Yamaha MTX Series Setup Manual page 75

NOTE
If you don't store the preset, alert number 61 will occur.
Up to this point, our settings use all of the microphones connected to MTX units of UNIT ID =01 and 02, but there
might be cases in which you want to use a different number of microphones. In such cases, you can limit the number of
microphones by turning off the channels of unused microphones in the "MAIN" screen.
Example 3) Using cascade mode to add MTX input channels (analog connection)
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MTX Setup Manual