iRobot Roomba 620 repair, side brush not spinning, water damage

[Chungalin]

I was about to refuse this repair, because it's an appliance that I don't know at all, but the customer insisted and, after all, he said a new unit costs 350€, so it's something worth working for a few hours.

Side brush doesn't spin and customer already changed motor, to no avail.

I disassemble the device following a Youtube video. When reaching main PCB I see two spots with mild liquid damage. The most interesting spot is near the H-Bridge that controls side brush motor (2 pairs of SS8050 / SS8550 transistors and 4 diodes). Q74 is shorted and no other shorts detected. I replace this SS8550 and try again, still doesn't work. Check again the other 3 transistors and Q74's complementary, Q64, isn't shorted but it has a lower resistance reading than its neighbor. I replace this SS8050 and now it's fixed!

It seems that some water drop made its way to the PCB and caused a short circuit, damaging these two transistors. The PCB is fully protected with white paint and no traces were apparently damaged.

All motors are controlled with H-Bridges, 2SD882 / 2SB772 pairs for wheels and main brush, and SS8050 / SS8550 pairs for side brush.