LG LM3400 spontaneously cycles on then off continuously

Sep 24, 2018 9:41AM PDT

This is a 5 year-old TV. As soon as it is plugged in, it comes on with the LG splash screen, but then turns itself off after a few seconds, and then back on, over and over again.
I opened it up, looked for any obviously blown components, found none. I tried to determine if it was the power or main board. So I detached the main board from the power, and plugged the TV in. Obviously, the TV didn't come on, but the front LED power light did come on and then off repeatedly. So, convinced it was the power board, I ordered one for about 40 Euro. This didn't not solve the issue - it continued to do the same thing, but now the power LED doesn't even go on and off with the main board detached like before.
I can send the power board back for a refund (I pay the ~15 Euro shipping), but I am now debating whether to order the main board (70 Euro), as it seems to be the main suspect. I am reluctant to have it serviced, as I have already bought a new TV and they charge ridiculous rates here.
Is buying the main board worth trying? At worst, I'd be out the 30 some odd Euro for shipping both boards back. What are the chances that it is something else? Thanks in advance for any input.

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Clarification Request
The other usual.
Sep 24, 2018 11:24AM PDT

There is an LED strip that shorted out. You either find the connection to the LED strips and unplug for a test run (all the sets I've tried this on power up if the LEDs are unplugged) or you get out the (required) service manual.

A short in those LED strips does seem to lead to a rather common symptom of power cycling. Thankfully there is a lot of research on this (hint: Google it.)

Post was last edited on September 24, 2018 12:03 PM PDT

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Thanks - will try
Sep 24, 2018 11:56AM PDT

I have googled the "on/off issue" extensively, but have never seen anything about the LED strips - Thanks. I have found the service manual, but to be honest, I can't make heads or tails of it. I'll have another look, but I think that even if I do find it is an LED strip, I wouldn't know how to fix it. I'll google that too Wink So that probably leaves me with the eternal dilemma - "to service, or not to service". Thanks again.

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Here you go.
Sep 24, 2018 12:11PM PDT

Many instances of failed or shorted LEDs. You can choose to not believe me here but it's one of the usual things and EASY to diagnose then move on to other areas. The number of times we've seen shorted LEDs in the strips is "too often."

I can't claim this is it. The TV is not on my test bench. I can only give you ideas to work, dismiss or once in a while, find what it is.
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Seen This ?
Sep 24, 2018 9:46AM PDT
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Seen it - but thanks
Sep 24, 2018 11:57AM PDT

As per subject - I googled around a lot before coming here and yes I saw (and enjoyed) the video. Thanks.

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