LG-50UH5500 flashes on for a moment but doesn't show a logo. Not LEDs?

I'm trying to diagnosis and repair my sister's TV. I've already read about the common issue with this and other LG TVs - bad LED strips, but don't want to make any investments just yet.

A quick visual inspection of the power board and main board didn't turn up bulging or leaking, and I stumbled on a few (more like a lot) of posts about the LEDs. After taking it apart and checking all of the LEDs individually, I came across a single bad one, which in the cases that I've seen is enough to cause the issue of turning on and then off. However… In all of those cases, they get an LG logo before the TV shuts down. I don't get a logo, so I'm curious as to whether or not there might be another issue besides the one bad LED.

Surely, as in their cases, the LED would have to be replaced, so that investment could potentially be made - but I'd rather test anything else that I can to get a more accurate representation of the cost of repair and any other potential issues before doing that.

My biggest problem is that before both testing and installing a guitar input jack and testing the LEDs today, I have zero experience with a multimeter or soldering iron. I have no problem with learning more by doing things hands-on, but in this circumstance, with no background or knowledge in the subject, I don't really know where to start with testing the boards.

Any help with that would be appreciated.