One problem that's been apparent on both my AX-990s was the incredibly poor video quality. Colours would be distorted and sometimes the TV would lose the video signal entirely. The culprit is the toggle switch on the rear that changes between MSX and Sega modes. Often just touching it lightly was enough to make the video signal dance and flicker. With only a small amount of work this can be fixed and perfect composite video is the result!
So what does the MSX/Sega switch actually do and why is it a problem? It's a six-throw dual-pole toggle switch and after so many years the contacts become corroded and fail to pass signals correctly. The reason such a large switch is needed is that it physically switches the red, green, blue, composite sync, and mono audio signals from the MSX and Sega chips into the CXA1145 composite video encoder chip. It also switches the +5V power supply between the Sega and MSX halves of the machine. Since the video signals are analogue this was probably cheaper than using an integrated circuit to handle the switching.
It's an easy replacement -- the most difficult bit of the whole operation was locating a compatible part. I found the "Toneluck PBN-S6A1.5-AAG Standard 6 Pole Changeover Switch" was a perfect fit. I managed to source them from Rapid for £0.93 each.
Fitting the new switch is a matter of breaking up the old switch (pliars work well here) and removing the through-hole pins. Then just solder the new switch in place. Make sure to keep the plastic cap from the end of the old switch! It just pushes onto the end of the toggle switch.