Hitachi 42HXT12U - Not working - Standby LED Flashes

[Jack828]

Hello fellow tech geeks!

I've recently come into possession (a freebie!) of a Hitachi 42HXT12U LED TV which does not work.

It does the following:
- Plug in
- Standby LED flashes alternating RED and BLUE approx 6-7 times
- Does this approx twice
- Standby LED stays solid BLUE

I've opened it up and had a look at the PSU board (which is a 17IPS20 23145828) for any bulging caps and there aren't any.

Any ideas folks? If you suggest replacing the PSU board then I'll go ahead and do it - only about 30 quid off ebay.

Thanks!

[yokoono]

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

Find software for main 17MB95 and panel VES420UNDL-N01

[dick_barton]

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

Take some voltage readings from CN4 and report back.

[Jack828]

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

Find software for main 17MB95 and panel VES420UNDL-N01

Not sure I follow what you mean?

Take some voltage readings from CN4 and report back.

It's pretty wacky to be honest. I unplugged the cable so I could get readings and I got this:

1 - PIN_1 - 0v
2 - GND - ok
3 - ST_BY - 0v
4 - GND - ok
5 - BL_ON-OFF - 0v
6 - GND - ok
7 - PWM_DIM - 0v
8 - GND - ok
9 - +5V_STBY - 0.1v
10 - +12VCC_S - varies between 7v and 11v, not stable at all
11 - +12VCC_S - same as above
12 - +12VCC_S - same as above

Hope that helps, thanks for your input.

[Jack828]

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

Sorry for the double post but I figured a way to measure whilst the unit is plugged in.

Here's the readings:
1 - PIN_1 - 0v off, 24.29v when powering up
2 - GND - ok
3 - ST_BY - floating, ~2v when powering up
4 - GND - ok
5 - BL_ON-OFF - 0v
6 - GND - ok
7 - PWM_DIM - 0v of, 3.3v when powering up
8 - GND - ok
9 - +5V_STBY - 5.3v
10 - +12VCC_S - can't reach whilst plugged in
11 - +12VCC_S - 12v bang on
12 - +12VCC_S - can't reach whilst plugged in

EDIT: Tested what I think is the LED inverter power (no idea, the black/red 2 wire connector at top of board) and it just floats between -1v/+1v.
Double EDIT: Tested between red wire + chassis as ground and I get a bunch of different readings. Sometimes 180-190, sometimes just falling from 120, can't seem to get it stable

[dick_barton]

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

It looks like you have no BL-ON-OFF (Backlight On) control signal being fed to the power supply from the mainboard.

What dc voltage do you have across C105 (ie between the two legs of that capacitor.)

What voltage do you have on each wire of CN501 as you power up.

Edit

Double EDIT: Tested between red wire + chassis as ground and I get a bunch of different readings. Sometimes 180-190, sometimes just falling from 120, can't seem to get it stable

Those results are generally indicative of faulty led(s) or connection between the led strips.

[Jack828]

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

It looks like you have no BL-ON-OFF (Backlight On) control signal being fed to the power supply from the mainboard.

What dc voltage do you have across C105 (ie between the two legs of that capacitor.)

What voltage do you have on each wire of CN501 as you power up.

C105 - 393v
CN501 - Black - Started at 80v, goes down to 11v, then jumps back to 80v after another power on
CN501 - Red - 118v, then jumps around between 180-190v

Those results are generally indicative of faulty led(s) or connection between the led strips.

Roger that, lets hope not.

[dick_barton]

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

C105 is one of the large capacitors 47uF 450V

[Jack828]

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

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

[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

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

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

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

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

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.

[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.

[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

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

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

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