TCL 40S325-CA backlights, sound, no picture

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Hello, another TV found on the sidewalk here.

I have a TCL 40S325-CA ROKU LED TV. The backlights turn on, I can hear the typical Roku click sound when pressing the power button (no remote unfortunately) but nothing is displayed on the screen.

Power supply:

Main board goes into standby, so no reason to suspect anything here. Backlights turn on, etc..

Driver boards:

I have 12V going on both sides of the fuse on the panel driver boards.
VGH and VGL are around 1V when powered on.
No shorts to GND on any of the test pads or any of the caps on the driver boards.

Main board:

See pictures - a number of the caps seem shorted, however they're about 90Ohms to GND. In diode mode, red probe on GND, black probe on non-GND side of the caps then I get about 0.05V.

With the TV turned on, there's about 1V on the caps.

My FLIR doesn't show any excessive hot spots on the board except for the main processor.

Thoughts? Main board toast?

[dskall]

Re: TCL 40S325-CA backlights, sound, no picture

Vgh and Vgl are both bad.

[nomoresonys]

Re: TCL 40S325-CA backlights, sound, no picture

^^^ I agree, very bad.

[lotas]

Re: TCL 40S325-CA backlights, sound, no picture

90 Ohm is a normal resistance, it can be even lower, and 1 volt is the power supply of the processor core (there is always low resistance on this bus).

[nomoresonys]

Re: TCL 40S325-CA backlights, sound, no picture

vgh should be 20-30, vgl -5 to -12.

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Re: TCL 40S325-CA backlights, sound, no picture

Ugh ok, I'll track down why VGH and VGL are gone.

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Re: TCL 40S325-CA backlights, sound, no picture

90 Ohm is a normal resistance, it can be even lower, and 1 volt is the power supply of the processor core (there is always low resistance on this bus).

This is very good information. Thank you!

[dskall]

Re: TCL 40S325-CA backlights, sound, no picture

Might try unhooking one side of panel board and see if voltages come up. Some good pictures of panel boards would help.

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Re: TCL 40S325-CA backlights, sound, no picture

Okay, I did a bit of troubleshooting. Of the two FPC cables that join the two driver boards, I found where VGH/VGL are and disconnected that ribbon cable.

I turned on the TV but VGH/VGL is still missing, so it's not the secondary (left-side) driver board.

Since this is a trash TV, I had nothing to lose so I found where VGH was going to to the COF flex on right-side driver board and cut the trace. I turned on the TV and got VGH=23V and VGL=-7.98V. A bit of success!

But still no picture.

CKV=1.6V
STV=0V

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Re: TCL 40S325-CA backlights, sound, no picture

Attached are some more pictures of the panel driver board that I've been poking at.

Apologies, but the last two pictures seem to have been uploaded and rotated 180!

[nomoresonys]

Re: TCL 40S325-CA backlights, sound, no picture

hmm, well if it's a shorted panel, maybe the tape cu-off method will work?

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Re: TCL 40S325-CA backlights, sound, no picture

hmm, well if it's a shorted panel, maybe the tape cu-off method will work?

I don't think it will... there's no dedicated TCON board. The signals seem to be passed-through one driver board to the other.

I think the tape cut off method _might_ work if the problem were on the panel driver board that's daisy chained off the main panel driver board (see first set of pics). However, the VGH trace that I severed is on the "main" panel driver board that has the LVDS connector going to it from the mainboard.

[dskall]

Re: TCL 40S325-CA backlights, sound, no picture

Probably need to inspect panel traces for short. And look at side cofs. If panel short you can jumper around it

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Re: TCL 40S325-CA backlights, sound, no picture

Probably need to inspect panel traces for short. And look at side cofs. If panel short you can jumper around it

Any YouTube videos, or previous posts that I can use to help educate myself about doing this?

[nomoresonys]

Re: TCL 40S325-CA backlights, sound, no picture

there's one of many videos, you might even find one in English but it might take some looking, why youtube doesn't have a choice to search for a preferred language is a mystery: https://www.youtube.com/watch?v=wQA02AvViRE

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Re: TCL 40S325-CA backlights, sound, no picture

Okay, I jumpered VGH to the side COF and got picture. Unfortunately huge horizontal lines are running across the screen. Spent enough time on this one, so I’ll scrap it for parts and take the rest to the dump!

RIP!