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

[budwich]

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

Interesting.... it would appear that maybe there is some base level coms between the tcon and main board... but my "pay grade" at this point is way past.

The display patterns that you posted especially ones where there are "significant" difference from a "perfectly" even black / white screens might be screen that are attempting some sort of screen but the panel is in trouble. Further, since appears there is some sort of start up sequence with the main->tcon, it is possible that perhaps there is also some sort of similar "sequencing" between the tcon and panel / edge board and that is not complete.

Not sure how much further time you have, but if you get the panel to display again like in the previous posts (blotchy white / blacks), check some of the voltages ... vgl / vgh / vcom on both sides to see if you can spot anything. Again, you might try the tape cutoff method to see if the panel comes up.

still further, if you get a "half panel" result like previously (or otherwise), during your "playing", trying using your finger as a "touch probe" around some of the smd componets (eg caps) on the edge board. A few have found that the "added capacitance" may trigger things. This is no worry of body safety as the voltages are all small.

[ngth82]

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

For curiosity sake, i tried the original main board that i was seeing "instant on" with (as discussed in Post 27... it works with the start up blink now. I didn't change anything else, just swapped the boards.

After discovering this, i tried all four combinations of T-COns and main boards:

Original Main board, original T-CON -- start up blink, no video.
Original Main board, replacement T-Con -- start up blink, no video.
Replacement Main board, original T-Con -- start up blink, no video.
Replacement Main board, replacement T-Con -- start up blink, no video.

So maybe this wasn't a "married" Main-TCON issue? Just likely a display problem?

[ngth82]

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

From what I see on these pictures, that doesn’t look promising.

I'm kinda that that point ... BUT as long as you guys have ideas, i'm going to give it a try!

OK... so it would at least appear that the "smashed set good board" might actually be good. In terms of the blinks, the power button might actually be a "macro" which has a few ir / whatever commands "stacked" which the status led is pulsing to.

Did you try pressing other buttons on the remote once the "sequence" has completed? For instance, menu. Try this with both cables on the tcon and then again with one or the other.

If I understand the manual correctly, the main and tcon are "talking" so it is quite likely that if the tcon is in trouble (or what's behind the tcon), that the main gets hosed... or perhaps it reality, the main thinks the display is ok and its just a bad display.

Interesting... i thought TCONs just merely receive instructions from the Main board.

In any case, the Main and TCON from the smashed TV works when i salvaged them for parts - just that the display was messed up in the middle. Someone tossed a controller at it.

Back on this TV: After I waited the power up sequence (pulsing) to complete, i tried pushing buttons on the remote - no observable changes to the display when both LCD cables care connected, and one (then the other) cable is disconnected.

I looked at the main board differential pairs past the DC blocking caps with my scope - the lines don't toggle at all. Just like before.

[CapLeaker]

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

From what I see on these pictures, that doesn’t look promising.

[budwich]

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

OK... so it would at least appear that the "smashed set good board" might actually be good. In terms of the blinks, the power button might actually be a "macro" which has a few ir / whatever commands "stacked" which the status led is pulsing to.

Did you try pressing other buttons on the remote once the "sequence" has completed? For instance, menu. Try this with both cables on the tcon and then again with one or the other.

If I understand the manual correctly, the main and tcon are "talking" so it is quite likely that if the tcon is in trouble (or what's behind the tcon), that the main gets hosed... or perhaps it reality, the main thinks the display is ok and its just a bad display.

[ngth82]

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

OK... so the "onion" might have a lot of different skins... :-)
It would certainly appear that you have at least a startup problem. That problem could be causing an image problem IF the reason for the startup issue is somewhat related to image output from the main to the tcon.... although maybe also a panel issue.

Did you try your other main board now that you knwo what to look for on the status light?

On lots of sets, the main coms with the tcon and if things are not right, the main might hose up or just shut down... don't know what a ruku OS does.

With the mainboard from the smashed screen TV:

1. Unplugged unit
2. Plugged it in
3. The status light is "BRIGHT" - solid.
4. Four seconds later, the status light brightness cuts to about half. The unit doesn't do anything else at this point. I watched it for a good minute. The LED stays 'on' in this condition.
5. This time, the unit slow pulses/blinks until the start up completes. (Still no video on the screen). Then the LED stays off when the panel is "ON"

6. Pressing a button on the remote causes the status light "ON-OFF-ON-OFF-ON-OFF-ON-OFF" in very rapid succession. It makes the corresponding clicking sound when a menu item is selected or the volume level is changed. This must equate to the "dims on/of once" for a remote action.

[budwich]

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

OK... so the "onion" might have a lot of different skins... :-)
It would certainly appear that you have at least a startup problem. That problem could be causing an image problem IF the reason for the startup issue is somewhat related to image output from the main to the tcon.... although maybe also a panel issue.

Did you try your other main board now that you knwo what to look for on the status light?

On lots of sets, the main coms with the tcon and if things are not right, the main might hose up or just shut down... don't know what a ruku OS does.

[ngth82]

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

No, this would be the first time to go through the manual. lol. I downloaded it from TCL. Page 41 shows the status light combinations... This is what i observed:

1. Unplugged unit
2. Plugged it in
3. The status light is "BRIGHT" - solid.
4. Four seconds later, the status light brightness cuts to about half. The unit doesn't do anything else at this point. I watched it for a good minute. The LED stays 'on' in this condition.
5. I pressed the power button on the remote and took video of it in slow-motion. The LED state change is:
"ON-OFF-ON-OFF-ON-OFF-ON-OFF" in very rapid succession (less than a second).

According to the manual, the LED is supposed to "dim on/off once" when it receives a remote command.

There is no "slow pulsing until start up completes" when i start it from the "off" (unplugged to plugged in) state.

[budwich]

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

OK... we are still backing up. First, does the status blink with each remote button press?
Next... we are going to watch the status light closer... depower the set (unplug). Once it has settled, plug it back in... what's happening with the status light?

Have you gone thru the user manual to look at the startup sequences / stati?

[ngth82]

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

Yes sir, I have the remote for this TV. The LED on the front acknowledges input from the remote. The status light is on

[budwich]

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

Let's back up for a moment... do you have a remote for this set? If so, does the set acknowledge the remote withe button presses flashing the status light? Is the status light actually on?

[ngth82]

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

No movement yet. I don't plan to unless there are no other alternatives.

I picked up this TV from a Craigslist post three weeks ago. The original owner said it stopped working one day. When i got it, it fired up fine the first time. Then a few minutes later the LCD no longer displayed anything even though the backlight was on. Turning the unit off/on or power cycling by unplugging it made no difference.

Leaving the TV off for a while seemed to have better odds of getting it to work again - i got the unit to boot properly three times, but it would ultimately go back to the blank screen state after a few minutes. It hasn't worked properly since.

Original thought was the TCON was bad. I swapped it from an exact unit with a broken screen. No difference in behavior. Same with swapping main boards from that same smashed screen TV. In both cases - no video output differences, but i can still "hear" the clicks when the menu selection changes.

I'll give it a go and see what happens with the tape method on the VGL/VGH (right) side of the T-CON cable.

How many signals am i really targeting to tape off? Control signals only, or differential pairs as well?

[budwich]

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

DON'T go too fast... removing side tabs IS the last resort. History of the set might be important as to how did it get to this "state"?

Your potentially best "no destructive" alternative is the tape cutoff method. You move a small bit of tape around blocking signals on one tcon cable. Usually, the area is near one side like where the VGL VGH powering is done. Is experimental and I am not sure if anyone has ever tried it on a newer tv.

[ngth82]

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

:-P This TCL is a ROKU TV - when the unit turns on, it's supposed to show a "desktop" upon power up with the different streaming services.

There are side tabs on this TV - four on each side. With that in mind.. What's the liklihood the LCD is just bad and no longer recoverable? Would cutting the side tabs be a doable "last resort"?

Updates to follow this afternoon.

[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.

[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.

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.

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.

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.

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