Original post by: Andre ,
Hi, I've just taken apart two different failed LED lamps. The typical failure mode on these as the array is normally a 3*8 or similar array and one or more diodes goes high resistance due to overheating or just plain bad design. The series drive chip detects this and shuts down, however sometimes the capacitor goes bad as well. Depending how vigilant your lamp manufacturer was this should just shut down or blow a fuse, but sometimes the circuit goes into runaway and blows all the diodes. I have also seen a similar failure in LED TVs, if you have a dim third of the picture then this is quite likely. Often changing the bad diode (darker than the rest) then testing will bring it back to life. With S**s**g LED TVs the board housing the bad LED can be replaced individually if you can find a scrap panel with good backlight unit(s). LEDs can vary between 3 and 9V Vf which can be determined by voltage testing with a 9V battery and 330 ohm resistor in series, and a voltmeter across diode under test, also handy for colour matching. Obviously if you're going to repair these the good old hob-reflow-at-75%-heat technique works well but YMMV, if metal core PCB this works brilliantly (no pun!) Best to use brand new emitters and lots of no clean flux when changing these as you normally get away with one reflow per diode. If you can't get new emitters its probably worth buying a cheap bulb and harvesting unused ones at a pinch.