Toshiba 50" 50L3400U blinking backlight

[ngth82]

**I need to get to my desktop to upload a few pictures. Stay tuned **

Model: Toshiba 50L3400U
Manufacture date: 2014 Feb.
Original complaint: Original owner said the TV blinks after about 15 minutes of watching it. All HDMI inputs exhibit this behavior. Haven't had a chance to verify if this occurs on the TV ANT connection, or the Composite inputs yet.

Boards:
Power supply board: FSP107-3FS03 (PK101W0350I)
Main board:

Diagnostic Notes:

• No bulged caps found from visual inspection.
• Backlight completely shuts off and then turns back on (I confirmed it wasn't the video source since you can see the backlight turn off through the holes on the back of the panel)
• All the connectors and ribbon cables are seated; and re-seated to double check.
• The power supply board generates a steady 12V2, 5V1 to the main board.
• The PW_ON signal is a steady 3.3V
• BL_ON signal is a steady 3.3V from the main board. I'll grab my portable oscilloscope and verify it's not turning off for a small toff time.

Backlight voltage are 55V nominal according to the power supply silk screen.

• Vled1 measured = 55.8V
• Vled2 measured = 55.8V
• Vled3 measured = 55.8V
• Vled4 measured = 55.8V

I reset the TV to defaults and disabled the auto brightness function of the TV.
When the dynamic brightness control is enabled, the screen blinks more/less depending on the brightness of my room.

The blinking frequency increases as you manually adjust the TV brightness.

• At backlight=100%, it blinks about 3-4 times per second.
• Backing the backlight down to 75 makes it blink slower to ~1 time per second
• Backing it down to 60 seems to make the issue go away (at least in the last few minutes of observing the TV)

I may just leave the brightness at 60. It's plenty bright for this room, but does anyone have any suggestions?

Thank you,
Thomas

[ozzy214]

Re: Toshiba 50" 50L3400U blinking backlight

Sounds like you may have bad bulbs. They are just like flourscent bulbs as they age they draw more juice. Sending a error signal to psu to shut it down. Hence why reducing backlight brightness cures it.

I had a lg that was doing this and tried a new to me used inverter and tv has been working fine ever since. You can try inverter, but I doubt thats the issue. I dont have a manual, but since voltages are all the same on inverter, I would think bulbs.

They sell laptop lcd inverters on ebay universal. You can get one of those and rig it to each bulb to find the weak one. You may also see black bands left to right where the bulb is burnt out compared to others.

I had a vizio like that, bad inverters and no backlight. Bought a universal invrter kit off of ebay and got tv working only to find out 4 black bands where the bulbs died. I think the bad bulbs caused the inverter failure.

Luckily you can replace the bulbs if the case, if you feel comfortable stripping the tv to get at them.

[ngth82]

Re: Toshiba 50" 50L3400U blinking backlight

Ozzy214 - Thanks for the input. I'll keep this in mind. At least that is a direction that I can start with and that would make sense -- the PS is probably over current limiting as each of the four 55V rails are rated for 240mA.

I don't see any obvious black bands on the screen. The far left and far right edges have their standard light/dark pattern as the screen isn't evenly lit there (about the same as my other LCD TVs).

I'll look for more information on testing the panel LEDs then report back and assess the trade off of the brigntness setting at 60 vs full. If i have the time, i'll also look into hooking up my current probe for each of the rails and see what happens.

The person that I got the TV from just wanted it out of the house. So, I'm going to keep it... my application would be to use it as a large picture frame + Chrome Cast and would replace the last freebie TV above the fireplace.

Thank you,
Thomas

[ozzy214]

Re: Toshiba 50" 50L3400U blinking backlight

OO I didnt realize...its a LED tv? Then most definitely probably a failed led. My bad I was thinking older ccfl tv. Probally one or two is failing or dead causing the shut down.

[ngth82]

Re: Toshiba 50" 50L3400U blinking backlight

OO I didnt realize...its a LED tv? Then most definitely probably a failed led. My bad I was thinking older ccfl tv. Probally one or two is failing or dead causing the shut down.

Yeah, LED TV. It's going to be a total pain in the butt to strip the panel down that far. Still going to try and find the service manual and exploded diagram for it though. We'll see how adventurous I get with it since it "works" (!!!) right now. I'd be more inclined to rare it apart if the screen showed no sign of life

[ozzy214]

Re: Toshiba 50" 50L3400U blinking backlight

Its not that hard, just be very careful in the order of the sheets and dont break the black liquid crystal sheet. I also recommend rubber gloves to prevent dirt and oil from your fingers getting on screen. Also remeber the order of the sheets

[ReeceyBurger123]

Re: Toshiba 50" 50L3400U blinking backlight

Probably the led link connectors have broken down and corroded failing to make contact. Is this an Lg screen too ?

[ngth82]

Re: Toshiba 50" 50L3400U blinking backlight

Rah... My concern is cracking the panel while lifting it out. Done it before, but it's touch and go lol.


The panel vendor is InnoLux. The part number was obscured by the main board.

[ReeceyBurger123]

Re: Toshiba 50" 50L3400U blinking backlight

Probably a rebadged Lg screen then, also when taking it out fold the tab bond boards over the screen and use your nails to lift it slightly then grab the edges with your hand. Easy as anything as long as you're slow I have never cracked one

[ngth82]

Re: Toshiba 50" 50L3400U blinking backlight

Probably a rebadged Lg screen then, also when taking it out fold the tab bond boards over the screen and use your nails to lift it slightly then grab the edges with your hand. Easy as anything as long as you're slow I have never cracked one

Lol... good to know. I'll take on this project should the blinking comes back with the brightness set to 60 Thanks for the replies everyone.

I'll report back if i do anything to it. I hope this thread helps someone else as well!

[doh13]

Re: Toshiba 50" 50L3400U blinking backlight

I have this exact problem
Model: Toshiba 50L3400U
Manufacture date:June 2014
Tried updating all settings etc..
It blinks faster as i increase the backlight.
I read somewhere that cutting or unplugging a yellow wire from (pin 15?)would keep the board from sending the signal to shut down the screen. I am wondering if this might be a valid workaround fix.. of course the led bulbs would burn out eventually but i am wondering if disabling it from shutting off could cause things to melt or cause a fire? in anycase i cannot find a pin 15 but i do see 2 yellow wires on the power board going to the video board..

[doh13]

Re: Toshiba 50" 50L3400U blinking backlight

I just removed Pin 15 (DIM?) from the power board going to the video board the tv turns on and is working. I have maxed out the backlight to 100 and have not messed with any other settings to see what could be affected by unplugging the pin and i will observe it for failures or blinking and report back as of now 5 mins being turned on its working normally. PS i am not too concerned about frying the tv as long as it does not burn my house down in the process from removing the wire lol.. I could do all of the checking of the leds etc but it seems like alot of effort and im not sure if i want to get into not to mention im not sure where to get replacement led strips.

[dick_barton]

Re: Toshiba 50" 50L3400U blinking backlight

Have you just tried turning off DynLight in the menu before disconnecting Pin 15

[budm]

Re: Toshiba 50" 50L3400U blinking backlight

You may be able to dim down the backlights so the LEDs will not burn out due to running at maximum by using as resistor between GND and DIM pin on the LED driver section. You can start with 100K and go down from there or use 100K Pot.

[doh13]

Re: Toshiba 50" 50L3400U blinking backlight

Have you just tried turning off DynLight in the menu before disconnecting Pin 15

Yep i did that as i said in my other post i tried all settings that could be related to the problem & also updated to latest firmware with no joy..
I have noticed in the bottom right of the screen it looks to be darker when there is a white screen showing, this could be where the burnt LED or leds are located I am assuming that this is what is the actual problem with this TV I may have to order some new strips If i can figure out where to get them as this TV might be worth saving. I notice that this thread hasn't had any posts for awhile i am curious as to if the OP fixed his issue by replacing the bad led strips.

[doh13]

Re: Toshiba 50" 50L3400U blinking backlight

You may be able to dim down the backlights so the LEDs will not burn out due to running at maximum by using as resistor between GND and DIM pin on the LED driver section. You can start with 100K and go down from there or use 100K Pot.

Thanks for the suggestion budm .
From my research the burning out led strip problem seems to be a common issue for this Tv maybe the board is putting too much voltage to the leds or they are just cheap and burn out easily? I will keep the backlight settings lower and use it with pin 15 disconnected at least it doesn't seem to be blinking anymore and the picture seems pretty decent other than the spot i mentioned that is not really noticeable . I am not sure i want to invest money into this tv if putting a new strip in is pointless because it will burn out quickly i may try your suggestion budm if i decide to order new led strips..
Thanks.

[budm]

Re: Toshiba 50" 50L3400U blinking backlight

Too much current (constant current power source) is what causing early failure, they drive them at max or beyond the spec of the LED with poor thermal management.

[doh13]

Re: Toshiba 50" 50L3400U blinking backlight

Too much current (constant current power source) is what causing early failure, they drive them at max or beyond the spec of the LED with poor thermal management.

Thanks budm so what type/kind of 100K Pot would you recommend? could use a link if possible please?
Another question does turning down the backlight brightness in the tv menu do the same as adding the 100K Pot I am assuming it doesn't reduce the current or not enough? Also how will I know a safe limit to run the Leds at? i appreciate your help and this info will also be relevant to the OP so i don't feel as bad about hijacking his thread.
This youtube link seems to explain why the led strips burn out so fast in some tvs and verify's what your saying. https://youtu.be/O2R3ZfCsYW4?t=150

[budm]

Re: Toshiba 50" 50L3400U blinking backlight

Thanks budm so what type/kind of 100K Pot would you reccomend? could use a link if possible please?
Another question does turning down the backlight brightness in the tv menu do the same as adding the 100K Pot? also how will I know a safe limit to run the Leds at? i appreciate your help and this info will also be relevant to the OP so i don't feel as bad about hijacking his thread.

Since you disconnected the Dimming control line then the on screen backlights level will no longer have any effect.
Do you have any kind of resistors in stock, it may be easier than trying to explain how how to get the hook up the pot.

[budm]

Re: Toshiba 50" 50L3400U blinking backlight

BTW, what DCV do you have on the DIM pin for the LED DRIVER board?