RUNTK5351 TCON - defect analysis - ISL98602

The TCONs with the ISL DC/DC chip notoriously go bad. Sometimes they are fixable by swapping the chip, sometimes they are not. I had the chance to play with four broken boards. Two of them got a new ISL and still didn't work. However, I present you a few tips how to avoid fruitless work, because the main video chip may be measurably dead. Also I think I have measured the reference voltages that the ISL should produce.

The next image shows the voltages of a good ISL chip:

If the voltages are all there and there is still no image, the main chip is dead.

Normally though, the TCONs come with an ISL, which produces no voltages at all. Here are the tests you can do to asses whether it is worth changing:

  • Test 1: In diode mode, measure the breakthrough voltage of the 1.2V trace. It should be around 0.5V. If it is 0, forget the board.
  • Test 2: Attach a lab power supply with 1.2V and current limit around 100mA to the 1.2V supply. The main chip should draw about 0.01A. If not, it is broken.

Alas, I did not yet have a working board in my hand to know how much current the board is supposed to draw from the 12V line. The ones with the fixed ISL and no shorted main chip both pulled 0.39A. The main chip got pretty hot quickly. I guess this is not normal.

Changing the ISL

This is very difficult. I never managed to solder it properly with hot air only. It always took me an extra step with the soldering iron to get the solder to flow at the pins. I failed with a needle tip. It doesn not have enough heat capacity. Spade tips neither worked, even small ones, because they all were too clunky to reach the pins. The only tip that worked was the horse shoe with an excess amount of solder on it. A perfectly rectangular tip would be best. And lots of flux is required, of course.

Be extremely careful with the microscopic SMD parts around it, especially on the upper right corner (previous image). That one 0 Ohm resistor close to the edge gets pushed away easily.