Hitachi 42HXT12U No Backlight 17ips20

[london-freddie]

Hi all again
Working on this Hitachi 42HXT12U with failed backlight
First thought failed led or leds, + volts to backlight 190v
Stripped panel found one dead open led, replaced failed led, tested each led with 3v and all leds now working
Reassembled tv and working however after about 20 minutes backlights went back off
Stripped panel again and all leds seem to be working, maybe failing with higher current supplied by the powerboard
On testing powerboard output to leds am getting 117v rising to about 190v backlights flash on for a split second, then volts fall to 117.7v and then settles at that with no backlight
Question do you guys think I still have a faulty led that is failing under higher current from powerboard supply
OR Could this be the MPS chip on the powerboard failed or failing
If I disconnect the red/white wires from the powerboard to backlights and power the backlights with my cheapo backlight tester the backlights come on and stay on drawing a voltage from the tester of 134 volts
Any help appreciated please before I strip the panel again or try and replace this MPS chip
Thanks

[london-freddie]

Re: Hitachi 42HXT12U No Backlight 17ips20

Just powered on the tv to check voltages again and backlight has come on, been on for about ten minutes but am sure will go off again soon
Only thing I notice now with the backligh working is a noisy hum from the powerboard

[london-freddie]

Re: Hitachi 42HXT12U No Backlight 17ips20

Backlight gone off and not coming back on

[R_J]

Re: Hitachi 42HXT12U No Backlight 17ips20

Remove the power supply and resolder the connections on the transformers , they could have poor solder connections

[london-freddie]

Re: Hitachi 42HXT12U No Backlight 17ips20

Thanks R_J will check that, but will be tomorrow now when I get time

[london-freddie]

Re: Hitachi 42HXT12U No Backlight 17ips20

Made time and checked transformer joints, all seemed ok, but resoldered anyway no change, still no backlight and voltages the same 118 up to 190 back down to 117.7v

[diif]

Re: Hitachi 42HXT12U No Backlight 17ips20

Sounds like a bad LED to me. They should be tested at 300mA

[london-freddie]

Re: Hitachi 42HXT12U No Backlight 17ips20

Any easy way of testing the leds as to which one or which ones are failing under load

[diif]

Re: Hitachi 42HXT12U No Backlight 17ips20

I have a piece of the PCB the Vestel LED strips plug into, with a couple of wires soldered on.
I power it with my bench power supply.

[R_J]

Re: Hitachi 42HXT12U No Backlight 17ips20

You can also check the led (-) lead, pin 7 of led plug, if there is 0 volts when first turned on at the led return it is almost always an open connection for the led string in the panel, you checked each led but you need to check any led string plug connections also

[london-freddie]

Re: Hitachi 42HXT12U No Backlight 17ips20

Update*** stripped screen panel again checked led's again, led that I had replaced had failed, maybe I put too much heat to it when I resoldered . However replaced led again and backlight is now working, tv has been on now for about four hours, so fingers crossed that is a fix.
Strange thing though when I powered backlights using my cheapo chinese backlight tester and powering the whole backlight string from the connection at the powerboard connector (diconnected from the powerboard] the whole string lit up including the faulty led that I had replaced.
How I found the faulty led, I reconnected led connector and powered on the tv, checked the voltage at each strip, can't remember now but say 175v at first strip 150v at second strip 130v third strip etc. etc. until think it was fifth strip volts dissapeared 0v tracked back to the faulty led with no voltage passing through the faulty led.
Anyway as I type this the tv is still working
Thanks again everyone for your help again