LG 47LV550W audio and back light works / no picture

[Masba]

Hi, decided to start a own thread on this one.

Things I've measured this far:

Voltages from power board (first picture) TV on:

Upper left connector:
LED+: 54v
N.C: 0v
VC_2A...2D: 11.5v - 13.3v
N.C: 0v
VC_1A...1D: 10.9v - 11.2v
N.C: 0v
LED+: 53.3v

Upper right connector:
LED+: 53.7v
N.C: 0v
VC_4A...4D: 10.9v - 11.6v
VC_3A...3D: 13.6v - 18.8v
N.C: 0v
LED+: 53.7v

Middle right connector:
REVERSE: 0v
N.C: 0v
SCLK: 0,08v
GND: 0v
SIN: 0,15v
SCL: 3,2v
SDA: 3,2v
VSYNC: 0,02v

Bottom right connector:
N.C: 0v | ERROR: 0,01v
12v: 12,04v | PWM: 1,2v
12v: 12,04v | N.C: 0v
12v: 12,04v | DRV-ON: 3,3v
GND: 0v | VSYNC: 0v
GND: 0v | GND: 0v
3,5v: 3,54v | 3,5v: 3,54v
3,5v: 3,54v | 3,5v: 3,54v
GND: 0v | GND: 0v
GND: 0v | GND: 0v
24v: 24,1v | 24v: 24,1v
PWR-ON: 3,3v | 24v: 24,1v

Next moving to TCON board (second picture). First I measured the fuse when TV off, it gave 1 ohm and 9,9v at both ends. Then I turned TV on and measured 9,72v at both ends of fuse and following from testpoints:

P_VDD: 0v
GMA1...9: 0,02v
GMA10...18: 0v
VCC: 3,28v
VDD: 0v
VGL: -1,55v
UC1: 0v
VGH: 0v
VST: -1,55v
VGH_EVEN: -1,55v
VGH_F: -1,55v
GCLK1...6: -1,55v
VCOM: 0,26v
Z_OUT: 0v
VCOMRFB: 0,26v
GIP_RST: -1,55v
VGL_1: -1,55v
VGH_ODD: -1,55v
VGH_R: -1,55v
E0: 0v
GST: 0v
MCLK: 0v
GCLK: 0v
VCORE: 0,99v
VIN: 9,72v
HVDD: 0v
SOE: 0v
VST_IN: 0v
OPT_N: 0v
VCOMLFB: 0,26v
PDL: 0,04v
H_CONV: 0v

Then as Caleb suggested in other thread I tried pulling the connectors while TV was on but didnt see any reaction on screen.

Last thing I tried was hooking it up with computer and changing images on screen. When I put up black image the back lights would dim and likewise on white image back lights amped up.

Earlier owner tried changing TCON board without success. Could it be that he had bad luck and the new part was DOA? Or could some other part be busted?

Kind regards,
Masba

[Masba]

Re: LG 47LV550W audio and back light works / no picture

All heatsinks warm up after awhile. TCON makes faint buzzing sound the minute I plug in mains, hearable under 10cm. No component warm up in TCON.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

Quess I'll order new TCON and main board just to rule out dead panel...

[budm]

Re: LG 47LV550W audio and back light works / no picture

What DC voltage do you have at either end of the SMD white fuse on the T-CON board?
"VIN: 9,72v" I expect it to be 12VDC.
What do you get if you have one of the two flat cables between the T-CON and the LCD panel connected one at a time? Do not disconnect or connect the cable when TV is plugged in.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

TV off: 9,9v at both ends. TV on: 9,7v both ends.

[budm]

Re: LG 47LV550W audio and back light works / no picture

The dc voltage feeding the T-CON is through a switched MOSFET which is turned on when the TV is on , and turned off when the TV is turned off.
9V does not sound right for the voltage for running the T-CON.
Can you see the LCD panel model number? We need to look up the spec. I see the LG DISPLAY sticker in the picture. At this point it sounds like the main board is not working properly.
Did you try disconnecting those two flat cables yet?

[Masba]

Re: LG 47LV550W audio and back light works / no picture

I it this one: LC470EUF (SD)(A1) ?

[budm]

Re: LG 47LV550W audio and back light works / no picture

http://wenku.baidu.com/view/c81f763310661ed9ad51f332

The T-CON runs on 12VDC

[Masba]

Re: LG 47LV550W audio and back light works / no picture

Do you mean the ones leaving from TCON to panel? Those disconnected I get 10,2v at both ends of fuse.

[budm]

Re: LG 47LV550W audio and back light works / no picture

Yes, those two white flat ribbon cables with big GREY(?) band at the end. 10.2V is still low.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

According to datasheet pins 48-51 are Power supply +12v to TCON. I toke main board off, here are pictures above and under the mentioned pins. Could the mosfet be one of those in the second picture? Any way to measure them?

BTW, I appreciate you all who spend your time helping strangers in distress!

[budm]

Re: LG 47LV550W audio and back light works / no picture

I cannot tell from the pictures, the 4 pins on the T-CON board are where the 12V is fed into, I cannot see well enough what they are connected to on the main board side. Can you see or mark it for me? you will have to sue the Ohm meter to check for <1 Ohms on one end of the furs to the other end of the connector on the main board to see where it goes.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

They connect to pins 48-51, marked with red circle. Tried measuring those four pins and got same results from all as from the fuse. That is 9,72v TV on, 9,9v TV off and 10,2v ribbon cables in TCON detached.

[budm]

Re: LG 47LV550W audio and back light works / no picture

OK, then there should be 3-PIN MOSFET close by that one of its pin will be connected to the pins on the connector, just start looking for <1 Ohm from those pins to the device.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

Part # Q507 gives 0,8 ohms. Marked in the picture. There are markings X72A and 21 on the part.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

Apparently it is this one if I read the schematics of SM right: https://cdn.badcaps-static.com/pdfs/...fa1ec3aa2d.pdf

[budm]

Re: LG 47LV550W audio and back light works / no picture

OK, lets take the DC Voltage readings on all 3 pins, at least one of them should have 12VDC.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

Yeah marked values in picture.

Top right: TV ON 12.03v/11.98v TV OFF
Bottom right: TV ON 9.58v/9.91v TV OFF
Middle left: TV ON 9.67v/9.91v TV OFF

[budm]

Re: LG 47LV550W audio and back light works / no picture

OK, looking at the picture of Q507, this is a P-CH MOSFET, the top pin is SOURCE (S) pin, the middle pin (on the left of the body) is the Drain (D) pin, the bottom pin is the GATE (G) pin.
So lets check the resistance between S and D, S and G, D and G.

[Masba]

Re: LG 47LV550W audio and back light works / no picture

S-D: 101ohms
S-G: 120ohms
D-G: 20ohms