Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

[dicky96]

Hi guys, sorry to have a few threads all on the go at once right now. I have a spate of Toshiba TVs with back light problems - but this is a general LED question really.

So I have on the bench now a Toshiba 40HL933G

It has four chains of LEDs in parallel.

The fault is the Power supply comes up OK (5V 12V 24V)
PS_ON goes to 3.3V
INV_ON_OFF goes to 3.3V
BRIGHT_ADJ goes to 3.3V

The back lights flash on very briefly and go out. BRIGHT_ADJ goes to 2.9V

Pressing the standby button repeats the process

When I test the LED chains with a back light tester they all illuminate but I see I have three chains with almost the same voltage 78.4V - 78.7V and the other chain is 84.9V

I would have expected they would all be the same or at least much closer than that.

Would such a large voltage difference on one chain cause the LED driver IC (in this case SSL110SN) to detect a fault condition on the LEDs and shut down hence the back lights flash briefly?


Thanks
Rich

[budm]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

They are not connected directly in parallel, each string has its own PWM controlled MOSFET's (Q100 ~ 104) for maintaining constant current for each string. The Anode of all 4 string are tied to the same output of the Voltage boost converter circuit.
What is the LCD panel model number?

[dicky96]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

Its a Samsung LTA400HM22 Bud

[vinceroger69]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

lcd panel specs link
http://www.panelook.com/LTA400HM22_S...iew_15903.html

[budm]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

https://www.badcaps.net/forum/attach...3&d=1518445891

1) Turn the TV off, and then leave the black probe of the meter on the chassis.
2) Put red probe on the test pad LED-1 of the LED driver board.
3) Turn on the TV while watching the meter to see how high the Voltage goes up to and then wait 5 seconds to see how low it goes down to.
4) Repeat 1, 2, and 3 step again but with test pads LED -2, LED-3, and LED -4.

Report what you have on all four test points.

[dicky96]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

Bit hard to measure as my Fluke DMM can't react fast enough to the peak voltage even with auto range switched off. I can read the low voltage quite accurately

On the DMM it looks like

LED1 1.49V -13.6V - 0V
LED2 1.86V - 14.9V - 0V
LED3 1.68V - 6.8V - 0V
LED4 0.85V - 4.6V - 0V

However on the analog scale on the Fluke I can see the peak voltage goes higher than those readings

On my scope it looks like

LEDS1, 2 & 3 1.8V - 30V - 0V with a square wave on the 30V which has a high % on duty cycle

LED4 0.8V - 30V - 0V with a square wave on the 30V which has a low % on duty cycle

Rich

[dicky96]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

Hi guys
I'm still struggling to trace the problem on this one

I tried disconnecting each LED chain in turn. With any of them disconnected I get a very brief flash on the backlights

With all four connected I get back lights for maybe a second then they go out.

So it would seem the SSL110SN LED driver can detect if a chain is open circuit which makes me wonder if it is kinda working and maybe detecting a false error condition?

I know all the LED chains work OK on my back light tester albeit one has a larger voltage drop than the other three.

Problem is I can't seem to find any meaningful data sheet on the SSL110SN with pin description.

I guess I could just order a new SSL110SN and see what happens but i'm not convinced the driver chip is the fault here. Any suggestions?

Rich

[diif]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

The tester tests at low current and has indicated a string is not the same voltage as the others.
That is what I'd check first.

[dicky96]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

OK I got it apart without breaking anything

The LEDs are on a single strip down one side, there are 14 LEDs per segment

Alol segments light up and interestingly they all read the same voltage on the backlight tester 78.5V for a short while but then the top segment went back to approx 85V

I also don't like the look how some LEDs look a bit 'burnt' and they don't all come on at the same brightness until the current ramps up.

As there are the same number of LEDs in each segment it would seem I do have a problem with the LED strip. Can anyone tell me the part number, there is no writing on the strip just a funny little square symbol

[diif]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

The part number is on the aluminium in photo 2.
https://www.aliexpress.com/item/orig...728500735.html

[Naruto-sama]

Re: Toshiba 40HL933G - LED Back Light voltages - should all chains be the same?

Hi, did you solve the problem by replacing the LEDs?