LG 60LB7100 Intermittent No Backlighting

[Steveakus]

Any help and input much appreciated, Its been a tough one for me to diagnose so any help there is also welcomed.

So I have an LG 60LB7100-UT and it will occasionally loose backlighting, I can shine a flashlight at the screen and see the picture perfectly, so I have tried to catch it to diagnose and so far all my test readings are as follows, but also these readings so far are without failure:

(I will update as soon as I get it to fail again as it is getting more frequent )

The following readings are the TV staying on for 20min after I took the back off during a failure:

VC6 - .925
VC5 - .920
VC4 - 2.08
VC3 - .926
VC2 - .930
VC1 - .928

LED 1-6 where all 68.9

I did see LD701 blink 4 times (I dont know what that code means ? Does that mean the 4th set of LED's with the higher reading of 2.08 is bad ?) but I didn't have my MultiMeter ready to test any voltages and by the time I set up and plugged the TV back in and it has worked since so far.

EDIT:

After 4 hours I finally caught it acting up...... here are the readings from that :

Initial ON - LED Out Failure (about 5 seconds after turning on)
VC6 - 11.5 - 8.80
VC5 - 11.5 - 9.09
VC4 - 11.5 - 12.98
VC3 - 11.5 - 9.08
VC2 - 11.5 - 8.78
VC1 - 11.5 - 9.13

[dick_barton]

Re: LG 60LB7100 Intermittent No Backlighting

Any sharp focused pictures of the boards?

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

If different pics are needed please let me know, thanks

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

These may be better pics?

[dick_barton]

Re: LG 60LB7100 Intermittent No Backlighting

The pictures are not focused well so I can't quite make it out but it looks like the circuit board legend says output (leds) 56.3V at 1.2Amp.
Could you take readings as given below when the backlight is on and when they go out.


These are readings from an earlier thread for An LG 60LB7100-UT
Backlights off:-

SCLK / .086
Vsync / .011
Sin / 3.19
P-Dim1 / 2.44
P-Dim2 / 1.735
Dry on / 3.128
Pwr on / 3.14
24v , 12v , 3.5v working

Voltages to backlights connector
Led + \ 55.5v
VC1 / 9.96v
Led + \ 55.5
VC2 / 9.05
Led + \ 55.5
VC3 / 11.9
Led + \ 55.5
VC4 / 8.82
Led + \ 55.5
VC5 / 9.3
Led + \ 55.5
VC6 / 9.3


Backlights on
Ok so running tv with no picture for almost an hour then turning it off then back on I get a picture back . Here are voltages that are different when it works .
Sin / no picture 3.189v - with a pic .128v
P-Dim2 / no picture 1.735v -with a pic 3.53v

On the led output connector :
Led+'s \ when not working 55.5v when has a pic 72v
VC 1 thru 6 / when not working voltages range from 9v to 11.9 - when has a pic they range from 9v to 18v

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

Here is close up of that legend

And a pic from ShopJimmy of the same model board just for clarity. I cant seem to get a good enough pic.

[dick_barton]

Re: LG 60LB7100 Intermittent No Backlighting

The leds should have 69.3V when running but it would be worth your while taking the measurement as above when backlights on and then when they go off.

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

The leds should have 69.3V when running but it would be worth your while taking the measurement as above when backlights on and then when they go off.

Yes, I do have that as well

LED 1-6 ON = 69.0

LED 1-6 OFF = 55.6

[dick_barton]

Re: LG 60LB7100 Intermittent No Backlighting

What about the following pins when backlight on then off

SCLK
Vsync
Sin
P-Dim1
P-Dim2
Dry on
Pwr on

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

What about the following pins when backlight on then off

SCLK
Vsync
Sin
P-Dim1
P-Dim2
Dry on
Pwr on

_______ on ________ off
SCLK = 1.0 _____ 87.3
Vsync= .9 _____ 11.2
Sin = .9 _____ 3.27
P-Dim1= 2.66 _____3.28
P-Dim2= 1.76 _____1.75
Dry on = 3.13 _____3.13
Pwr on = 3.41 _____3.41

[dskall]

Re: LG 60LB7100 Intermittent No Backlighting

Do you notice any dark spots on screen when it is working?

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

Do you notice any dark spots on screen when it is working?

No looks great when working

[dskall]

Re: LG 60LB7100 Intermittent No Backlighting

Looks like vc4 string has problem.

[budm]

Re: LG 60LB7100 Intermittent No Backlighting

It looks like VC4 string may have leaky/shorted LEDs.
Try turning down the backlights to as low as possible to see if the backlights will stay on longer than 20 minutes.

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

It looks like VC4 string may have leaky/shorted LEDs.
Try turning down the backlights to as low as possible to see if the backlights will stay on longer than 20 minutes.

How hard are these to replace ? also not finding parts readily available anywhere ?

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

Also yes that seemed to help by turning the backlight down to minimum ... so you still think the VC4 LED String is the main culprit ?

[Steveakus]

Re: LG 60LB7100 Intermittent No Backlighting

stayed on for almost 2 hours after turning the backlight down to minimum, but now shuts off every few minutes after turning back on.