TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

[ngth82]

Good morning Badcaps -

I finally have some time to look at a TCL 55S405TBBA that's been sitting in the corner.

Symptoms: I turn on the TV, backlights turn on, the LCD glows, but i don't see anything on the screen. When i press the volume buttons or directional arrows on the remote, i hear the normal menu clicking sounds. The panel appears in this state probably 95-99% of the time.

When i leave the TV off for hours/days, then plug it in, i can get the video to display -- but it only works for several minutes. If I press power and turn it on again, i'm back to the same broken state.

What I've done:

• Checked the supply voltages from the SMPS - looks nominal compared to the silkscreen voltages. I'll document them in this thread when i get back to it.
• I checked the fuse on the T-Con. It's good.
• Replaced the T-CON from known working and exact match from set with a broken screen. No change.
• Replaced the main board and LVDS cable - from the set with a broken screen. Also, no change in behavior.
• I disconnect one LVDS cable at a time - left only, and right only. No change. Cables were reseated as well.
• Measured the impedance of the caps on the panel side PCB (past the T-CON LVDS cables) - no shorts or low impedance paths found. Everything was several kOhm or higher.

Also, 1/8th of the screen is slightly a different shade of "black" when it initially powers on, but this evens out and the screen is uniform in look after a few seconds. Not sure if this is significant. I checked the panel caps in this region - no corrosion, no shorts. Please see the attached picture.

Sounds like a bad panel to me... but i'd like your opinion.

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I'll measure the voltages across the caps on the tab bond PCB after work today. Maybe there is a bad cap that is dragging the voltage down.

[CapLeaker]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I'd be looking at the LVDS pairs with an oscilloscope, while flicking the menu on and off. That tells you the main board is o.k. Check for proper voltages on the tcon. On your TV there after the Tcon another board that has tabs to the panel. Either that or a bad panel.

[dick_barton]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

What dc voltage do you have at the tcon fuse?

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

What dc voltage do you have at the tcon fuse?

I see 12.1V on both sides of the T-CON fuse.

I'd be looking at the LVDS pairs with an oscilloscope, while flicking the menu on and off. That tells you the main board is o.k. Check for proper voltages on the tcon. On your TV there after the Tcon another board that has tabs to the panel. Either that or a bad panel.

I'll measure the voltages on all of the VR outputs today and get you a list and annotate a photo. I have two T-CONs. I'll give you an A-B result as well in case i see a difference. I'll do the same for the tab boards past the T-CON.

I only have a 100MHz scope. What's the frequency of the LVDS lines?

[budwich]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I would "normally" expect that when you disconnect one tcon to panel cable, that the corresponding half of the screen shows some form of white IF there is backlights.

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I would "normally" expect that when you disconnect one tcon to panel cable, that the corresponding half of the screen shows some form of white IF there is backlights.

Hi budwich - it does. I re-read my original post. I'm sorry i was unclear on it. - What i meant to say is that when i disconnect one LVDS cable at a time, the corresponding side goes white. I swapped T-CONs that I have from a broken screen TV (exact match) and repeated the same test. No difference in TV behavior there.

Measurements are in progress. I'm getting all the test points plus the main rails from the VRs on the T-CON.

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

T-Con voltages are below. Measured with the TV on. Please reference the image for locations. Generally taken from upper left corner working down, then towards the right bottom corner.

Does anyone have the TCL panel LVU550ND1L spec? Measuring the panel voltages is kinda in the dark without the spec sheet. I can only assume the GMxx voltages would be in the same order and similar to the values found on the T-CON.

R_VAA	15.26
L_VAA	15.26
GM22	0.208
GM21	0.468
GM19	2.5
GM17	3.54
GM15	4.49
GM13	5.75
GM12	7.01
GM11	7.42
GM10	8.9
GM8	10.33
GM6	10.76
GM4	12.35
GM2	14.27
GM1	14.61
VREF	14.62
VCM_CB	5.86
VCM_XB	5.86
VIN12	12.1
STV	0.006
STVR	0.006
OE	3.27
SQINV	2.995
CKV	0.006
GVON	3.28
T_RST	3.255
RST	3.255
TEST	0.005
PCK	0.005
BK_SEL	0.005
TP1	0.005
VDD1V8	1.821
VDD1V3	1.377
VDD33	3.29
VDD_IO_33	3.285
HVAA	7.31
VGL	(-5.508)
VGH	33.25
VGHF	33.25

While i was at it, here's the SMPS output. Format is:

Pin #, Signal name, value when off, value when on.

1	P_DIM	0.4	1.992
2	BL_ON	0	3.314
3	GND	0.06	0.06
4	P_ON	2.578	2.575
5	NC	0.02	0.02
6	A_DIM	0	3.41
7	GND	0.07	0.07
8	GND	0.07	0.07
9	12V	12.17	12.16
10	12V	12.17	12.16
11	12V	12.17	12.16
12	12V	12.17	12.16
13	12V	12.17	12.16
14	N/C	0	0

Interestingly enough - this TV appears to have 12V on all the time. The only difference between "off" and "on" is the P_DM and BL_ON signals for that "instant on" feature.

[CapLeaker]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I see 12.1V on both sides of the T-CON fuse.



I'll measure the voltages on all of the VR outputs today and get you a list and annotate a photo. I have two T-CONs. I'll give you an A-B result as well in case i see a difference. I'll do the same for the tab boards past the T-CON.

I only have a 100MHz scope. What's the frequency of the LVDS lines?

100Mhz is fine... that's all I have...
Watch this: https://www.youtube.com/watch?v=WeYFzugvH4c

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

100Mhz is fine... that's all I have...
Watch this: https://www.youtube.com/watch?v=WeYFzugvH4c

Thanks for the link. I'll view the entire video then remove the bezel from the TV. I haven't done that yet due to the lack of space in the front room.

Also.. "watch this" is usually followed by "hold my beer"

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I've tried using both Main boards that i have (the original one, and one from an exact set with a broken screen) - No output is observed on any of the LVDS differential pairs on the main board side. Interesting. No video output even though i can hear the clicking sounds when i mash the buttons on the remote?

Additionally, i placed the TV face up on the table and removed the front bezel (poked at each of the bottom tabs and the tabs on the side. No change. All of the tabs and ICs on the side physically look okay. No signs of melted, burned, or water indication.

[budwich]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

Hi budwich - it does. I re-read my original post. I'm sorry i was unclear on it. - What i meant to say is that when i disconnect one LVDS cable at a time, the corresponding side goes white. I swapped T-CONs that I have from a broken screen TV (exact match) and repeated the same test. No difference in TV behavior there.

Measurements are in progress. I'm getting all the test points plus the main rails from the VRs on the T-CON.

well that's a bit of "difference". Your eye and the post's eye now see the same thing. :-)

OK. So your problem is ..... I don't know exactly... :-) BUT your measurements which you had promised and finally showed up provides some "light". My "guess" is the VGL values are not correct. You need to provide some sort of picture of how the tcon to panel cables connect... ie. what kind of termination boards are on the panel. Go From there.

[CapLeaker]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I've tried using both Main boards that i have (the original one, and one from an exact set with a broken screen) - No output is observed on any of the LVDS differential pairs on the main board side. Interesting. No video output even though i can hear the clicking sounds when i mash the buttons on the remote?

Additionally, i placed the TV face up on the table and removed the front bezel (poked at each of the bottom tabs and the tabs on the side. No change. All of the tabs and ICs on the side physically look okay. No signs of melted, burned, or water indication.

Maybe remove the panel from the tcon. Did you set up your o'scope correctly?

[budm]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

What is the T-CON IC 1 P/N?
What DCV do you have on D3 Cathode pin? you show 12V on the anode so the Cathode should be higher.
Do you get the same Voltage reading on the TC-ON if the cables between T-CON and the LCD panel not connected?
I cannot find AVDD test point on the T-CON, can you see it any where?

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

well that's a bit of "difference". Your eye and the post's eye now see the same thing. :-)

OK. So your problem is ..... I don't know exactly... :-) BUT your measurements which you had promised and finally showed up provides some "light". My "guess" is the VGL values are not correct. You need to provide some sort of picture of how the tcon to panel cables connect... ie. what kind of termination boards are on the panel. Go From there.

• Heh ... sorry... working from home, dealing with the house and kids.. time is scarce
• Please see the third and fourth pictures in Post #1. Is this the kind of photos you are looking for? I can provide higher resolution photos too...
• Yeah, the negative VGL value threw me too...i remeasured it; still (-5.508V) with or without the LCD connected to the T-CON.

Maybe remove the panel from the tcon. Did you set up your o'scope correctly?

• Scope was setup at 50mV/div, 1sec/div and I'm probing the LVDS pairs on the main board past the DC-blocking caps (towards the connector side. Scope waveform stayed at zero. I left it on auto scroll. Just to make sure the probe was working, i hooked it up to the probe comp on the scope - i get the proper 5V square wave.
  
• This doesn't appear to make a difference if i have the cables connected between the T-CON and panel.

What is the T-CON IC 1 P/N?
What DCV do you have on D3 Cathode pin? you show 12V on the anode so the Cathode should be higher.
Do you get the same Voltage reading on the TC-ON if the cables between T-CON and the LCD panel not connected?
I cannot find AVDD test point on the T-CON, can you see it any where?

• T-Con IC (IC1) markings are T09L (lot code) / 5567S. Some kind of PMIC regulator.
• The cathode of D3 on the right measures at 15.28V.
• New voltages with the T-CON to panel cables disconnected is shown below.
• I did not see a labeled silkscreen location for the AVDD testpoint on this T-Con either.

Formatting: TP name, first reading (with LCD connected to T-Con), second reading (LCD disconnected from T-CON)

R_VAA	15.26	15.27
L_VAA	15.26	15.27
GM22	0.208	0.206
GM21	0.468	0.391
GM19	2.5	2.519
GM17	3.54	3.511
GM15	4.49	4.518
GM13	5.75	5.799
GM12	7.01	7.01
GM11	7.42	7.42
GM10	8.9	8.83
GM8	10.33	10.34
GM6	10.76	10.76
GM4	12.35	12.34
GM2	14.27	14.34
GM1	14.61	14.61
VREF	14.62	14.61
VCM_CB	5.86	5.86
VCM_XB	5.86	5.86
VIN12	12.1	12.1
STV	0.006	0.005
STVR	0.006	0.005
OE	3.27	3.27
SQINV	2.995	3.281
CKV	0.006	0.003
GVON	3.28	3.281
T_RST	3.255	3.254
RST	3.255	3.254
TEST	0.005	0.005
PCK	0.005	0.003
BK_SEL	0.005	0.003
TP1	0.005	0.003
VDD1V8	1.821	1.818
VDD1V3	1.377	1.376
VDD33	3.29	3.291
VDD_IO_33	3.285	3.283
HVAA	7.31	7.3
VGL	(-5.508)	(-5.511)
VGH	33.25	33.25
VGHF	33.25	33.25

As always, thank you for the help

[budwich]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

The pictures of the tcon and bottom edge board do not show enough detail / labelling to be read. Further, with no power on the set, using continuity testing, trace / track where the vgl originates from / goes in "both directions"... ie. towards the bottom edge board or otherwise. It is somewhat strange that this value doesn't change with disconnection of the tcon to panel cables.

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

The pictures of the tcon and bottom edge board do not show enough detail / labelling to be read. Further, with no power on the set, using continuity testing, trace / track where the vgl originates from / goes in "both directions"... ie. towards the bottom edge board or otherwise. It is somewhat strange that this value doesn't change with disconnection of the tcon to panel cables.

Good morning budwich -


Please see the attached close up pictures. Lets see if this will work better. Let me know what else you'd like to see.

--> I'll trace VGL after my morning conference calls.

Pics 1-2 show the T-CON to tab boards.
Pics 3-5 work its way "left" from the current TV orientation
Pics 6-9 works it way "right" from the current TV orientation

[budwich]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

well that's a bit of "difference". Your eye and the post's eye now see the same thing. :-)

OK. So your problem is ..... I don't know exactly... :-) BUT your measurements which you had promised and finally showed up provides some "light". My "guess" is the VGL values are not correct. You need to provide some sort of picture of how the tcon to panel cables connect... ie. what kind of termination boards are on the panel. Go From there.

I guess the question about this is... was the screens uniform white and uniform dark... pictures? My initial guess relooking at the first pictures of the screen with the "darness" on the left side (1/8) is that side has issue. Hence, the question above about any differences in the "cable disconnect test".

Further with your pictures that you just posted, there are some VCM test points on the bottom edge boards... can you measure those and compare across boards?

[ngth82]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

I guess the question about this is... was the screens uniform white and uniform dark... pictures? My initial guess relooking at the first pictures of the screen with the "darness" on the left side (1/8) is that side has issue. Hence, the question above about any differences in the "cable disconnect test".

Further with your pictures that you just posted, there are some VCM test points on the bottom edge boards... can you measure those and compare across boards?

Hi Budwich -

Yikes, new photos of the LCD screen with the "cable disconnect test" attached. This was NOT like this before. The only thing i saw previously was a white screen with the respective sides disconnected and only that 1/8th banding on the side. Looks like things are getting worse!

Pictures of the TV with both cables connected and respective sides disconnected are attached. I attached pics with the TV result when it's intiially powered up, and what it looks like a few seconds later.

Also, the requested voltage readings are attached in the spreadsheet. Please refer to the third and fourth tabs of that Excel file. I did not measure the differential voltages on the panel tab board.

[budwich]

Re: TCL 55S405TBBA - backlights on, no video; T-con and main board replaced.

Yikes... you don't say. Hmmm we seem to have some lag in the "observations" and descriptions ... :-) Its not looking good. The fifth picture appear to show some form of "display" .... possibly some sort of menu or otherwise.... maybe even a "no input selected" type screen and resulting timeout of the display. Have you tried hitting a "menu" button on the remote when the set is in this "mode" to see if you get some sort of screen?

the third picture is kind of what is expected when no signal has made its way to the panel.... nice black on the "connected side" but the white should more uniform. Not sure what the fourth is but my guess is now some sort of signal made it way to the panel and is trying to display... the white side getting closer to expected.


The makeup / connections of the boards is somewhat different than other manufacturers. History of the set might help a bit. With two different tcon, its hard to believe they are both that bad... but maybe if the subtending panel is distress.
You might want to start measuring resistances in and around the edge boards and comparing across side (of course, with power off). You could try the "tape cutoff" method for blocking signals at the tcon connector going to the panel. It might be successful although if there are side tabs, that method usually does not work.

oops ... I see the main board picture now and also read where you did measure components on the edge board.