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

[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?

[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?

[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.

[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.

[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"

[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.

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.

[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.

[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.

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?

[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?

[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.

[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.

[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.