Panasonic TX-L50B6B | High pitched noise and black lines

[oldmanhouse]

Hi folks, I have an issue with my 7 year old Panasonic TX-L50B6B that has appeared within the last few days.

The symptoms are:

• A high pitched "buzzing" / "whining" noise coming from the TV when it is turned on. There is no noise during standby, and the noise persists with all AV connections removed.
• There are noticable horizontal lines in the picture. Upon closer inspection it looks like every few rows of pixels are dead/black.

Attached is a close-up of the picture in "test mode", to demonstrate the rows of black lines.

A few other observations:

• Unplugging the TV at the mains for a few hours made no difference
• Adjusting the picture settings (mode, brightness, backlight, etc) made no difference. Backlight appears to be working correctly.
• The noise is most audible on the left hand side of the TV (from the back) which makes me think it is the LD Board (diagram from service manual attached)
• The TV turns on fine and is otherwise fully operational, albeit with a diminished picture. The further you sit away, the less notiable the "lines" are so it is not a huge problem but the high pitched noise is very annoying. It may come and go, but that could be my ears getting used to it!

Any pointers much appreciated!

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

A little further information...

I found out about a "LCD Panel Test Mode" from the service manual, which I ran last night. It displays test patterns on the screen for observation of visual defects. Quote from the repair manual:

If the abnormal picture is displayed, go into LCD Panel test mode to display the several test patterns. And then, judge by the following method.
Still abnormal picture is displayed: The cause must be in LCD Panel.
Normal picture is displayed: The cause must be in A board.

I observed visual defects during the test mode, so based on the above it must be a LCD Panel fault. My question is, could that also include the LD Board or the ribbon connections from the panel to the board?

Regarding the high pitches noise. Is that a symptom of a faulty LCD Panel? The bad panel puts additional load on the PCB components, causing the whine? Or the other way around - the faulty PCB causes the defect in the Panel?

[budwich]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

do the "standard test" to help isolate where the problem might be and go from the result.... basically disconnect, one at a time, the cable going from the tcon to panel. What is the resulting display and the resulting noise.

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

do the "standard test" to help isolate where the problem might be and go from the result.... basically disconnect, one at a time, the cable going from the tcon to panel. What is the resulting display and the resulting noise.

Thanks - I've done as you suggested and removed each ribbon cable from the T-Con to the Panel. I did this whilst the TV was in test mode displaying a picture. The results were:

with left cable unplugged: left side of screen was white (no lines), right side still displayed the picture with lines

with right cable unplugged: right side screen was white (no lines), left side had a garbled picture with lines

The noise was present the whole time, except once I plugged the right cable back in (so both cables were plugged in). The noise stopped, but the picture was just black on the screen.

I have some photos, which I'll upload shortly.

The T-Con isn't listed in my service manual, I guess because it's part of the panel which is a LG. The part number is 6870c-0452a (high res picture here).

[budwich]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

well, hopefully, you weren't unplugging / plugging in cables while powered ... :-(
will wait for pictures. the other thing to check is ensure the cabling between the main board and tcon is correctly seated and the connectors are clean / free of oxidation.

Your test appears to indicate a problem with the left side. further, investigation will likely require measuring voltages at the tcon (check the labelling on the card for hints as to what is where). The "whine" is a result of some loading placing on the powering component. You should be able to determine if it is coming from the tcon or power supply card... use a tube to your ear to isolate / locate the source.

Still, also check the tcon to panel cable. what is the history of the set?

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

well, hopefully, you weren't unplugging / plugging in cables while powered ... :-(
will wait for pictures. the other thing to check is ensure the cabling between the main board and tcon is correctly seated and the connectors are clean / free of oxidation.

Your test appears to indicate a problem with the left side. further, investigation will likely require measuring voltages at the tcon (check the labelling on the card for hints as to what is where). The "whine" is a result of some loading placing on the powering component. You should be able to determine if it is coming from the tcon or power supply card... use a tube to your ear to isolate / locate the source.

Still, also check the tcon to panel cable. what is the history of the set?

I've had the TV from new, for 7 years. It's never had a repair performed on it before, and has never had any knocks or physical damage.

Thanks for your suggestions. I checked the A-Board to T-Con connection and it seems OK, firmly seated and not corroded. There's 11.9v on the A-Board side of the LVDS cable.

As the T-Con specs aren't in the service manual, I don't have a reference for what "good" voltages should be, but here they are...

These are from various test points on the board

VCC 3.29
HVDD 7.92
VDD 15.76
VGH 28.8
VGL -4.92
VCC18 1.78

and these are from test points before the T-Con to Panel connections

VCOML_FB 7.13
VCOMR_FB 7.13
VCOM 7.14
VST -4.86
VGH_E -4.9 to 28.2 *
VGH_F -4.91
CLK6 6.5
CLK4 6.7
CLK2 6.61
VGL_1 -4.9
VGH_0 -4.9 to 28.3 *
VGH_R 28.2
CLK5 6.58
CLK3 6.67
CLK1 19.27

* switched between these two voltages

I hope i've labelled those right, it wasn't completely obvious to me from the printing on the PCB.

Nice tip with the tube. I listened at it does appear to come from the T-Con. As the "driver board" (not sure on proper name) is also nearby it could be that but I don't think so.

[budwich]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

thanks for the info... I don't see any pictures of the display with the "cable disconnected" test.... just ensure your eyes and our eyes are seeing the same thing. :-)

I am thinking that the VCOM voltage seems a bit low. I was expecting something in the 15-20v range... maybe.

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

thanks for the info... I don't see any pictures of the display with the "cable disconnected" test.... just ensure your eyes and our eyes are seeing the same thing. :-)

I am thinking that the VCOM voltage seems a bit low. I was expecting something in the 15-20v range... maybe.

Certainly - I've attached them here. Hopefully they are useful, it was hard taking a picture with one hand!

I did the test twice, once where I unplugged the connections whilst power was on (I guess that wasn't the best idea...) and then again turning the power off whilst I removed the connections.

The results were slightly different, when I left the panel on and removed the connection, the picture stayed intact on one side. When I turned the TV on with only one connection, the connected side was all garbled

One thing common amongst all the tests was the unplugged side was white and had no horizontal lines

[budwich]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

ah... your "garbled" is more of "no picture, just vertical lines"... :-)
note that this result is somewhat different from your earlier description as you indicated that you could have a "garble" in one cable configuration and in the other a "lined picture".
Now you are saying that you always get the "garble" with a side disconnected from power up.

At this point, I am thinking it could be a tcon issue but more likely a panel issue. You could try using the "tape cutoff" method (use a piece of tape to block signals at the tcon connector... read about it).... but since your screen is responding the same way with either side connected / disconnected, the tape cutoff method is likely not to work as you need to have one side doing something good and the other side bad which you then focus on removing those signals. This is why I think you have a tcon problem possibly.

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

ah... your "garbled" is more of "no picture, just vertical lines"... :-)
note that this result is somewhat different from your earlier description as you indicated that you could have a "garble" in one cable configuration and in the other a "lined picture".
Now you are saying that you always get the "garble" with a side disconnected from power up.

At this point, I am thinking it could be a tcon issue but more likely a panel issue. You could try using the "tape cutoff" method (use a piece of tape to block signals at the tcon connector... read about it).... but since your screen is responding the same way with either side connected / disconnected, the tape cutoff method is likely not to work as you need to have one side doing something good and the other side bad which you then focus on removing those signals. This is why I think you have a tcon problem possibly.

Thanks, I've ordered a recycled t-con so I will see if that rectifies the problem. I'll also do some reading on the tape trick. As I understand it, I need to find the pins which carry video signal and block those, so I'll try and find a pinout for the t-con to panel connections...

Regarding the vertical lines, I've not seen that during normal operation - only when powering the TV up without one of the t-con to panel cables disconnected.

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

I found some references to normal t-con voltages on a blog: https://translate.google.com/transla...search&pto=aue



So comparing my readings, they look OK. So I suspect t-con is OK but a "new" one is cheap so worth a shot. I could try replace the ribbon cables to panel (they are removable), but poking/moving them doesn't change the picture so they are probably not the cause. So the most obvious cause of the LCD gate driver seems likely now.. I was doing some research and on some panels, it seems a single failing gate driver IC can affect all of them which is perhaps why my horizontal lines appear across the full height of the picture.

[budwich]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

certainly the probability of a panel / driver problem exists, but it is unusual to impact both sides the way that yours is and thus will make trying the "tape cutoff" method difficult as not knowing which side is the issue to focus on. In terms of cabling, both ends should be checked as the end at the driver board can be compromised in its seating / positioning.

Further, the tabs to the panel from the driver boards should be checked visually. This needs to be done from the front of the panel by removing the plastic bezel / trim. It is possible that "window cleaning" might have caused a problem. :-)

The links that you provided indicated issues with replaceable components at the driver boards (ie. smd capacitor, resistor, etc). You can try measuring / checking those areas. Normally, with the noise that you are hearing, a suspected device would be shorted / lower impedance than normal.

[budm]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

Just curious about your post #6:
These are from various test points on the board

VCC 3.29
HVDD 7.92
VDD 15.76
VGH 28.8
VGL -4.92
VCC18 1.78

Are those Voltages measured with all the cables in place?

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

certainly the probability of a panel / driver problem exists, but it is unusual to impact both sides the way that yours is and thus will make trying the "tape cutoff" method difficult as not knowing which side is the issue to focus on. In terms of cabling, both ends should be checked as the end at the driver board can be compromised in its seating / positioning.

Further, the tabs to the panel from the driver boards should be checked visually. This needs to be done from the front of the panel by removing the plastic bezel / trim. It is possible that "window cleaning" might have caused a problem. :-)

The links that you provided indicated issues with replaceable components at the driver boards (ie. smd capacitor, resistor, etc). You can try measuring / checking those areas. Normally, with the noise that you are hearing, a suspected device would be shorted / lower impedance than normal.

Thanks budwich, I think I'll remove the LVDS cable and t-con cables and give them a clean next time I have the TV apart. If the t-con replacement doesn't work, I'll disassemble further down to the driver board and have a look for shorts/burnt out components there..

I ran the internal screen test on the TV again last night, and noticed the problem is more pronounced when it displaying a red or blue screen (problem less noticable on the black, white or green screens). Also I noticed although the horizontal lines are displaying from top to bottom of the screen, they are more noticable on the bottom third/quarter of the screen... I'll try get a picture later

Just curious about your post #6:
These are from various test points on the board

VCC 3.29
HVDD 7.92
VDD 15.76
VGH 28.8
VGL -4.92
VCC18 1.78

Are those Voltages measured with all the cables in place?

Hi budm, yes those voltage were taken with all the internal connections (LVDS cable, the two t-con to panel cables) attached and the TV powered on. As for external connections- there was no hdmi/coax/scart devices connected at the time providing an AV input

[oldmanhouse]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

Hello, I'm back with an update. I received my replacement T-Con board, but before fitting I took voltage measurements again, just to make sure I read them correctly first time. They were all recorded correctly as I posted above, including the most suspect one which was CLK1 reading 19.27v.

I've been doing a lot of searching, and managed to find some good resources for my T-Con board (or very similar ones). Turns out CLK1 through to CLK6 should all be around 6.5v, which makes my reading for CLK1 very suspicious indeed. I suspect whichever IC controls the output for CLK (I'm guessing that refers to some sort of "clock"?) was faulty, and was probably the component making the noise.

I replaced the T-Con and the noise was gone, and the picture restored. Problem fixed, so it would seem

Original source for annotated image: https://electronikasoftware.blogspot...-pantalla.html

Many thanks to everyone for their help, especially budwich

[budwich]

Re: Panasonic TX-L50B6B | High pitched noise and black lines

great work.... I am glad you got it going and further that it wasn't a "panel issue".