LG 42LS345Y Backlight with no image

[s_henya]

Hi,

i found this set today,

powers on with backlight but no image.

tested voltages on psu with mainboard connected.

got 12v, 3.5v, and 24v.

got 305v on main filter caps.

on the tcon the fuses are good.

im guessing either the mainboard or the tcon is bad.

i have a DSO5102P oscilloscope, just need to know how to test them.

[budm]

Re: LG 42LS345Y Backlight with no image

"on the tcon the fuses are good." So you do have 12VDC on the fuse?
Can we see the pictures of the power supply board?

[s_henya]

Re: LG 42LS345Y Backlight with no image

YES got 11.95v on both sides of the main fuse of the tcon board.

[budm]

Re: LG 42LS345Y Backlight with no image

YES got 11.95v on both sides of the main fuse of the tcon board.

Good, did you also try having one cable between the T-CON and the LCD panel connected one at a time to see what you will get?

[s_henya]

Re: LG 42LS345Y Backlight with no image

with the right side disconnected got no diffrence still backlight with no image.

BUT with the left side disconnected got half screen white and on the other strips- i can take a picture if that helps.

[s_henya]

Re: LG 42LS345Y Backlight with no image

any ideas?

[budm]

Re: LG 42LS345Y Backlight with no image

Remove both cables and start testing the DC Voltage at the test points on the T-CON board, there are many test points (copper pads) with names next to them, I.E. I can see VGL, VG, VDD, etc.

[s_henya]

Re: LG 42LS345Y Backlight with no image

VDD 16.71
VCC18 1.807
VIN 12.04
HVDD 8.35
VCORE 1.780

I have no refrence to compare the results so i don't know if the voltages are correct.

[budm]

Re: LG 42LS345Y Backlight with no image

How about the VGL, and VGH? These two voltages are used by the driver ICs on the panel.

[s_henya]

Re: LG 42LS345Y Backlight with no image

VCOM 7.34
VCMOR_FB 7.34
GID_RST (-4.91)
VST (-4.93)
VCL_1 (-4.99)
VGH_EVEN 26.97
VGH_ODD 26.97
VGH_F 26.97
VGH_R (-4.99)
GCLK1 7.4
GCLK5 7.4
GCLK4 7.4
GCLK3 7.4
GCLK2 7.4
GCLK1 7.4

don't know if thats good or bad

[budm]

Re: LG 42LS345Y Backlight with no image

VGH is fine, not sure about the VGL should be > negative than that or not.
Example of the IC for generating VGH, VGL. Look at the application circuit.
You may have to locate the similar IC on the T-CON board.
I hope TOM66 will chime in.
Example of IC.
https://cdn.badcaps-static.com/pdfs/...12ef5e563a.pdf

[s_henya]

Re: LG 42LS345Y Backlight with no image

The IC i have is TPS65178A

[s_henya]

Re: LG 42LS345Y Backlight with no image

because i have no additional knowlage of what is the solution,

shuld i gamble and order a used tcon?

[budm]

Re: LG 42LS345Y Backlight with no image

If it is cheap enough.

[fzabkar]

Re: LG 42LS345Y Backlight with no image

This TV uses the same TPS65178 IC:
ftp://helpedia.com/pub/temp/datashee...E_47LM6200-UE_[SM].pdf

According to TI's short form datasheet, the TPS65178 IC combines the TPS65168 and the BUF08630 ICs:
https://cdn.badcaps-static.com/pdfs/...7db0781811.pdf

Here is the datasheet for the BUF08630 which has an application circuit that includes the TPS65168:
https://cdn.badcaps-static.com/pdfs/...746ad8e194.pdf

The information is in dribs and drabs, but ISTM that there should be a VCC (aka VIO) voltage of 3.3V at the L2 coil and adjacent capacitors.

The Vcore voltage in the examples is 1.0V, so the OP's measured voltage of 1.78V seems out of place. Of course the subject T-Con PCB could be using an MCU which requires this higher Vcore, but it's something I would check anyway. The other voltages appear to be in the ballpark.

[s_henya]

Re: LG 42LS345Y Backlight with no image

i hace ordered a new board from Aliexpress for 14$.
all i have to do is wait until it will arrive from china to israel.

i will update as soon as i get it.

[s_henya]

Re: LG 42LS345Y Backlight with no image

i got the tcon but got the same outcome, light with no image.

if i take the left flat cable out i get half white screen and half strips.

i guess this means that the mainboard is the fault?

[s_henya]

Re: LG 42LS345Y Backlight with no image

Sorry to update, i got the mainboard today.

The fault is the same,

the TV responds to the remote, i have sound and backlight but no image.

If i replaced both the mainboard and tcon board.

I guess this means that the panel itself is bad?

[s_henya]

Re: LG 42LS345Y Backlight with no image

any more ideas before i scrap it?

[zwarder]

Re: LG 42LS345Y Backlight with no image

https://www.badcaps.net/forum/showthread.php?t=37823

I have the same problem this thread may help