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

[ngth82]

:-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]

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]

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]

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]

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]

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]

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]

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]

Originally Posted by budwich

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]

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.

[CapLeaker]

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

[ngth82]

Originally Posted by CapLeaker

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!

Originally Posted by budwich

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.

[ngth82]

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?

[budwich]

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.

[budwich]

One other thing.... now that you know about the status light. There is also a power down sequence. Do you see this happening?
This is still trying to isolate where the problems lie, although the panel is not looking so good at this point.

[ngth82]

Originally Posted by budwich

One other thing.... now that you know about the status light. There is also a power down sequence. Do you see this happening?
This is still trying to isolate where the problems lie, although the panel is not looking so good at this point.

I'll go for the low hanging fruit first: on power down - the TV just powers off the backlights. This allows for "immediate on" when the user presses the power button at a later point in time.

When I press the power button on the remote or front panel, the TV set makes the "power off sound", the status LED does that fast blink sequence as noted in prior posts. Then the backlight turns off. The BL_ON signal goes low. 12V is still present.

From this state, pressing the power button asserts BL_ON==HIGH again and the backlights turn back on.

I'll do the tape test on the LVDS cables and poke around at 4pm PST.. i have a "listen only" conference call that i can just put a headset on and do other stuff..

[ngth82]

Originally Posted by budwich

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.

I had a few minutes... i powered on the TV and the screen came right up (see first pic). I powered it down and disconnected the "left" side of the screen. The right side stayed up - left side was perfectly white. (pic 2).

I powered it down again and swapped the cables (left connected, right disconnected) - no dice; i reconnected it again - no dice. No video displays again.

While the full screen was "working" and actually displaying something, i ran my fingers across each tab -- pushed on it a little bit to see if i can get the image to change. Nothing changed. Did this twice.... no difference.

With the unit not displaying video again, i ran my fingers across the tabs once more - no difference. Slowly pushed on each one hoping something would happen. But nope.

I'll try the tape method today - soon, but what's the chance of success on this TV since it has side tab drivers?

[budwich]

what.... sorry for get this post... I didn't see the pictures and missed the part about seeing a picture... wow.... forget this ->
not sure I understand the "power down" statement... oh, you mean cord disconnected in what ever state the set was in (ie. if operating, then it kind of keeps that state so that if you plug back in all you need is to do a "power button" start as opposed to wait for the thing to initialize... right?

anyway, it does appear that the main board kind of thinks things are hunky dory and appears to power up and down, in terms of state lights, OK.... so perhaps things from the tcon out are a target.

I think is you track down the vgl (which I will take as voltage gate low), that value should be more in line with -12v (guess). It is being loaded down or the driver has an issue.

[budwich]

Originally Posted by ngth82

I had a few minutes... i powered on the TV and the screen came right up (see first pic). I powered it down and disconnected the "left" side of the screen. The right side stayed up - left side was perfectly white. (pic 2).

I powered it down again and swapped the cables (left connected, right disconnected) - no dice; i reconnected it again - no dice. No video displays again.

While the full screen was "working" and actually displaying something, i ran my fingers across each tab -- pushed on it a little bit to see if i can get the image to change. Nothing changed. Did this twice.... no difference.

With the unit not displaying video again, i ran my fingers across the tabs once more - no difference. Slowly pushed on each one hoping something would happen. But nope.

I'll try the tape method today - soon, but what's the chance of success on this TV since it has side tab drivers?

why the hell did you touch any cables ????? :-))))))
YOU HAD A PICTURE

[budwich]

IF you ever get back to the picture state again... leave it alone and check some of the voltages that you made note off way back... the ones dealing with VGL VGL VCM..