Hisense 40H3E No backlight

[hanswr]

Hi everyone.

I'm trying to fix a Hisense 40H3E TV.

Symptoms:
- Back light is not turning on.
- You can see images with a flashlight pointed to the screen.
- Standby light is present.

I opened it up to get the voltage reading in the back light led connector. I'm getting 37 volts. There is no information printed on the board front or back regarding normal voltage ranges.

I don't know if this should be the correct voltage, I tried looking for a service manual or schematics to no avail.

How may I proceed with this?

Thank you very much in advanced.

[dick_barton]

Re: Hisense 40H3E No backlight

The SN51DP is the led driver chip which should be driving the gate of a mosfet which I cannot see but could be on the underside of the board. Can you post a clear sharply focused picture of the underside of the board.

If you could measure the voltages on the pins 1 to 10 would be usefull.

[hanswr]

Re: Hisense 40H3E No backlight

The SN51DP is the led driver chip which should be driving the gate of a mosfet which I cannot see but could be on the underside of the board. Can you post a clear sharply focused picture of the underside of the board.

If you could measure the voltages on the pins 1 to 10 would be useful.

Thank you for taking the time to reply and try to help.

Regarding the measure of the led driver chip, unfortunately I don't have the appropriate tools yet to do that without shorting the chip (magnifying glass, and super thin testing probes), but working on it.

I have attached a new picture of the back of the board as requested.

Also while measuring the voltages again in the let connector and the TV is off (stand by) the voltage is 37.4, then pressing the power button the voltage goes up to 100 and slowly decrease and stabilize in 47v.

Even during this voltage spike I never see the led to light up.

Is it possible that the led stripes are bad?

[dick_barton]

Re: Hisense 40H3E No backlight

Also while measuring the voltages again in the let connector and the TV is off (stand by) the voltage is 37.4, then pressing the power button the voltage goes up to 100 and slowly decrease and stabilize in 47v.

Your description is sounding more like you have a faulty led or led connection.

[hanswr]

Re: Hisense 40H3E No backlight

Your description is sounding more like you have a faulty led or led connection.

Shouldn't the voltage on the main/power board connector remain stable and in normal range when the led stripes are disconnected?

It does exactly the same while the leds are disconnected, goes up to 100v and slowly stabilize to 47v.

[dick_barton]

Re: Hisense 40H3E No backlight

The led driver chip monitors the current flowing through the leds.

With an open circuit led or led sting connector, the result is the same as not having the leds connected.

The voltage rises to the driving voltage required by the leds, no current is detected flowing or incorrect current is detected and the led driver switches off.

The 47V is the voltage at the led driver chip which is boosted to the voltage required for the leds to operate. When a fault condition occurs the the led driver does not boost the 47V

[hanswr]

Re: Hisense 40H3E No backlight

The led driver chip monitors the current flowing through the leds.

With an open circuit led or led sting connector, the result is the same as not having the leds connected.

The voltage rises to the driving voltage required by the leds, no current is detected flowing or incorrect current is detected and the led driver switches off.

The 47V is the voltage at the led driver chip which is boosted to the voltage required for the leds to operate. When a fault condition occurs the the led driver does not boost the 47V

I understand now. Thank you for the explanation.

I will try to buy some suction cups to properly handle the panel and check the led stripes.

[hanswr]

Re: Hisense 40H3E No backlight

I finally managed to get 2 suction cups to properly remove the panel. I have access to the leds now.

Connectors seems to be fine.

These are the voltages on every led strip from too to bottom when the TV is on :

15.4v
16.4v
23v
17.4v

I have also recorded a video when I press the power button here is the link:

https://youtu.be/NacRDqu4Ckk

[budm]

Re: Hisense 40H3E No backlight

At least two pf the strips have shorted LED's, you can see some of the LEDs are not lit up on two of the strips. The other two strips that do not light up at all probably due to open circuit, I.E. open LED's in the LED strip.

[hanswr]

Re: Hisense 40H3E No backlight

The original led strips have 11 leds each, I have some leds strips from a another hisense tv that only have 5 leds each. Both work with 94v. I disconnected the original strips and plugged the 5 led ones and the four of them light up to what it seems full brightness.

Is this an indicator that the original led strips need to be replaced?

[budm]

Re: Hisense 40H3E No backlight

You cannot just put LED strip with 5 LEDs in place of 11 LEDs unless you know what the forward Voltage of the LED is.
Where did you get the 94V rating from?

[hanswr]

Re: Hisense 40H3E No backlight

At least two pf the strips have shorted LED's, you can see some of the LEDs are not lit up on two of the strips. The other two strips that do not light up at all probably due to open circuit, I.E. open LED's in the LED strip.

Sorry. I guess we were posting at the same time.

So new led strips it is.

Thank you again for all the help and information.

[Hans Visser]

Re: Hisense 40H3E No backlight

Hi Hanswr, I'm working on a JVC 55" UHD ED TV LT55N776A.
This TV was bound for the rubbish tip. On plugging it in, it has standby LED on, when switching on the TV the back light flashes for a split second. It has two groups of back light strips which I could test one by one. The left hand side didn't respond at all so dug deeper to find from all the back light LED's only one was failing. The TV seems to have a safety feature to shut down when this happens. Fingers crossed to just replace a single LED :-)

[hanswr]

Re: Hisense 40H3E No backlight

You cannot just put LED strip with 5 LEDs in place of 11 LEDs unless you know what the forward Voltage of the LED is.
Where did you get the 94V rating from?

The 94v is printed in both the original and the ones I have from another TV. That is why I went ahead to do the test, replacing one at a time. Unppluging tv power during each replacement and test.

[hanswr]

Re: Hisense 40H3E No backlight

Hi Hanswr, I'm working on a JVC 55" UHD ED TV LT55N776A.
This TV was bound for the rubbish tip. On plugging it in, it has standby LED on, when switching on the TV the back light flashes for a split second. It has two groups of back light strips which I could test one by one. The left hand side didn't respond at all so dug deeper to find from all the back light LED's only one was failing. The TV seems to have a safety feature to shut down when this happens. Fingers crossed to just replace a single LED :-)

HI Hans! Cool name BTW .

Thanks for your reply. How did you test the led strips? Do you have a led strip tester?

[budm]

Re: Hisense 40H3E No backlight

The 94v is printed in both the original and the ones I have from another TV. That is why I went ahead to do the test, replacing one at a time. Unppluging tv power during each replacement and test.

94V is the UL rating for the PCB flame retardant rating, not LED Voltage.You can easily damage LED if you do not set the current limit for the LED.
https://en.wikipedia.org/wiki/UL_94

You should buy LED strip tester if you are going to do more TV works.

[hanswr]

Re: Hisense 40H3E No backlight

94V is the UL rating for the PCB flame retardant rating, not LED Voltage.You can easily damage LED if you do not set the current limit for the LED.
https://en.wikipedia.org/wiki/UL_94

You should buy LED strip tester if you are going to do more TV works.

Thank you very much for that information. Wow I guess I got lucky then that I didn't blow up all the LED from the other strip. Lesson learned.

I will definitely try to buy a LED strip tester.