The typical image symptoms of a defect AS15 chip on TCON boards are:
- solarization - colors, especially darker ones are pink or green, for example
- faded colors
Many TVs with AUO panels built around 2009 have this defect.
First quick test: does it get hot? It must not. According to the datasheet, its idle current is 10-20 mA. That's not enough to heat up a chip. The last TCON I fixed reduced its total idle current by 150mA with a new chip.
Besides apparent overheating, there is a technique to verify the chip's performance.
Locate testpoints named VGA or VGMA, numbered 1 to 14 or up to 22. Measure those against ground. They must produce a consecutive sequence of decreasing voltage of around 15 down to 0. If there are "holes" or big jumps in between, the chip is defect.
However, the exact values vary between panels. There is no rule.
Also, locate the VGMAREF test point. A typical voltage level there is 15.6V. I had a couple of defect TCONs where VGMA0 measured slightly higher than 15.6V. Apart from that everything looked ok. This is not possible in a working configuration! The reason is the AS15 pulling too much current through the reference resistor ladder and so skewing the reference values.
Note that some TCONs have 0V holes in the testpoint sequence because they only produce 14 or 18 voltages for 22 channels. Those TCONs have pads for up to two 4xOPAs, which are the buffers for voltages #15 to #22. The panels are wired to pick the active outputs accordingly. You must trace the origin of the testpoint voltages whether they lead to the AS15 or to the OPAs.