Sony Bravia KDL-40EX720 no backlight

[olegau]

It seems to me all work except the backlight.
The red standby and the green power on lights are stable, no flashing.

I try to identify which board or the screen is faulty.

Connections on LD board:
CN1001 (4pins)+122V each
CN1000 (5pins) four pins+28V each

What confuse me is CN1001 voltages. They suppose to be negative but it is +122V each.
I think the problem is somewhere around here.

I appreciate any opinion.

[dick_barton]

Re: Sony Bravia KDL-40EX720 no backlight

There are no negative voltages. The "+Out" indicates the power to the leds and the "-Out" is the return connection from the leds.

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

Thanks for the clarification.

So it looks LD board and screen leds are okey.

Who knows how to identify a faulty board?

[dick_barton]

Re: Sony Bravia KDL-40EX720 no backlight

Unfortunately the power supply schematics within the above pdf are not readable which would have been of help.

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

I checked supply 12V on conections CN6151 and CN6801. All of them 12.5V.
Power on/off 3V/0V works.

[Biruslapio]

Re: Sony Bravia KDL-40EX720 no backlight

Hi,i had a 40EX725 but i broke its display,i still have all the boards except signal board,this one that you're having problems with is the exact same,and it was working when i pulled out,if you need measurements from it i can do it and you compare to see what is faulty,still have the LEDs and power supply also,but i don't know how to hotwire the PSU for it to supply voltage to this board.
edit:you should also check the LED stripes inside the panel,it is not rare for them to burn out for this model.

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

It would be great but a lot of hassle. Maybe take measurements only the power board without the load. But it may not work because the PSU will not get feedback from other boards.

[budm]

Re: Sony Bravia KDL-40EX720 no backlight

Can we see the pictures of the whole back side of the TV showing all the boards?
What DCV do you have on
CN1002:
LD_ERR
LED_ON
GL_CTRL

CN1003:
1-PWM1
2-PWM8
3-PWM1
4-PWM8

Also leave the probe on one of the (+OUT) and record what you read when the TV is OFF and when the TV is turned on to see if it goes up to more than 122v and then drops down to 122V or less.

[budm]

Re: Sony Bravia KDL-40EX720 no backlight

Is this your power supply board?
Sony 1-474-300-11 (APS-293, APS-293(CH)) G6 Power Supply.

Pictures provided by shojimmy.

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

Yes, that is my power supply board.

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

Pics of the TV:
General view
Connections of PCU
Connections of T-CON

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

Today I was following an advice to disconnect one after another the T-CON two flat ribbons going down to the screen. Nothing happened.

[Biruslapio]

Re: Sony Bravia KDL-40EX720 no backlight

This model generaly fails the main board,LEDs,display(crap quality) or ribbon cables between the T-CON and display,the power supply and inverter sells for nothing because they are reliable,maybe the T-CON fuse is blown.
Do you have sound AND picture?Aiming a flashlight at the display works,but you need to put back the Display<>T-CON flat cables.

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

This model generaly fails the main board,LEDs,display(crap quality) or ribbon cables between the T-CON and display,the power supply and inverter sells for nothing because they are reliable,maybe the T-CON fuse is blown.
Do you have sound AND picture?Aiming a flashlight at the display works,but you need to put back the Display<>T-CON flat cables.

There is sound but I cannot say about the picture because the screen is black.

[budm]

Re: Sony Bravia KDL-40EX720 no backlight

Read Post #8.

[olegau]

Re: Sony Bravia KDL-40EX720 no backlight

Measurements:

CN1002: off/on

LD_ERR 0/0V something moving around 0 Volts
LED_ON 0.1/3.3V
GL_CTRL 0.7/0.0V

CN1003: off/on

1-PWM1 0/3V goes up slowly from 0V to 3.0V
2-PWM8 0/3V goes up slowly
3-PWM1 0/3V goes up slowly
4-PWM8 0/3V goes up slowly

"Also leave the probe on one of the (+OUT) and record what you read when the TV is OFF and when the TV is turned on to see if it goes up to more than 122v and then drops down to 122V or less".
All measurements of connection SN1001 show quick jump voltage from approximately 0V to 140V and then quick down to constant 122V.

[budm]

Re: Sony Bravia KDL-40EX720 no backlight

Measurements:

CN1002: off/on

LD_ERR 0/0V something moving around 0 Volts
LED_ON 0.1/3.3V
GL_CTRL 0.7/0.0V

CN1003: off/on

1-PWM1 0/3V goes up slowly from 0V to 3.0V
2-PWM8 0/3V goes up slowly
3-PWM1 0/3V goes up slowly
4-PWM8 0/3V goes up slowly

"Also leave the probe on one of the (+OUT) and record what you read when the TV is OFF and when the TV is turned on to see if it goes up to more than 122v and then drops down to 122V or less".
All measurements of connection SN1001 show quick jump voltage from approximately 0V to 140V and then quick down to constant 122V.

The booster circuit is working, I need to look at the LED driver board circuit in more details to give you the test points.