Originally Posted by Stanton
Highly likely it's the Main Board; there was a major revision/replacement after October 2015 (I have one). If (you know) the panel is good, it might be worth the trouble.
No amount of lab equipment will help you diagnose the problem without a service manual, but if you really want to "fix" it, contact an authorized LG repair shop. Otherwise, you can replace the Main Board at your own risk (doesn't require much more than a few screw drivers).
Thank you so much for posting. Since I made the post, I did find a couple of resources. Posting them here in hopes they are useful for someone else:
The LG OLED 65EF9500 service manual available for free download: https://www.manualslib.com/manual/11...-65ef9500.html
These excellent slides on testing the power supply board: https://www.scribd.com/document/3522...esting-pdf-pdf
(note the slides are free to see but need to make (paid) account to download pdf)
I have not looked through the service manual in much detail. I started testing the power supply board per the second link. When I apply AC power to the TV, no click is heard. The power supply output on P201 only showed 3.5V going to the main board. I disconnected the connector at the main board and shorted the pins for PWR ON and 3.5V. This signal from the main board tells the power supply to switch on 12V and 24V outputs. With this jumper in place, when I apply AC power, the relay clicks ON and I measure 3.5V, 12V and 24V on P201. Getting somewhere now.
Points to me that the main board may be the culprit since its not sending the PWR ON signal back to the power supply. I still need to run through the remainder of the power supply test to make sure other outputs to the T-CON board are OK. I suspect they are though.
Unfortunately, I have not yet found a way to tell if the panel is working or not. I am of the mind to finish troubleshooting the power supply board. After I make sure that's fully functional, try to diagnose what may be wrong with the main board and if it can be fixed. Unfortunately, looks like the board is not in stock anywhere.
I came across a couple posts here where people had replaced the main boards without success. May they'd be willing to sell me theirs if they still have it.
Thank you so much again for posting @Stanton