In this particular mixer I found that the ADC chip, AK5390 VP, was faulty, and causing the channels to not work. I was able to follow the path of the signal to the inputs on the 5390 (pins 3,4, 25,26), and noted that it was arriving clean. However,I listened to the digital outputs of that 5390 and it was full of noise and distortion. Once I removed the circuit board and looked underneath, my suspicion that the 5390's were the culprits was confirmed by the fact that there were burn marks on the circuit board underneath of the suspect chips. I replaced the bad chips with sockets, so that new ones can be put in easily, or the unit can be used as is with only 12 inputs.
I suspect that dead and distorted channels on the 03D can often be attributed to this failure.
CS5390 is supposedly the same chip, and a working substitute for the AK5390-VP.
Hope this helps someone in a similar situation! The adc chips can be seen in this photo... notice the burn marks on the circuit board revealing how they have seen some heat.