LG 42LT75 no backlight

[Bilbo44uk]

Re: LG 42LT75 no backlight

Did you get this working as i have one with same fault nothing seems to get it going tried replacing loads caps too. Tim

[tn245]

Re: LG 42LT75 no backlight

Hi Tim. No I didn't get any further with this and after spending a lot of time on it decided to just replace the PSU. Sorry to not be able to help. If you get any further I would love to hear about it as I still have the board kicking around.

[Bilbo44uk]

Re: LG 42LT75 no backlight

Hi, i re capped my set it all came on fine for a few mins, then the screen went off, sound stayed on, i have checked all caps, esr, swapped parts in and out, you name it i have done it, the drive voltage to the pwm IC Fan7382 is about 14.5v which is normal between pin 1 and 4, the two other pins seem to be for a logik pulse, i am in middle doing this as im bit annoyed with it as the TV has only come in for what i thought was a cheapo re cap, half a dozen puffed up caps and i thought easy peasy, then this happened, there is no logik whatsoever on this panel, i tested the backlights so know they are good, the inverter isnt turning on at all, i am going to check all small smd resistors for an open one, hold onto that board i may need cross reference. Tim.

Update: Do you have a TTL Logic Probe, if so check the hi Lo logic state to the chip, pin 2 and 3, use 4 as ground, should get a hi state lo state off of those, if not go back one stage, i am in middle doing this now.

[Bilbo44uk]

Re: LG 42LT75 no backlight

Further to my last message, the set is now working, i have spent hours on this as your see by my post times, here is what I did.

As the O/P stated this is a weird fault, but in the end was logical, it boils down to a combination of bad solder joints capacitors and luck, replace all obvious bulging caps, ESr the rest, replace the 22nf ballast caps on the inverter output, this may resolve the backlight issue, it says 6kv, but 3kv are fine as it only hits 1800v on run, still no good? resolder the 1uf 600v box cap at the back of the inverter transformer, or replace if not 1uf on the dot, this got mine going tonight, obviously check the diodes, especially the one next to the FAN7382 as this was a crappy joint on mine too, check the chip voltage pins 1 and 4 look for around 14.5 volts, if you have a logic prob or modern digital scope check pin 2 and 3 for Hi and Lo logic states, right if you have all that done it should run, worth checking the tracks with a low ohms meter or even your esr meter, this is how i found at least 4 suspect solder joints, ultimately i believe it was the blue box capacitor on the back of the inverter, I also checked ever SMD resistor and diode on the board around the main switching transistors and the drive to the IC FAN7382. I think you just have to be very persistent on this board, when the Tv is working, SELL IT asap!! Tim.

[budm]

Re: LG 42LT75 no backlight

"replace the 22nf ballast caps on the inverter output, this may resolve the backlight issue, it says 6kv, but 3kv are fine as it only hits 1800v on run, still no good?"
I would not use 3KV since startup voltage will be higher than that. Also are you sure the ballast cap is 22nF not 22pF? 22nF is very high capacitance for ballast cap.

[Bilbo44uk]

Re: LG 42LT75 no backlight

sorry 22pf, i was tired, the sets now working and been collected, in the end it was the blue box cap that caused my issue, tim

[tn245]

Re: LG 42LT75 no backlight

As I'd said in a post earlier in the thread, with mine I tried swapping the blue box cap with the one from the working board I had and it didn't solve it. Maybe I'll try the other suggestions though if I get any time to go back to that board.

Glad to hear you got your one going.

[Agent24]

Re: LG 42LT75 no backlight

I would not use 3KV since startup voltage will be higher than that..

x2

Manufacturers are cheap. They aren't going to use a 6kV capacitor where a 3kV one would do.