Toshiba 40L3433DG backlight OK but no picture

[Pierre95]

Hello,

I got a faulty Toshiba 40L3433DG TV. When turned ON, no logo is displayed. There is no way to enter any setup menu. A permanent dark picture is displayed as if no signal was even found.

I tried connectnig a PC through the VGA socket. The TV name is correctly retrieved on PC side, but again there is no picture. My first thought was a faulty firmware, but as the keyboard works correctly and the TV turns off after a few minutes, the main and only CPU seems to be running correctly.

This seems to be a quite uncommon issue, did someone experience that?

[nomoresonys]

Re: Toshiba 40L3433DG backlight OK but no picture

Test connector from mainboard to powerboard on powerboard side with all boards hooked up, test with tv plugged in but off then hit power, report both conditions here, give pin number, pin description/name and condition both off and when you hit power on. So we can see all the voltages and what they are doing. example in second link below if needed.

[Pierre95]

Re: Toshiba 40L3433DG backlight OK but no picture

OK, I will do that and let you know. Should I also test 17MB95's locally generated voltage from U19 (TPS65251)??

[nomoresonys]

Re: Toshiba 40L3433DG backlight OK but no picture

Couldn't hurt I guess, testing those powerboard connector pins is usually a good place to start, when done properly lets us know if mainboard and powerboard are working and communicating correctly.

[Pierre95]

Re: Toshiba 40L3433DG backlight OK but no picture

I also noted there is no change in picture apparence when the main PCB is disconnected from the TCON which most likely means the mainboard doesn't produce any video signal at all.

[Diah]

Re: Toshiba 40L3433DG backlight OK but no picture

Flash MBOOT.bin ONlY for U10 IC

[budm]

Re: Toshiba 40L3433DG backlight OK but no picture

What Voltage testing have you done? Is 12 VDC present on both ends of the SMD fuse on T-CON?
You also need to upload the pictures of the whole back side of the TV showing all the boards so we can see what we are dealing with.

[R_J]

Re: Toshiba 40L3433DG backlight OK but no picture

Use your scope and check on the lvds cable to see if there is any video signal, you may need to press the menu to have something show up. If you have video signals going to the T-con, your main board is likely ok

[Pierre95]

Re: Toshiba 40L3433DG backlight OK but no picture

Here are the measurements of voltages on the 2*6pins PSU connector:

TV in standby mode (red LED):

12,11V- -12,11V
5,42V- -12,11V
0V - -GND
0V- -GND
2,24V- -GND
0,22V- -GND

TV ON (green LED):

12,11V- -12,11V
5,38V- -12,11V
0,28V- -GND
3,30V- -GND
0V- -GND
23,4V- -GND

White fuse on the TCON is OK (12V on both sides)

I also took measurement for several mainboard voltages (1,8V, 2,5V, 5V_VCC), all are OK.

OK R_J but I can only manage a 100MHz bandwidth scope which may be too low for 1920*1080 mode signaling being far beyond 100MHz. Is there any lower frequency signal within the LVDS I may check?

[R_J]

Re: Toshiba 40L3433DG backlight OK but no picture

The scope will be fine to check the signal, even a lower frequency scope will work. It won't be that much different from a regular video signal, and if it is there (should be on multiple lines) you will see it change when you press menu or change the video signal.

[nomoresonys]

Re: Toshiba 40L3433DG backlight OK but no picture

.

[nomoresonys]

Re: Toshiba 40L3433DG backlight OK but no picture

What voltage do you get on the main filter caps, in standby and when you hit power on?

[Pierre95]

Re: Toshiba 40L3433DG backlight OK but no picture

I took measurements directly on the TCON and for each pair, signals are as shown on the attached pictures which are for the TX_1_0_N/TX_1_0_P pair but other pairs show similar signals, except for the 2 clock pairs which are with no intervals (see 2 other pictures).

And as you suggested R_J, when I depress a menu button I see changes in the displayed signals which most likely means changes in the displayed picture.

The clock signals are lower in amplitude as compared to the data signals.

So the mainboard is likely to be ok and the issue on the TCON.

Shall I take picture of the TCON? What more may I check on the TCON?

I also had reading of the voltage on the main's filtering caps: 333V in standby, and 330V when ON (main is 220V AC).

[Diah]

Re: Toshiba 40L3433DG backlight OK but no picture

.

[budm]

Re: Toshiba 40L3433DG backlight OK but no picture

The LCD panel has integrated T-CON, so you already get to the T-con board which on the bottom edge of the LCD panel assembly?
BTW, are backlights on?

[Pierre95]

Re: Toshiba 40L3433DG backlight OK but no picture

Yes backlight is ON and yes I've got to the T-CON board which has an unusually wide shape (see picture).

[budm]

Re: Toshiba 40L3433DG backlight OK but no picture

https://www.badcaps.net/forum/attach...5&d=1565287394
I cannot see all the Voltage test points, I see VCC_1.8V, VCC_1.???, etc.
Start looking for VDD, VGH, VGL, etc.
I can see the LCD Bias switching power supplies the right side where the two inductors with 220 printed on top, you can also power supplies on the left side also where the two inductors with K printed on top. You should also look for the spec sheets of the the two IC's on those two locations.

[Pierre95]

Re: Toshiba 40L3433DG backlight OK but no picture

As you suggested budm, I took several measurements on the TCON when TV is ON. I read:

HVIN_12V: 0,17V
AVDD: 0V
VCC_1.8V=0V
VCC_1.15V=0V
VCC_3.3V=0V

Only VIN_12V (provided from the mainboard) is correct. The IC located between the coils labelled 220 is RT6917GQW from Richtek but I cannot find any spec sheet for it. Even Richtek website doesn't provide any reference to it. I only managed to find RT6908 datasheet.

If those voltages according to the labels near the test points have incorrect values, something wrong is to be found regarding how those voltage are actually generated.

As I don't have schematics for the TCON, I don't know how HVIN_12V is derivated from VIN_12V...

[budm]

Re: Toshiba 40L3433DG backlight OK but no picture

HVIN_12V: 0,17V = Bad
AVDD: 0V = Bad
You cannot find VDD, VGH, VGL test points?
Check the resistance between GND and AVDD, same for the HVIN_12V to see if they show shorted circuit or not.
Bottom line: The problem is in that T-CON board.

[Pierre95]

Re: Toshiba 40L3433DG backlight OK but no picture

I don't have VDD, VGH, VGL or maybe they use different naming. What do VDD, VGH, VGL stand for? Maybe I can find some equivalency with different naming.

I have VON, L_L, L_H but they are close to the LCD bias IC so most likely not involved with RT6917GQW.

I read 74 Ohms between GND and AVDD (independently of multimeter's connection direction) and 11,24 KOhms between GND and HVIN_12V for one direction and 14 KOhms for the other direction.