I am a retired electronics engineer trying to fix my black screen Toshiba LCD/LED 40" TV. I have watched various videos showing how to test individual LEDs by scratching the track on either side. I have tried using 3 AAA batteries in series with and without a 100 ohm resistor but cannot get any to light. Where am I going wrong? Would 3 AAAs blow them either way? Or is it the opposite problem - insufficient current? (I have swapped the VESTEL 17IPS20 board for one known good. Mine had a very brown look around the Driver circuit.)
Are you sure you're not trying to power a bunch of LEDs in series (i.e., a whole or partial column or row?) The LEDs' voltage drops will sum in series, so if you had a column of, say, 10 LEDs in series, each with a 2.5 V drop, you'd need at least a 10 * 2.5 V = 25 V source just to get them conducting.
Also, since LED brightness is current-controlled, you'd need to estimate the voltage left after all the LED diode drops and figure the resistor you'd need to deliver the required current.
For instance, if you have a 30 V source with the 10 LEDs above, you'd be left with
V_source - V_diode_drops_combined = V_leftover
30 V - 10 * 2.5 V = 5 V.
The LEDs might take around 20 mA apiece, and since they're all in series, they all get the same current, so:
V_leftover / I_led = R_current_limiting_res
5 V / 0.02 A = 250 Ω.
Also, these are diodes, which only conduct in one direction. Are you applying the source in the correct polarity?