Hitachi 50HYT62U Backlight brief flash

[mutts]

Hi repairers I have a confusing one (for me).

A friend has asked me to repair his 2 year old tv, is this a led driver fault?...
Tv boots normally but no backlights other than a very brief flash. Tested with GJ2C tester all leds come on at 186v. During boot get 162v at connector, brief flash from backlight as voltage slowly rises to 236v, if the set is left on this drops to around 60v over the next 5 minutes.

All diodes test ok, all capacity and esr of caps is good, lcd panel is unmarked and menu etc can be viewed with a flashlight. Psu is a Vestel 17IPS20, set has done 12.5k hours.

I'm thinking that regardless of having 2.6v at each of the backlights 63 leds during boot the current is not enough.. and then the climbing voltage?? it's got me.

If anyone more knowledgeable has any ideas I would be most grateful.
Thanks

[Andrew F. Ali]

Re: Hitachi 50HYT62U Backlight brief flash

I still suspect bad backlight LEDs. From what I gather the LED Driver seems to be working.

[mutts]

Re: Hitachi 50HYT62U Backlight brief flash

Yes that would make sense. I probably missed one or more when checking with the lcd still attached.
Many thanks.

[zomi]

Re: Hitachi 50HYT62U Backlight brief flash

https://tinyurl.com/yc4accjm

these should be the strips for yours

[mutts]

Re: Hitachi 50HYT62U Backlight brief flash

Thanks zomi, perfect.

[ducky29]

Re: Hitachi 50HYT62U Backlight brief flash

Did you strip the whole screen down and check all the led strips?
Or just test them from the connector that connects with the power supply?

[mutts]

Re: Hitachi 50HYT62U Backlight brief flash

They were just tested from the connector, stripping a large panel apart wasn't an option at friends house.

[ducky29]

Re: Hitachi 50HYT62U Backlight brief flash

They were just tested from the connector, stripping a large panel apart wasn't an option at friends house.

Well sometimes only few of the leds lit up and give you the illusion that the whole strip works.

[budwich]

Re: Hitachi 50HYT62U Backlight brief flash

did you try unplugging the strings and then checking the initial open circuit voltage to see where it gets to?

[mutts]

Re: Hitachi 50HYT62U Backlight brief flash

Yes, iirc it was around 80v.

[GAStuntman]

Re: Hitachi 50HYT62U Backlight brief flash

Did you figure out the issue? I’m having the same problem with my tv. I opened it up & tested the LEDs & they all worked using the LED tester. I reassemble the tv & once the power button is pressed the LEDs flash and go back out. I have replaced the power supply board & the LED driver board.

[budm]

Re: Hitachi 50HYT62U Backlight brief flash

Did you figure out the issue? I'm having the same problem with my tv. I opened it up & tested the LEDs & they all worked using the LED tester. I reassemble the tv & once the power button is pressed the LEDs flash and go back out. I have replaced the power supply board & the LED driver board.

Did you use the LED tester to test the whole LED string from the LED connector side? What Voltage does it show on the LED tester?

[GAStuntman]

Re: Hitachi 50HYT62U Backlight brief flash

I tested the LEDs all the voltage was good & all the LEDs did light up with the tester. I don’t remember the exact voltage reading on the LEDs. Normally with LEDs you can shine a flashlight into the screen and see the image. In my case that didn’t work. Like I stated before I have already taken the tv apart and tested the LEDs and they all worked. I’m thinking there maybe a shortage somewhere or something else is causing the tv to shut off the LEDs as a safety precaution.

[budm]

Re: Hitachi 50HYT62U Backlight brief flash

LED tester only puts out 30mA compared to about 300 ~ 400mA when driven by the TV LED driver circuit. I do have LED tester that I use as a guide but not for 100% verification of the condition of the LED's. Bad LED will fail when driven at high current.
You can disconnect the LED connector from the power supply board and use your tester to light up the LED string and verify for sure if you do not have any image on the screen or not, if not then check the fuse on the T-CON board and see if you do have 12VDC at both ends of the fuse with ref. to chassis GND.

[GAStuntman]

Re: Hitachi 50HYT62U Backlight brief flash

Thanks, I took the tv back apart & found one burnt LED. Replacing the bad LED fixed the issue.