Toshiba LED TV, blank screen

**tibimakai** · 07-10-2015, 06:31 PM

Re: Toshiba LED TV, blank screen

Thanks Bud, for helping me.
I don't understand the resistor part, though.

**budm** · 07-10-2015, 06:53 PM

Re: Toshiba LED TV, blank screen

The analog dimming (not used by TOSHIBA, the TV uses the PWM dimming) has pull -up to full 5V for max backlights level, so I use the resistor (between analog dim and gnd) to drop the Analog dimming line down to about 3.5VDC or so. When PWM wire is disconnected, the ANALOG dim becomes active so you can vary the backlights by varying the ANALOG diming Voltage.
If the PWM line stuck low, then it will force of the backlights off. If you monitor the PWM line and see <1V then the PWM line is not working.
The one shown in the pictures, when you first turn ont he TV, the PWM line show around 4V (it is actually AC 200 Hz PWM, 0~5V pulse, 10% pulse width = min brightness, 90% pulse width = max brightness) which you will see the screen flashes on for second then the PWN drops down to 0V which then the baclikghts go out, in my case I have two problem, one is the PWM signal and anther one are bad LEDs strip that goes out if they driven full on.

**tibimakai** · 07-12-2015, 09:16 PM

Re: Toshiba LED TV, blank screen

I have just removed that yellow wire and the TV keeps working. I have left it over night and in the morning, it was still working.
I will keep monitoring for a couple of days, but to me it seems like it is repaired.
The right side(watching the TV), it's very hot though.

**budm** · 07-12-2015, 10:21 PM

Re: Toshiba LED TV, blank screen

"The right side(watching the TV), it's very hot though." That the LED's strips are getting hot, that is why I lower the backlights level to extend the life of the LEDs for awhile.

**tibimakai** · 07-12-2015, 10:40 PM

Re: Toshiba LED TV, blank screen

I have changed it to 75%, but it is still to hot.
I'm thinking, that it would be a good idea, to stick some heatsink on the metal surface?

**budm** · 07-12-2015, 10:54 PM

Re: Toshiba LED TV, blank screen

With that PWM_dim is disconnected, you no longer have control for the backlights dimming, it is on full blast, the user menu has no idea that the wire is not connected.

**tibimakai** · 07-12-2015, 11:16 PM

Re: Toshiba LED TV, blank screen

It seemed to me that when I was adjusting it, nothing changed on the screen, so you are right, that it runs on maximum brightness.
What size resistor did you use?
I'm a bit confused, on what did you do.
Did you remove the wire, or do you have it connected?

**budm** · 07-13-2015, 09:08 AM

Re: Toshiba LED TV, blank screen

Take the DCV reading on pin 13, then start by using 100K resistor between pin 13 and GND, take the reading, to see if the voltage does drop down. I do not remember the value of the resistor I use but I end up with around 3.8Vdc on that and it is still bright enough for me and the LEDs stay lit all day. You can use a pot too for adjustment, if the Voltage goes too low you will see the screen flash on for seconds then goes off.
Leave the wire off, see the wire I removed.

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**tibimakai** · 07-13-2015, 09:25 AM

Re: Toshiba LED TV, blank screen

The lead, from #15 has to be removed, when the resistor is soldered there?
This TV, you have kept it for yourself?
It can be sold, with this repair?
I would rather keep the Sony Google TV, after we repair it.

Yesterday, I have scored a Fluke 189 in very good condition from CL. Only meter, leads and a bag(not Fluke).
The only problem that I have found out after I have tried using it home, that the LEADS was showing up on all positions of the switch. At the purchase location, it was working fine(weird). I have tested the amp side when on switch was on ohm and showed 1.000, so it was accurate, no internal problems.
The problem it was, that it was "dirty" inside the meter's connectors.
At the amp connectors, was showing high resistance, in the mega ohms range.
Un-soldered(was not necessary) it and poured some 98% IPA in them and left it over night.
In the morning, the "short" it was completely gone. Re-soldered the connector and it was good to go.
Found this fix at eevblog.
I have just purchased, some better test leads from Prob Masters:
http://www.probemaster.com/product_i...roducts_id=207
It is so much better to use these on tiny components. The tip is much smaller, then the original Fluke one.

**budm** · 07-13-2015, 09:33 AM

Re: Toshiba LED TV, blank screen

No need to remove the wire from pin 15 on the LED driver board, I remove the wire at the power supply board as shown in picture #2. You already removed the PWM wire so no need to remove the pin 15 wire at the LED driver board, as you can see I leave it connected.
"This TV, you have kept it for yourself?" I gave it to my friend daughter to use in her bed room, she left it on 24/7 so far for two weeks now, I gave it to her so I can keep track to see what happen.

"It can be sold, with this repair?" You will have to tell the customer hat the backlights level cannot be adjusted any more.

**tibimakai** · 07-13-2015, 10:06 AM

Re: Toshiba LED TV, blank screen

We still don't know, what is the real fix right?

**budm** · 07-13-2015, 10:18 AM

Re: Toshiba LED TV, blank screen

It is the main board problem, when the TV is first turned on the PWM signal is present (>4VDC) for seconds (backlights flash on) and then it drop down to <0.5V. I never spend the time to troubleshoot the main board to locate the root cause of the problem.

**ReeceyBurger123** · 07-13-2015, 10:55 AM

Re: Toshiba LED TV, blank screen

Got to youtube and search for Norcal715 he has a video of a similar set its his latest upload which shows that the tv had a bad led inside of the panel wich was causing the issue that you have described.

**budm** · 07-13-2015, 10:59 AM

Re: Toshiba LED TV, blank screen

If it has BAD LEDs, then removing the PWM dimming control line will not make the BAD LEDs to come on and stay on, but bad LEDs are common failure for backlights LED TV.
The one I worked on has two problems, one is the PWM signal, and one of the LED strip goes out when hot (you can see the LEDs flicker) then after about an hour the circuit will shutdown when that LED strip stops working.

**ReeceyBurger123** · 07-13-2015, 11:00 AM

Re: Toshiba LED TV, blank screen

Ahh right I see sorry still learning and haven't read most of the thread.

**tibimakai** · 07-13-2015, 11:01 AM

Re: Toshiba LED TV, blank screen

He has, way to many videos. Could you please, let me know at least, which brand TV it was?

**tibimakai** · 07-14-2015, 09:49 PM

Re: Toshiba LED TV, blank screen

OK, I have added two resistors in series, total value is 231Kohm and the voltage is 2.8V.
That is, a 80% value from 3.5V.
I will leave it at that.
To me, it looks good.
Thank you everybody for the help, specially Bud.
Now, I need to move on, to the Sony Google TV.

**budm** · 07-14-2015, 09:53 PM

Re: Toshiba LED TV, blank screen

Did you notice that it does dim down the backlights and runs a little cooler too?
I told people that want to buy new TV to set the backlights level as low as possible to extend the life of the LEDS.

**tibimakai** · 07-14-2015, 10:55 PM

Re: Toshiba LED TV, blank screen

Yes, it runs cooler.
That is a good idea.

**Agent24** · 07-14-2015, 10:59 PM

Re: Toshiba LED TV, blank screen

I run my LCD monitors at about 40% brightness. Unless you have bad eyesight I don't know why people run them at 100%.

Toshiba LED TV, blank screen

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