LG 42LV355T Blue screen No picture

[notallbad]

Powers on, blue screen only. no picture, no sound, no menu.
Does respond to remote and keypad or at least the front led flashes when key pressed.
Stays on for sometime before turning off in stby.

Disconnect t-con makes no difference.
Remove t-con left ribbon cable no change.
Remove right t-con ribbon cable no change.

Chassis : LD11U
Power bd : LGP424T-11SPL
Main : EAX63972702(0)
Panel : T420HW08 V.1
T-Con : 46T03-COK T460HW03

Whilst tv powered on
0v - 0v
12v - 1.1v
12v - 2.4v
12v - 3.27
0v - 0v
0v - 0v
3.5v - 3.5v
3.5v - 3.5v
0v - 0v
0v - 0v
24v - 24.4v
3.4v - 24.4v

When tv is stby all voltages drop to 0v except the 4 lines with 3.5v.

Mainboard or fw?

[budm]

Re: LG 42LV355T Blue screen No picture

How about Voltage on both ends of the fuse on T-CON board?
Pictures of the T-CON?

[notallbad]

Re: LG 42LV355T Blue screen No picture

I did check the fuse for continuity on the T-con, its ok. Didn't measure voltage though! Nothing, 0v on both sides!!

Which one is the fuse? Small blue smd above 'VGL' marking? Assuming this is F102 12Vin.
This is shorted to chassis gnd even when the cables are disconnected.

[notallbad]

Re: LG 42LV355T Blue screen No picture

There is no fuse on F101

D109 (ON 034 LG.) reads as a short both ways

[budm]

Re: LG 42LV355T Blue screen No picture

There is a small device with 'D' printed on top sitting right below VGL test pad. Do you get 12V dc on each end on that device?

https://www.shopjimmy.com/auo-55-42t...-con-board.htm

[notallbad]

Re: LG 42LV355T Blue screen No picture

The SMD between C109 print and VGL print?

Thats the F102 12Vin with '0' printed on it, it is blue but can't really see that in the photo.

0v on both sides.

[budm]

Re: LG 42LV355T Blue screen No picture

Hmm, that protection diode D109 (ON 034 LG) Cathode should be connected to the output pad of the not-installed F101. Is it? And does it show 0 Ohms when check between Cathode of D109 and Gnd?
It looks like this T-CON board is made to be used with either F101 or F102.
I also need close up pictures around the LVDS connector of the main board to locate the switched MOSFET for the switched T-CON 12V to see if the 12V is present on the main board or not.

[notallbad]

Re: LG 42LV355T Blue screen No picture

Around lvds on main.
Thanks budm.

[notallbad]

Re: LG 42LV355T Blue screen No picture

Aha, on the reverse!

[notallbad]

Re: LG 42LV355T Blue screen No picture

D109 cathode side to gnd 1.6ohm
D109 anode side to gnd 0.7ohm
D109 cathode side to F101 0.7ohm (both sides)

These readings are without the main connected.

[notallbad]

Re: LG 42LV355T Blue screen No picture

Lifted the anode side of the diode D109 on the t-con, diode definately shorted.

So cathode side is connected to F101 (both sides even though theres actually no smd fuse there) and to F102.

Anode pcb pad of D109 (with diode lifted) reads 0 ohms to gnd.

On the main bd its L702 and Q409 burnt out. Nothing left of them on the top to identify them.

[budm]

Re: LG 42LV355T Blue screen No picture

So the protection Diode (Anode connected to GND, Cathode connected to +12VIN) shorted out which caused the switched MOSFET Q409 (P-CH) and inductor to burn up for the switched 12V for T-CON to burn up.

[notallbad]

Re: LG 42LV355T Blue screen No picture

Yep, pretty much sums it up, thanks.

Any idea whats needed to replace them.

[budm]

Re: LG 42LV355T Blue screen No picture

Since the shorts circuit on the T-CON is now gone when the protection diode is removed, so for testing purpose you should remove the burnt part from main board first then you can use may be 3 ~ 4A fuse to feed the 12V to the T-CON board by connecting the fuse between the output side of the inductor L702 and the Source pin of the Q409.
BTW, you should verify first after removing the burnt out part that you still have 12V on the Source solder pad.

[notallbad]

Re: LG 42LV355T Blue screen No picture

Ok. shall report back results.

[notallbad]

Re: LG 42LV355T Blue screen No picture

Yep, 12v still present and that gets it back up and running again.

Will an SK310A diode be ok for the T-con bd?

Thanks.

[budm]

Re: LG 42LV355T Blue screen No picture

Great! I cannot find the info on D109 to find out the spec to see if it is TVS diode or just Schottky diode.
Any luck reading the numbers on the burnt MOSFET? More likely 50V or higher and Vth of 3V or so, and able to handle at least 5A or so (According to your LCD panel spec, the T-CON uses 1.2A max, 5A inrush, see page 6: ) when fully turned on and will not exceed power rating which depends Rds of the MOSFET.
https://cdn.badcaps-static.com/pdfs/...0be94ea103.pdf

[notallbad]

Re: LG 42LV355T Blue screen No picture

Unfortunately not, damaged too badly.

[budm]

Re: LG 42LV355T Blue screen No picture

I update my post #17.

[budm]

Re: LG 42LV355T Blue screen No picture

Service manual: see PDF page 21 and 28 for the burnt out parts.