LG 42LE7500 TV Backlighting but no Picture

bwilliams60 · 01-31-2020, 07:47 PM

Good Evening All. I have a 42" LG TV that was given to me and my background is more automotive electronics so was hoping for some direction on this. I can get around the TV okay, just need pointing in the right direction. This TV starts up okay and backlighting comes on with no problem but there is absolutely no picture. I have ran an infrared monitor over the power supply and most areas that I think should get hot, are, but that is obviously no measure of anything so where should I start with this one?
It has two boards in it. Mainboard and Power Supply I am assuming. I don't have any pictures yet but if you need them, I can upload some later on.
Thank you in advance.

**nomoresonys** · 01-31-2020, 07:58 PM

Yep, throw up some good straight on pictures of all the boards and one of the whole back and how they are connected, try test number 3 below.

bwilliams60 · 01-31-2020, 08:19 PM

Okay here are the voltages I got with Power Off and then Power On
ON 0.00 VDC 3.25 VDC 20V 0.00 VDC 21.4 VDC
20V 0.00 VDC 21.40 VDC 20V 0.00 VDC 21.4 VDC
GND 0.00 VDC 0.00 VDC GND 0 0
GND 0.00 VDC 0.00 VDC GND 0 0
3.5V 3.48 VDC 3.40 VDC 3.5V 3.3 3.3
3.5V 3.48 VDC 3.40 VDC 3.5V 3.3 3.3
GND 0.00 VDC 0.00 VDC GND 0 0
GND 0.00 VDC 0.00 VDC V-SYNC 0 0
12V 0.00 VDC 12.28 VDC DRV-ON 0 2.9
12V 0.00 VDC 12.28 VDC SCLK 0 0
12V 0.00 VDC 12.28 VDC PWR DIM 0 3.2
SIN 0.00 VDC 0.02 VDC Error Out 0 0

Pictures to follow momentarily

bwilliams60 · 01-31-2020, 08:23 PM

Here are my pics. I suspect that 20V circuit may be a problem?

**tvtimmy** · 02-02-2020, 04:42 AM

The cpu chip is in need of a re-balling, or a quick reflow with a heat gun on low for about 45 seconds. Remove the big black heatsink, then heat it evenly.

bwilliams60 · 02-02-2020, 01:37 PM

Thank you @tvtimmy for your response. I have seen where people have heated their mainboards in an oven at 380 for ten minutes. What you are saying is to just heat the cpu? Is that the only part that is losing connection? I can do that with my hot air station. Any recommended temperature? I generally run my station at 225 for most things. Thoughts?

**tvtimmy** · 02-02-2020, 04:13 PM

Just heat up the chip under the black heatsink. A hot air station will work very well, only heat the chip . I put a towel under the board to prevent the surface mounted parts from falling off. Set the temp just over what it takes to melt leaded solder. Let it cool and try it out.

arbitarytwist · 02-19-2020, 11:45 PM

Did this fix your issue?

01-31-2020, 07:47 PM	#1
bwilliams60 Member Join Date: Aug 2014 City & State: London, Ontario My Country: Canada I'm a: Knowledge Seeker Posts: 99	LG 42LE7500 TV Backlighting but no Picture Good Evening All. I have a 42" LG TV that was given to me and my background is more automotive electronics so was hoping for some direction on this. I can get around the TV okay, just need pointing in the right direction. This TV starts up okay and backlighting comes on with no problem but there is absolutely no picture. I have ran an infrared monitor over the power supply and most areas that I think should get hot, are, but that is obviously no measure of anything so where should I start with this one? It has two boards in it. Mainboard and Power Supply I am assuming. I don't have any pictures yet but if you need them, I can upload some later on. Thank you in advance.

01-31-2020, 07:58 PM	#2
nomoresonys Badcaps Veteran Join Date: Jan 2013 City & State: midwest My Country: U.S. I'm a: Knowledge Seeker Posts: 5,895	Re: LG 42LE7500 TV Backlighting but no Picture Yep, throw up some good straight on pictures of all the boards and one of the whole back and how they are connected, try test number 3 below. __________________ 1) https://www.youtube.com/watch?v=4F2J1Dkf4Is HIGH VOLTAGE HAZARD demonstration. Video by Electronic Doberman. Always happy to help decent people save stuff from the landfill and I'm always open to a good stock tip, Thanks.

01-31-2020, 08:19 PM	#3
bwilliams60 Member Join Date: Aug 2014 City & State: London, Ontario My Country: Canada I'm a: Knowledge Seeker Posts: 99	Re: LG 42LE7500 TV Backlighting but no Picture Okay here are the voltages I got with Power Off and then Power On ON 0.00 VDC 3.25 VDC 20V 0.00 VDC 21.4 VDC 20V 0.00 VDC 21.40 VDC 20V 0.00 VDC 21.4 VDC GND 0.00 VDC 0.00 VDC GND 0 0 GND 0.00 VDC 0.00 VDC GND 0 0 3.5V 3.48 VDC 3.40 VDC 3.5V 3.3 3.3 3.5V 3.48 VDC 3.40 VDC 3.5V 3.3 3.3 GND 0.00 VDC 0.00 VDC GND 0 0 GND 0.00 VDC 0.00 VDC V-SYNC 0 0 12V 0.00 VDC 12.28 VDC DRV-ON 0 2.9 12V 0.00 VDC 12.28 VDC SCLK 0 0 12V 0.00 VDC 12.28 VDC PWR DIM 0 3.2 SIN 0.00 VDC 0.02 VDC Error Out 0 0 Pictures to follow momentarily Last edited by bwilliams60; 01-31-2020 at 09:00 PM..

02-02-2020, 04:42 AM	#5
tvtimmy Badcaps Veteran Join Date: Mar 2015 City & State: milwaukee,wi My Country: usa Line Voltage: 120VAC/60HZ I'm a: Knowledge Seeker Posts: 972	Re: LG 42LE7500 TV Backlighting but no Picture The cpu chip is in need of a re-balling, or a quick reflow with a heat gun on low for about 45 seconds. Remove the big black heatsink, then heat it evenly.

02-02-2020, 01:37 PM	#6
bwilliams60 Member Join Date: Aug 2014 City & State: London, Ontario My Country: Canada I'm a: Knowledge Seeker Posts: 99	Re: LG 42LE7500 TV Backlighting but no Picture Thank you @tvtimmy for your response. I have seen where people have heated their mainboards in an oven at 380 for ten minutes. What you are saying is to just heat the cpu? Is that the only part that is losing connection? I can do that with my hot air station. Any recommended temperature? I generally run my station at 225 for most things. Thoughts?

02-02-2020, 04:13 PM	#7
tvtimmy Badcaps Veteran Join Date: Mar 2015 City & State: milwaukee,wi My Country: usa Line Voltage: 120VAC/60HZ I'm a: Knowledge Seeker Posts: 972	Re: LG 42LE7500 TV Backlighting but no Picture Just heat up the chip under the black heatsink. A hot air station will work very well, only heat the chip . I put a towel under the board to prevent the surface mounted parts from falling off. Set the temp just over what it takes to melt leaded solder. Let it cool and try it out.

02-19-2020, 11:45 PM	#8
arbitarytwist New Member Join Date: Nov 2016 City & State: BC My Country: Canada I'm a: Knowledge Seeker Posts: 13	Re: LG 42LE7500 TV Backlighting but no Picture Did this fix your issue?

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