Hi Chris,
Don't pretend to be an aficionado on LED TVs, but the panels seem to have an escalating failure rate.
As the LEDs are in series, the failure (o/c) of one LED will extinguish all LEDs in that chain, and as the return path for the LEDs is monitored for current drain, the full backlighting arrangement will fail.
A shorted LED may allow the backlighting to continue functioning but suspect it too will activate protection.
Your observations are correct. LED backlights don't always give "2 seconds to black", but can do. Of course a fried LED is a dead giveaway.
To check LED strings, I now use a Variac with a full bridge rectifier and filter capacitor. This gives me an adjustable DC supply. You need to figure out how the LED strings are wired, then test each string in turn by advancing the Variac towards the required DC volts, allowing 3V per LED and not exceeding the combined volts of each string. Seems to work.
My first effort was to whack the workshop variable DC power supply on the end of a string with no regard to current limiting. Result: one fried LED string. Live and learn. Bruce
