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**R_J** · 08-29-2020, 11:27 AM

Re: Toshiba 49L621U - Backlight protection shutdown

It could be ONE bad led in the panel (likely -VLED 1) and that will be enough to shut down the circuit, Unless you removed the lcd panel to see allthe led's you cannot tell if one ore more led's are bad.

**neilc6** · 08-29-2020, 03:18 PM

Re: Toshiba 49L621U - Backlight protection shutdown

Those Toshiba models have bad backlights that can potentially damage the LED controller on the power supply. You will likely need to replace both the backlights and the power supply.

**Cardioman** · 08-30-2020, 08:16 PM

Re: Toshiba 49L621U - Backlight protection shutdown

So I pulled the panel and found as thought ALL leds are good. They light at about 19.2V for all strips. (7 leds per strip, 4 strips per set, 12 strips, 84 total leds) Have been looking at power supply disconnected. The 3 power mosfets (for leds) all look good in circuit, at least not shorted. I suspect the 28 pin SMD device HAI7207CPC which is I think the shutdown monitoring circuit. Of course I cannot find any datasheet for the device. I am leaning towards getting a power supply board. Anybody have any input. Again thanks for any help offered. Help Please!

**jjanssen7** · 08-31-2020, 12:28 AM

Re: Toshiba 49L621U - Backlight protection shutdown

In my experience, on this model the Led strips will test good with a tester but fail when powered up with the full current of the power supply. I believe you'll need to replace the backlights. Best to do them all as a set and then turn down the backlight level in settings.

**Cardioman** · 08-31-2020, 07:55 AM

Re: Toshiba 49L621U - Backlight protection shutdown

Can you power up set momentarily with power supply board and main board but without TCON and screen, to verify the leds are bad at full current of PS? I would think not! I haven't seen anyone due that before.

**jjanssen7** · 08-31-2020, 07:02 PM

Re: Toshiba 49L621U - Backlight protection shutdown

I do it all the time, but I'm not sure what that test would confirm as you already know the issue is not with the Tconn or screen.

Once you've replaced the LEDS I would do that to make sure you've got all the connects tight etc.

Top Tip: Before testing this way, make sure you disconnect the LVDS cable at the Main Board. If the ribbons from the Tconn touch the metal case while powered up (because you've removed the screen) they can short out the Tconn .

**Cardioman** · 09-01-2020, 01:06 PM

Re: Toshiba 49L621U - Backlight protection shutdown

JJanssen7, thanks for input! Plug in just main logic, and PS with LVDS cable off. Did momentary test over and over again watching leds. All light bright on flash. Is this valid to rule out leds or can these bad boys flash and still be bad thus shutting down the fet?? Does anyone have a source with stock on a set of 12 strips of leds (7 per) Toshiba 4708-K49WDR-A1213K11 LED Backlight Strips. Or equivalent ??

**neilc6** · 09-01-2020, 01:29 PM

Re: Toshiba 49L621U - Backlight protection shutdown

Intermittent LEDs can draw higher or lower voltage triggering the shutdown. Without a LED tester, you may be able to probe the voltage going to the strips.

**jjanssen7** · 09-01-2020, 06:32 PM

Re: Toshiba 49L621U - Backlight protection shutdown

I have found a LED tester to be less than helpful with these models. I have repaired many of the 43 & 49" versions with the symptoms above. (All strips light up with a tester and flash once when powered up or stay on for between 3 seconds to 15 minutes or more and then shut off, image still on screen.)

A new set of strips resolves the problem in 95% of my cases. (I say 95% because no "fix" is 100% of the time).

It is usually just one strip that is failing and is not revealed by a tester... I've even tried two different testers, one that is supposedly able to supply more current. The testers simply cannot reveal which strip is failing.

My solution (given the failure of LED testers to reveal the problem with this model) is to swap out the strips one by one with a known working strip and power up the TV (with the screen and tconn disconnected of course) and inevitably there is just one strip that has been pulling down the system.

But here's the thing. IF you replace just the one strip, the TV will work fine for a month to 3 months and then another strip will fail, requiring the whole procedure again.

**budm** · 09-01-2020, 07:01 PM

Re: Toshiba 49L621U - Backlight protection shutdown

Originally posted by Cardioman View Post

So I pulled the panel and found as thought ALL leds are good. They light at about 19.2V for all strips. (7 leds per strip, 4 strips per set, 12 strips, 84 total leds) Have been looking at power supply disconnected. The 3 power mosfets (for leds) all look good in circuit, at least not shorted. I suspect the 28 pin SMD device HAI7207CPC which is I think the shutdown monitoring circuit. Of course I cannot find any datasheet for the device. I am leaning towards getting a power supply board. Anybody have any input. Again thanks for any help offered. Help Please!

So how much current did you force through the LED? 100mA? 200mA?

**Cardioman** · 09-01-2020, 09:27 PM

Re: Toshiba 49L621U - Backlight protection shutdown

budm, dont know right now but i can measure it next. What current does one leg take normally?

**budm** · 09-02-2020, 12:12 AM

Re: Toshiba 49L621U - Backlight protection shutdown

Most TV LED's are driven around 150 ~ 300 mA of constant current..

**Cardioman** · 09-02-2020, 12:21 PM

Re: Toshiba 49L621U - Backlight protection shutdown

budm, thanks for reply. measured the Dc current, injecting the led tester (with PS led connector disconnected ) into VLED1- and appropriate VLED+ line one at a time. then moving to next leg, (VLED2- & VLED3-) measured 29MA for the 3 legs. (28 leds per leg)

**Cardioman** · 09-02-2020, 05:33 PM

Re: Toshiba 49L621U - Backlight protection shutdown

Need 12 NEW LED strips Toshiba #4708-K49WDR-A1213K11 for 49L621U. Only found in Canada at $120. Set isn't worth it. Anyone have a complete set to sell or have a source? Thanks!

**budm** · 09-02-2020, 05:36 PM

Re: Toshiba 49L621U - Backlight protection shutdown

Originally posted by Cardioman View Post

budm, thanks for reply. measured the Dc current, injecting the led tester (with PS led connector disconnected ) into VLED1- and appropriate VLED+ line one at a time. then moving to next leg, (VLED2- & VLED3-) measured 29MA for the 3 legs. (28 leds per leg)

Those LED tester only puts out about 30mA, the newer is supposed to be able to put out in hundreds of mA.
So if you can get to the user menu, can you lower the backlights level to as low as possible to see what will happen?

**Cardioman** · 09-02-2020, 05:51 PM

Re: Toshiba 49L621U - Backlight protection shutdown

budm, Down to LEDS no screen at present. Ran each strip with tester while tapping each led. Found an intermittent led.

**jjanssen7** · 09-02-2020, 06:30 PM

Re: Toshiba 49L621U - Backlight protection shutdown

Hey, nice catch. I have one of these sets to look at soon. I'll try that and see if that's a quicker way to identify which strip is bad.

**budm** · 09-02-2020, 06:37 PM

Re: Toshiba 49L621U - Backlight protection shutdown

Originally posted by Cardioman View Post

budm, Down to LEDS no screen at present. Ran each strip with tester while tapping each led. Found an intermittent led.

If you are able to drive it at high current the bad one will be shown, but tapping works too if all you have is low current driver.
Glad to hear you nail down the problem.

**Cardioman** · 09-03-2020, 09:18 AM

Re: Toshiba 49L621U - Backlight protection shutdown

Found out shop jimmy just got led strips back in stock should anyone need them. Thanks for ALL responses!!

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Toshiba 49L621U - Backlight protection shutdown

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