LG 50PS3000 noisy picture

[bbjunkie]

Not sure how you would describe this, the picture appears like there is noise getting onto the video - horizontal lines and flickers.

Video Here

Any thoughts where to start?

thx

Jim

[yokoono]

Panel problems. Isolate the ribbons leading to the panel one by one and see which side the correct image is on.

[Biruslapio]

If it were an LCD, trembling image would certainly point to a bad panel driver on either side of the panel, but since it is a plasma, seems to be something in between control board, LVDS and main board.

I suggest doing a AUTO_GEN test, this will make the TV operate without the main board and LVDS, and display some test patterns, if it's still bad I bet on the control board.

[bbjunkie]

AUTO_GEN test is still showing the flickering, tried to capture it on video but it's not showing up very well, in addition to this there was an odd horizontal bar during part of the test. link to AUTO_GEN video here

Checked some voltages while I was there;
Y sus
M5V = 5.06v
VA = 60v
VS = 194v
-VS = 194v (should this be negative WRT to ground? - double checked its definitley not)

Z sus
+15v = 18v
CTRL_OE 62.3v
EA_UP 176v
EA_DN 234v
YZB_CON = 3.22v
ZBIAS = 4.9v
SUS_UP 3.23v
SUS_DN 1.04v

Seeing another post here referring to Z_Bias being correct at 97v so wondering if my lower 4.9v is the problem, or perhaps a result of another issue?

[bbjunkie]

No reference to the Zbias voltage on panel,

[dick_barton]

Looking at th auto-gen video I wonder if the problem is with the control board addressing problem.

The picture looks fine other than the picture offset (rollover) so I doubt you have an issue with any of the supply voltages.

I'm no expert on plasma's so these are just my thoughts.

[Biruslapio]

Z-bias is the 105V value, -180 is -VY and 150 is VSCAN.

This is most certainly a control board fault, maybe dried up some capacitors and corrupted the memory, it's a cheap replacement.

[bbjunkie]

Great, thanks - I'll try replacing the caps ( see others mentioning a common issue of faulty 100uF 25v caps on the control board) if that doesn't work i'll try to find a board.

[Th3_uN1Qu3]

Oh I've seen that one before. Certainly a control board fault, not a panel or power supply problem.

[bbjunkie]

Decided instead of getting caps to get a replacement control board from eBay as it was just as cheap and less hassle, the fault is exactly the same. Thing is, the board could've been faulty. Also how coincidental would it be that the replacement board has exactly the same issue (noise on the picture and the horizontal bar at the bottom of the screen)

Perhaps I should try ordering another control board.

Before I do,

Originally Posted by Biruslapio

Z-bias is the 105V value, -180 is -VY and 150 is VSCAN.

ZBIAS measured at the plug on the Z board was 5v not 105v - am I measuring it at the correct location?

[bbjunkie]

Plug I'm referring to is bottom left, P100

[Biruslapio]

ZBIAS is measured from the test point at Z-SUS to ground, those resistors way up in the board with VZB written, your picture is fine looking at the video.


I didn't explain properly, but I think just replacing the capacitors wouldn't help, since they go bad and then corrupt the memory, it would need to be reflashed, it's just way simpler to replace the board with a working one, it's rare for this board to fail.

[bbjunkie]

Ok, that explains the low reading! Can confirm correct ZBIAS getting 104.3v there.

So despite the control board being a rare failure and the second showing the same fault you think I should try ordering another?

TIA

Jim

[Biruslapio]

It's really weird to still have the issue, when you did the testing you removed the LVDS cable and shorted the two pads on the new board also? Soldering those auto_gen pads is be a bad idea because it will probably void your warranty.

[bbjunkie]

Originally Posted by Biruslapio

It's really weird to still have the issue, when you did the testing you removed the LVDS cable and shorted the two pads on the new board also? Soldering those auto_gen pads is be a bad idea because it will probably void your warranty.

Yes, I removed the flat flex cable to the main board and bridged the 1st two pins on the control board plug P6 (labelled AUTO_GEN) with one end of a clip-lead.

[bbjunkie]

Should VS voltage have a fluctuating AC component? Was checking rails for ripple, VS appears to be swinging wildly while measuring on AC, it's showing a stable 194v DC though.

[Biruslapio]

All voltages that have a specific name on plasmas are DC, that goes for VS, VA, -VY, VSCAN and Z-BIAS.

Is the image fault exactly the same with the two boards? The original board had the image displaced vertically, part of the top appears on the bottom at one spot, does the new one does it at the same spot too?

Next thing to check would be the Y-Buffers, it's the only other boards that could cause this, maybe there's dust/dirt on the cables.

[bbjunkie]

Yes, the image is exactly the same with both boards. Without AUTO_GEN, and with the main board connected, I can see text in that solarised/noisy bottom part of the screen from a game console that was connected before I got the TV. Is this significant?

I'm sure I saw someone on the forum have a similar failure mode but can't find the post now.

I've tried removing the ribbon cables from the top Y buffer board one at a time, this didn't make any difference other than leaving a black bar across the screen.

Then, I removed the link cable between top and bottom Y buffer board - the bottom half of the screen obviously went black but again the issue was still visible with the flickering on the top half.

I've also got the loan of a Z board to try, that made no difference.

IMO I've only one option left and that's the Y-sus board. Any tests I can do before swapping it out?

TIA

Jim

[bbjunkie]

Using a method recommended by Tom66 in a different thread, (as shown here) I Removed Y buffers and checked for shorts, no input pins shorted to output pins.

In addition, after running for 10+ minutes, all heatsinks on buffers and other boards are warm but not hot to the touch.

[Biruslapio]

I have no idea what could it be now, but since process of elimination is at work, it can only be Y-SUS, control board (kinda ruled out by a substitute and nothing changed), or the X drive section at the bottom of the TV, these connect to the PDPs buffer ICs that control the vertical lines.