Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Good to know, thanks for the explanation. I'm not planning to remove the heatsink now as the TV works fine so far. We'll see later.
I had the doubts about removing the heatsing therefore I haven't rather touched it, looks like it was a good choice.
kubajs
Member
Last Activity: 08-11-2020, 04:11 AM
Joined: 07-08-2020
Location: Prague
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
I know I should remove the heat sink but was just lazy, would do so in case the TV didn't turn on. We'll see, the TV works now and I haven't "fixed" it for a typical customer (it's a family customer and the loss was only on my side - the time I spent with that TV
+ one 680uF cap I replaced on the PSU before) so no worries. If it stops working again, I'll give it more appropriate treatment.
The ram modules were among the components I heated the most so you could be pretty close with the estimation....Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Hi guys, the happy end is here. Today I took a heat gun and heated the main chip and other smaller chips at 275° C (or 527° F). Not for very long and with heat sink on the main chip so there is a high probability the main chip wasn't the root of the issue as it highly probably hasn't reached the tin melting point (one side completely covered by the big heatsink so I doubt the tin below the chip could melt).
Therefore I believe there was some cold connection on one of the other smaller chips (or close resistors...Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Alright. Anyway the chances I fix this are already pretty small but I'll try my best. I'll wait a few days in case somebody comes with any good idea including the idea to throw the TV from the window 
.
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
I'll probably desolder the bigger caps first and measure them by ESR meter.Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
The TV main red led is flashing so backlight doesn't help to anything. I think the normal operation would be indicated by a blue light.
Regarding the chip preheating, It will be challenging to estimate how much to preheat it. But I'll try to do it incrementally, in the way you suggest, then check whether it helped and if not, I'll desolder the chip completely and then position it back on the board and resolder by the heat gun. Not sure if this will be possible since the pads are directly below the chip (I...Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
I have no backlight and no picture. But with forced backlight on I still see no picture. No audio could be a secondary problem. Probably the whole circuit is prevented to be on. I have 3.3V on error pin so I guess some part of the circuit is purposely disabled by protection circuit or something.
Maybe I should also do a picture check with not forced backlight? (Using a flashlight or some source or light).
Do you guys have any other idea what part of the circuit I should check? Voltage on majority of...Last edited by kubajs; 08-02-2020, 01:40 PM.Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
I Found some switch on the mainboard. When pressed it shortly, blue LED slightly flashed and then the TV went to standby mode.
I really have no clue here. Lack of experience so I'll probably have to give up at this point.Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Danyx: yeah, the last chance. There is not much to lose. It's quite old TV so I don't want to spend money for a new mainboard but I still don't want to give up. At least this is some good experience.
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
I forgot to mention one thing (I just remembered words of my mother-in-law ;-) (not my TV) ). She mentioned that last time when the TV still worked it was difficult to turn it on, tried to power it on several times until in finally worked. Maybe this situation occurred a few times before it stopped working completely. I suspected it might be a bad capacitor but couldn't find any. I should probably measure it but it's not that easy task to check SMD caps, especially when there are sa many caps.
Caps on the...Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
I don't get any sound. Connected antenna cable, tried to put volume up but no reaction. I was able to turn the forced backlight on and off by the remote control but I doubt it will help us a bit. But I haven't tried to force audio high - A-dim is low (0.34V). Should I give it a try?Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Completely no image, only backlight is on.Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Backlight is fully operational when forcing BL-ON pin on PSU using the method budm suggested @post #18.Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Here are some detailed close-ups in original non resized photos:
[url]https://photos.app.goo.gl/v8vkNmFQmjCD4JY8A[/url]
I'm going to try the test.
Zero voltage on the contacts marked in red when the main board is powered on: [url]https://photos.app.goo.gl/8A85oQty4MAHEJQr5[/url]
Checked all 12V pins still have correct voltage on the 24 pin connector on the mainboard (from PSU).Re: LG 42ld750 TV not turning on (3.... some detailedLast edited by kubajs; 08-01-2020, 07:14 AM.Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
Thank you guys and sorry for my late reply.
All supply 12V voltages connectors have approximately 12V on the PSU. I checked the wires connection leading from PSU on the mainboard as well.
Not sure if I can force 3.5V on inv on/off pin (potential damage to the mainboard?)
Attaching some images of the TV plus voltages on the connector from PSU to mainboard.
I haven't inspected the T-Con yet since it's inside the TV frame, I'll try to remove it out later. Looks like there are 2 separate...Leave a comment:
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Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)
I have a similar issue, LG 42LD750 stand-by LED is slowly flashing, cannot turn the TV on, no audio.
Voltage on power source pins is:
3.4V Error pin
3.34V P-DIM
3.33V P-DIM2
0.34V A-DIM
0.037V INV-On/Off
3.41V standby
3.24V Power On
20V 20.5V
12V 12.26
386V on mains 82uF capacitors
Looks like a failed main board, right? Is anybody able to determine based on these nubmer what I should check on the mainboard?
Of course...Leave a comment:
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