LED Backlight Repair

A kind lady on Freegle gave me a load of broken electronic items including a 39-inch  ISIS brand LCD television. It turned out this was another Vestel rebranded chassis.

I rigged it up to a DVD player and used the bright torch trick to check that there was an active picture, but no backlight. There was about 120 volts being delivered to the backlight and a bit of research online suggested at least 1 of the LEDs might have gone open-circuit.

I dismantled the entire set as carefully as possible and found 5 strips of 9 LEDs, all 45 wired in series. This requires a voltage of about 135 volts, but means the current can be kept low and constant. With the strips separated I was able to apply about 25 volts to each strip. 2 strips were fine, 2 were illuminating, but had a dead LED each (LEDs had shorted out) and the last was completely dead meaning an open-circuit LED.

On the bench I was able to identify the open-circuit LED. I had some salvaged LED strips from a television that had suffered a cracked screen, but quickly realised that desoldering surface-mount LEDs, even using the hot air iron, was going to be impossible.

I hit on the idea of cutting down the salvage strips to pieces with a single LED. Then I ground the broken LEDs off the strips in the television and soldered the salvage pieces over the top of the gaps. A bit of an ignorant repair, but it worked.

Reassembly took a little while being careful not to bust the LCD panel connections which can be delicate, but the set is working fine with good even backlighting across the screen.