toshiba smart tv , only backlight , no picture

[layad]

hi,
as soon as i plug the power cable to the main , the screen comes on with bright screen(backlight) but no picture , no sound, and it stuck in stand by mode, would not responding to the remote control or the normal buttons on the back of the tv, i suspected the main board , i searched online i couldn't find any? is there anyway to diagnose the main board?
thanks in advanced

[dick_barton]

Re: toshiba smart tv , only backlight , no picture

Can give the pin legends and what voltage is measured at each pin?

[layad]

Re: toshiba smart tv , only backlight , no picture

Can give the pin legends and what voltage is measured at each pin?

thank you for your reply , ill check them when i go back home today after work

[CapLeaker]

Re: toshiba smart tv , only backlight , no picture

We need to have the exact model number of your TV. I suspect the Tcon board. Turn the TV on and check the fuse on both sides on the Tcon. If the Tcon is directly over or under the LCD directly tab bonded on, you can't replace the Tcon and the TV is more or less junk. I know you reported no sound.
Also could be the EEPROM on the main board. In this case freeze it with a can of air upside down and turn the TV on.
Measure voltages with as requested in post #2 with the TV on and off.
I think that is enough to chew on for now.

[layad]

Re: toshiba smart tv , only backlight , no picture

We need to have the exact model number of your TV. I suspect the Tcon board. Turn the TV on and check the fuse on both sides on the Tcon. If the Tcon is directly over or under the LCD directly tab bonded on, you can't replace the Tcon and the TV is more or less junk. I know you reported no sound.
Also could be the EEPROM on the main board. In this case freeze it with a can of air upside down and turn the TV on.
Measure voltages with as requested in post #2 with the TV on and off.
I think that is enough to chew on for now.

Hi, thank you. The tv is Toshiba 50l7355d I think . I'm not at home yet, I think the t-con is a separate board I don't know why you think I can not replace it!, is it possible that the t-con can freeze the tv an stop it from coming on ? Plus the tv powers on straight away with backlight only without touching any button. I'll check all the points have been asked to do . May you explain to me that part (In this case freeze it with a can of air upside down and turn the TV on) please. Thank you again

[CapLeaker]

Re: toshiba smart tv , only backlight , no picture

Some Tcon's are not made to replace, as they are tab bonded on to the panel. In this case, things do not look good at all.
You can check the LVDS with a 100Mhz oscilloscope and turn the menu on and off all the time... this way you can verify you got signal coming from your main board.
Bad Tcon = No picture but sound. ( Since you can't see anything I am not sure if you are on the correct input or not). So more investigations needed there.
The EEPROM on the main board can inhibit the TV to boot correctly. This case, no picture no sound, TV turns off again and other weird things. I have a 70% success rate by freezing the EEPROM to make the main board boot correctly. If there is a bad EEPROM, you need either a new main board, or replace the EEPROM with a programmed new one. So either you program the new EEPROM yourself or buy an already programmed one.
It also would be good to see the voltages on the PSU (bottom left connector) while the TV is on. Post pin out name with voltages measured.

So all in a nutshell right now we have to figure out which board is causing the problem.

[layad]

Re: toshiba smart tv , only backlight , no picture

Some Tcon's are not made to replace, as they are tab bonded on to the panel. In this case, things do not look good at all.
You can check the LVDS with a 100Mhz oscilloscope and turn the menu on and off all the time... this way you can verify you got signal coming from your main board.
Bad Tcon = No picture but sound. ( Since you can't see anything I am not sure if you are on the correct input or not). So more investigations needed there.
The EEPROM on the main board can inhibit the TV to boot correctly. This case, no picture no sound, TV turns off again and other weird things. I have a 70% success rate by freezing the EEPROM to make the main board boot correctly. If there is a bad EEPROM, you need either a new main board, or replace the EEPROM with a programmed new one. So either you program the new EEPROM yourself or buy an already programmed one.
It also would be good to see the voltages on the PSU (bottom left connector) while the TV is on. Post pin out name with voltages measured.

So all in a nutshell right now we have to figure out which board is causing the problem.

Unfortunately I don't have oscilloscope.by the way the tv stock on the stand by mode. Red light on and stays on.the red light is not not changing by Pressing on any button on the remote control or on the back of the tv . I don't know if that can make you ignore the t-con, may you explain to me how to freeze the EEPROM with extra details?

[fredaelsadany]

Re: toshiba smart tv , only backlight , no picture

hallo
i think your tv need upgrade or software .
before this check LDO voltages in the main board .
led stuck on red means processor not start (software) or bad voltages (not correct) in MB also possible stuck led and R/C , press bottun .
try voltages first .
thanks

[layad]

Re: toshiba smart tv , only backlight , no picture

hi again
pin 1: 12.15
2: 12.15
3: 12.15
4: 0
5: 0
6: 12.15
7: 0
8: 0
9: 0
10: 5
11: 0
12: 3.28
13: 3.15
14: 3.33
15: 0
16: 3.26

voltage on both sides of the fuse( on the t-con board) is 12.9

[layad]

Re: toshiba smart tv , only backlight , no picture

hallo
i think your tv need upgrade or software .
before this check LDO voltages in the main board .
led stuck on red means processor not start (software) or bad voltages (not correct) in MB also possible stuck led and R/C , press bottun .
try voltages first .
thanks

hi
thank you for sharing , for me its easy sometimes to diagnose the psb , but really my knowledge about the main board is limited? may you till me how to how to find the ldo regulator pin out on the main board and how to test the output please, thanks

[dick_barton]

Re: toshiba smart tv , only backlight , no picture

Not good news by the looks of it.

All the voltages you give appear to be what is expected. The 12V supply is on both sides of the Tcon fuse. The backlights work.

Looks like an EEprom problem or main board processor fault.

Hopefully someone else can give you better news.

Edit
Just taking another look at your reading. Should Pins 1,2,3 be 12.5V and Pin 6 be 12V

[layad]

Re: toshiba smart tv , only backlight , no picture

yes thats right , it might be the multitester i use is not good enough to give the right votage!!! or?

[CapLeaker]

Re: toshiba smart tv , only backlight , no picture

press on that black heat sink on the main board a little and turn on the TV. see what happens...

PSU voltages are correct. How did you get these? Is the PSU turning fully on just by plugging it in, or did you turn it on with the remote / button on front?

[layad]

Re: toshiba smart tv , only backlight , no picture

press on that black heat sink on the main board a little and turn on the TV. see what happens...

PSU voltages are correct. How did you get these? Is the PSU turning fully on just by plugging it in, or did you turn it on with the remote / button on front?

Thanks I'll do it today night after work, when plug to the main without touching the remote control or any button on the tv start blank for a 5-10 seconds , in the same moment(within the 5-10 seconds) the led infront the tv flash one or two times with green light then the screen goes bright with no picture and the green led goes red . So I read it the pins after that

[CapLeaker]

Re: toshiba smart tv , only backlight , no picture

yeah... main board problem...

[layad]

Re: toshiba smart tv , only backlight , no picture

pressed on that black heat sink on the main board still the same, thanks anyway

[layad]

Re: toshiba smart tv , only backlight , no picture

Just a silly question, what ever tv I have and I want to replace the main board, I must replace it with the same tv model main board number or the same main board number? For example on ebay I can find the same main board number looks exactly the same but the seller said been removed from different tv model which is smaller

[dick_barton]

Re: toshiba smart tv , only backlight , no picture

Just a silly question, what ever tv I have and I want to replace the main board, I must replace it with the same tv model main board number or the same main board number? For example on ebay I can find the same main board number looks exactly the same but the seller said been removed from different tv model which is smaller

From reading the Forum it appears that it is important to match the main boards. One main reason is that they may be set up for different screen sizes or even for a different LCD panel or panel maker.

[vinceroger69]

Re: toshiba smart tv , only backlight , no picture

maybe see if your lcd panel number matches the replacement it may work if it does. On the metal tv chassis theres usually a white sticker with the panel number printed on it.

[ReeceyBurger123]

Re: toshiba smart tv , only backlight , no picture

You need to match the board with the EXACT panel used in the Tv. These boards have the firmware written to the panel used in the set. So you cant just use any old one you need one for the exact screen.