can't even find my profile album. It's like I've been blocked or something.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I started a second thread regarding scoping the ysus and zsus. For the sake of those who might have this same issue in the future, I'm coming back to this original thread. Here's the other thread.

https://www.badcaps.net/forum/showthread.php?t=24715

At this point, I am suspecting the y buffer board again. I know someone said that a bad y buffer would cause different symptoms, but hear me out.

I just unplugged the y buffer board from the y sus and powered on the unit, which resulted in the exact issue as when the board was plugged in. Now I know that this doesn't confirm a bad y buffer. However, my thought is that it does confirm that the set will display the same symptoms with the buffer removed. If I had removed the board and the unit. Would no longer power up, that would suggest, at least to me, that the buffer was not part of the problem. This is promising, is it not?

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Do you still have the scope?

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

No, I took it back but I asked him if I could borrow it again if needed and he said that was fine.

[ghiotom]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Did you check the IPM on the Ysus board for shorts? The fact you aren't even getting a dim image I would think would point to the Y sus board.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Did you check the IPM on the Ysus board for shorts? The fact you aren't even getting a dim image I would think would point to the Y sus board.

There are no blown fuses on either y or z sus. The boards both get warm when powered on and I scoped both boards and was told that the waves looked fine. Everything, except the lack of glow, suggests the ysus is working fine.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I just read through this again. It took a second to set in. No, I didn't check the IPM for shorts. I will and will report back.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

First picture, these are the pins on the IPM from the back of the Ysus board. The pins in the yellow circle are the same pins in the yellow circle in the second pic. Of the pins in the yellow circle, they all show to be connected to ground though only the one says ground, I suspect that they are all supposed to be grounded. As for the rest of the pins, unless they say GND right next to them, there were none shorted to ground.





However, this thing, the grey can with the red vertical stripe, whatever it is, labeled L202 on the back of the board, is shorted to ground. I have no idea if it's supposed to be or not. It's counterpart, L203 just behind it and to the right, is not shorted to ground.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Tom66, if I can get the scope again tonight, what am I going to scope?

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

The copper traces are all connected, so they are all supposed to be grounded.
You can check to see if each buffer line is being driven (there are test points next to each line), but be VERY careful not to short them out, or the buffer will be blown up.