So what's this project all about? Well, the MX10 has the ability to key out elements based on luminance ("luma key"). However, you cannot use the other video channel as the background - only a solid color. This is pretty stupid, but luckily, there is a relatively easy mod that's supposed to get one around this limitation. Having tried it, I can say that not only does it work, it actually adds a lot more creative potential to the device than I ever hoped for, becase the way it works is that the "White" channel is replaced with either input channel. This means that in addition to using it with the Superimpose effect (to get luma key functionality), you can use it with the Fade control as well. By setting the Fade control out channel to White, you can fade between the two input videos, and use the wipes at the same time! Of course, you can still fade/key to white by using the background color.
Alright, let's get moddin'! Cracking open the mixer reveals the circuit boards. Our interest is in the mixer board, which is in the center in the picture.
Our two targets are IC 6 and connector 4:
original instructions tell us to "cut the grey wire that runs to connector 4 - pin 7". Here's a closeup of the connector:
The grey wire is connected to the center pin of a SPDT switch. The other pins connect to pins 4 and 7 of IC 6:
I've rehosted the original instructions (originally hosted at http://x.sysx.org/vsv/MX10mod.html, which is now dead, and modified by LEVEL6 of leper.org, which also seems dead at the time of writing), as well as a series of pictures by vjforums member Kyle.