LG 42LT75 no backlight

[tn245]

I have this LG TV open right now. Interesting TV - it has a built-in SATA hard drive for recording and pausing TV.

It turns on and boots up, but there's no backlights. Not even a flash. The caps all looked and tested fine but I replaced the usual troublesome ones with Panasonics anyway. Voltages all look pretty close to what they should be.

PSU is LGLP42SLPV2EV3 / EAY38640201

The inverter is integrated on the PSU with just one inverter transformer. The inverter uses a OZ9925GN controller ic.

ACD: 5.1V
PWR: 1V
ST 5V: 5V
INV-CTRL: 4.6V
ERROR: 0V
BURST-DIM: 0.6V
A-DIM: 0V
6.5V: 6.4V
3.4V ON: 1.4V

19V: 20.3V
3.4V: 3.4V
12V: 12.1V


I've read a lot of threads on it but I'm still not sure of the best way to test inverter outputs. Can I test this inverter with a CCFL as in this thread: https://www.badcaps.net/forum/showthread.php?t=19987 and if so, do I connect one end to each of the connectors where the pink wires plug in?

Any other tests I can do? I am planning to break the TV down more to look at the bulbs and connections but would like to rule out the PSU/Inverter before going further.

Thanks!

[tn245]

Re: LG 42LT75 no backlight

Hi! I'm back after a few months away. Moved house and lots of other stuff to do so haven't had much time to work on TVs. Got a backlog to clear out though so got to get working on them when I can.

I've just come back to this one. I picked up another TV with the same PSU and tested it in this one and everything works, so it's definitely the PSU at fault. Comparing resistance readings on the inverter transformers between the two PSUs everything looks good there.

Voltages on the working PSU measure the same as above except ERROR which I'm getting 5V where before I measured 0V on the faulty one.

Can anyone help me track down the fault? I haven't found a schematic for this PSU yet. Anybody know of one?

Any help greatly appreciated.

[tn245]

Re: LG 42LT75 no backlight

Hi guys! I'd really appreciate some help on this one if anyone can. I'm a bit stuck and not sure how to go forward. Please?

To summarise: I have two PSUs now - one borrowed from another TV. One turns on the backlights, the other doesn't. Voltages are the same on both (as in 1st post) except for ERROR, which is 0V on the faulty one and 5V on the working one. ERROR=5V only when backlights are connected up - not with PSU is isolated and jumpered on with a resistor.

Any ideas at all?

[Shinju]

Re: LG 42LT75 no backlight

(edit)

[tn245]

Re: LG 42LT75 no backlight

No? Not even a hint?

[selldoor]

Re: LG 42LT75 no backlight

Heres a couple of hints.
1Get a domestic cfl (NOT ccfl) 13 watt or less and solder an insulated wire to the centre contact.

You need a bare bit at the end. With the board on you just touch the bare end of the wire to output pins on the inverter transformer ( which does actually look burnt) and the bulb should glow. You need to take care as it could be 2000vac.

2 if you look closely at the back of the board there are some solder joints that dont look great down that side. I would re-flow them ( but see 3)

3 As the inverter transformer doesnt have many pins you could try swapping them or at least the known good one.

4 You could test the secondary winding resistances or even all of them on the two transformers. Is there a part number on it?

[stj]

Re: LG 42LT75 no backlight

change the small electrolytics around the invertor section.
you "pana'd" the rest - so why leave the invertor area?

[tn245]

Re: LG 42LT75 no backlight

Thanks selldoor and stj. That does give me something to go on.

I checked the transformer resistances and I get the same readings on both boards so assumed the transformer was good. I'll look into the other suggestions tomorrow when I get back to it.

Thanks again!

[budm]

Re: LG 42LT75 no backlight

Check the FUSE F700 and the capacitance of that blue rectangular cap C708 by the inverter transformer.

[tn245]

Re: LG 42LT75 no backlight

budm: F700 is good. C708 is 1012nF on the good board and 980nF on the bad one. Is that enough of a difference to cause a problem?

selldoor: The only CFL I have to hand is 20W which did nothing on either board, but.. I swapped the transformers over as you suggested and it made no difference to which board worked and which didn't, so I think the transformer is ok.

stj: I didn't have all the capacitor values to hand for those smaller ones but just tested them out of circuit for capacitance and ESR and all test within tolerance.

[selldoor]

Re: LG 42LT75 no backlight

The bulb trick might only work for an instant when the board is switched on but before it goes into protection mode.
Did you reflow the other joints - moving the transformer would fix those ones. You could
also swap the C708 to see. If they are meant to be 1000nF it depends on the tolerance
if it is +-20% then they are well within. Is there anywhere where it suggests there is a
blon pin on the connector from the main board?

Edit
Can you also check the solder on C226 (I appreciate it may be one you may have resoldered
but it looks a bit black in photo may just be shadow or flux.

[budm]

Re: LG 42LT75 no backlight

OK, the fuse F700 which is for feeding the B+ from the two large main filter caps to the backlights inverter circuits, now we need to find out if you do have 360~400VDC between the F700 and the negative leg of the main filter cap.

[tn245]

Re: LG 42LT75 no backlight

selldoor
You're right. I got the CFL to light with the working board. Nothing with the non-working one. I reflowed the other joints you suggested and also tried swapping C708 between the boards. No change so far. There is a INV CTRL pin which I have been using to force on the inverter circuits.

budm
I'm reading 397V between F700 and the negative lead of the main filter cap.

[budm]

Re: LG 42LT75 no backlight

That is good, I will need straight shot closed up pictures of the bottom side of the board in 4 sections so I can trace out the connections.
I also need the P/N of the IC701 I believe it is the PWM IC for the inverter circuit.
You can also try heating up those small 4 caps (C702, 113, 117, and one next to that small transformer) by that IC with hair dryer.

[tn245]

Re: LG 42LT75 no backlight

Are these ok budm?

[budm]

Re: LG 42LT75 no backlight

OK, that is pretty good. I will need that IC 701 number so I can see how those two MOSFETS for the Inverter transformer are driven.

[tn245]

Re: LG 42LT75 no backlight

U701 is FAN7382

[budm]

Re: LG 42LT75 no backlight

OK, here is the spec sheet for the FAN7382.
Since you have two boards, you can do the resistance testing (board removed from the chassis) on the IC using pin 4 as the ref point (black probe on pin 4) and read the resistance against the other 7 pins. Then put the board back in and take the DC voltage reading on the VCC pin 1 (using pin 4 as the ground ref pin).

[tn245]

Re: LG 42LT75 no backlight

Budm here are the measurements. I measured Vcc with the boards out of the chassis, but forced on with resistors.

     Faulty board         Working board

Pin 
1     Rises up to 155K      Rises to 164K
2     532K               530K
3     528K               529K
5     10K                10K
6     ~13M              ~12M
7     ~13M              ~12M
8     ~13M              ~16M

Vcc   14.27V             13.15V

The last three pin resistances fluctuated and seemed to change each time I went back to measure, but there was a significant difference between the readings on pin 8 between the boards.

[tn245]

Re: LG 42LT75 no backlight

There does seem to be a difference here but I'm not sure what to make of it. ?