Hitachi 42HXT12U - Not working - Standby LED Flashes

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Turn the backlight (not to be confused with the brightness or contrast) down as much as possible to reduce the wear on the leds and extend their lifetime.

[alfatv]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

That backlight uniformity is quite bad. Replacing all strips might improve it. LG LB series is another typical case, trying to stay away from them.

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Appreciate you sending over that link but I ended up buying the £33 pack off Ebay.

So thanks for all your help mate - we're all working again now!

Saved another rebadged vestel from the tip.

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

If your willing to wait for delivery then this may be of interest.
https://www.aliexpress.com/i/32925691852.html

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Awesome! Probably gunna have to buy a full set of LEDs as they seem to be cheaper than getting them individually.

Can't wait to get the dining room table back!

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Yes, that would seem to be OK.

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Yes, should've said.

I identify which traces are + and - by applying the multimeter in diode mode to each trace area using trial and error. When I've figured out which lights up an LED, I move to the next strip and try the same pads.

The marked LEDs do not light up when touching the same pads that lit up the LED on another strip.

(Of course, I had to do this twice, once for A strip and once again for C)

Hope that makes sense!

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Are they the ones with the purple marker alongside. Why do you believe they are broken? Did you double check your readings.

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

I did manage to test the LEDs individually - see attached for the believed broken ones.

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Do a Youtube search on how to test leds on the led strips. I would not be replacing the strips until I was certain I had found the fault.

https://www.youtube.com/results?sear...+tv+led+strips

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Sure, makes sense.

Just plugged it in now and no LEDs lit up.

I'm guessing next steps is buy a LED strip pack and replace those?

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Short circuit led will often allow the remaining leds in the circuit to light up momentarily and an over voltage will be seen by the led driver IC which switches off the drive to the leds to stop further damage.
Open leds (or open circuit led strip connector) will as you say not light up however the tell tale sign here is that the led voltage rises up quickly beyond the normal led driving voltage and then collapses just as quickly.

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

So I am aware (and sorry again for the double posting!) what does it mean if the backlights either DO or DO NOT flash when powering the unit up without the panel in?

I assume if I have one or more dead LEDs that the LEDs will not flash at all, and if they do flash we have a power issue.

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Yes, there is continuity between strips.

Haven't tried powering it up again yet but I'm pretty sure I've more than one blown led on almost each strip, just figuring out which is +ive and -ive traces is tricky - should have another update tonight.


EDIT: Looking at the moment that I need 3xA and 1xC strips.

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Do you have continuity between the two strips when testing with your multimeter.
If all the led strips are connected in series and they are 3v leds, then the driving voltage would be around 48 x 3.2V = 153V. There is a200V capacitor near to CN501.
Have you tried powering up the setnow that the panel is removed to see if the leds flash?

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

You may just have a bad connector or connection on one of the led strips interconnection so I would check for continuity between strips.

The panel uses a number of different strips to complete the array.
Type A and Type C are shown in this ebay listing. It's the way they fit together within the panel.

https://www.ebay.co.uk/itm/183802199613

so you would need to match any replacement strips accordingly.

Righto, managed to get the whole thing apart to check the LED strips.

Continuity between strips is fine, but putting the multimeter on diode mode and probing the little + and - test points doesn't make any of them light up at all.

EDIT: TIL, you can scratch the solder mask off and test each LED individually, just gotta figure out what goes where!!

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

You may just have a bad connector or connection on one of the led strips interconnection so I would check for continuity between strips.

The panel uses a number of different strips to complete the array.
Type A and Type C are shown in this ebay listing. It's the way they fit together within the panel.

https://www.ebay.co.uk/itm/183802199613

so you would need to match any replacement strips accordingly.

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Gotcha. Will look into that to see if I can't just replace the faulty ones or if I have to get the all the strips. Thanks so much!

[dick_barton]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

So it's not the PFC voltage being too low. It does unfortunately look like a led(s) or led strip problem.

[Jack828]

Re: Hitachi 42HXT12U - Not working - Standby LED Flashes

Sorry, my bad I managed to find it after looking again more closely - 393v.