LG42LA620V no backlight

[sordids999]

hello

Backlight flashes once on start up then no backlight but with a torch shining onto the screen you can see images ok

voltages seem fine also 58 volts going to the panel/LED`s not dropping

anyone come across this before or is it a bad led ?

thanks

[Diah]

Re: LG42LA620V no backlight

take it in parts till strips of LED, led shorted or open.

[budm]

Re: LG42LA620V no backlight

What is the Voltgae rating of those two lyrics cap by the bottom left of #2 picture where the boost converter for the LED driver circuit is?
58V seems low but I cannot tell yet until I have a chance to look closer at that boost converter section later on, I expect it to jump up higher than 58V at first and then drops down to 58V, but as history of LEDs backlights I will not be surprised if you have bad LEDs.

[sordids999]

Re: LG42LA620V no backlight

hi budm

do you mean the two caps i have circled in the picture ?

and yes when the tv first boots up voltages are 59.8 volts then drops to 58.4 volts and stays steady

[budm]

Re: LG42LA620V no backlight

NO, the two brown electrolytics caps to the left side of the board, and actually there are six brown lytics caps in the LED driver sections and also two inductors for boost converter circuits so the LEDs are not connected in one large LED string.
According to the Voltage chart next to the LED connector, the Voltage for the LED is 105V, 0.313A? I cannot see the chart that clear.
You need to leave the red probe on the L+ and black probe on GND then turn on the TV and watch the meter to see how high the Voltage goes up too, you need to do that twice since you have two L+.

[orinoco8]

Re: LG42LA620V no backlight

not sure whether this is allowed here: maybe you want to try the method posted on

http://www.forums.toshiba.com/t5/Tel.../154712/page/2

post 35.

[sordids999]

Re: LG42LA620V no backlight

NO, the two brown electrolytics caps to the left side of the board, and actually there are six brown lytics caps in the LED driver sections and also two inductors for boost converter circuits so the LEDs are not connected in one large LED string.
According to the Voltage chart next to the LED connector, the Voltage for the LED is 105V, 0.313A? I cannot see the chart that clear.
You need to leave the red probe on the L+ and black probe on GND then turn on the TV and watch the meter to see how high the Voltage goes up too, you need to do that twice since you have two L+.

correct

105V, 0.313A HIGH
70V, 0.313A LOW

and yes 105 volts when i first turn the tv on then drops so its pointing to a bad led i think

[sordids999]

Re: LG42LA620V no backlight

Fixed :

luckily had some spare LG LED strips from a cracked screen tv testing as we speak

[budm]

Re: LG42LA620V no backlight

Thanks for the follow-up on the repair, another failed LED backlights.
The trick when measuring the LED supply Voltage is to leave the probe on the Voltage supply pin for the Anode of the string before turning on the TV and have good meter that can response fast enough to capture the Voltage rise before dropping down due to protection circuit kick in. 58V is the feeding Voltage to the boost converter circuits.
BTW, what are the Voltage rating of those caps I asked?

[sordids999]

Re: LG42LA620V no backlight

BTW, what are the Voltage rating of those caps I asked?

No budm sorry i could not see any ratings on them

[dick_barton]

Re: LG42LA620V no backlight

The two brown capacitors C801 & C802 are both 33uF 200V