The first step is to switch local control to OFF on the Roland RD300SX. The instructions for that can be found in the RD300SX manual (English version) on page 46. Your RD300SX should not make any sound when you play the keys now as you’ve disconnected the keyboard from the sound generator. Note that to use the RD300SX without MainStage you’ll need to switch it back ON again to reconnect the keyboard to the sound generator.
Then, at the CONCERT level in MainStage (this means that in the patch list on the left you’ve selected the orange folder with the guitar player at the very top) create a new channel strip. When the dialog box pops up choose External MIDI at the top. On the right side of the main portion of the window choose “Output: RD300SX Channel 1-16.” Then click create. You’ll probably get a warning that you are creating this channel strip at the concert level. Click Create.
Verify that playing the keyboard on the RD300SX once again makes sound. Select this new channel strip in the mixing board. Now in the tabs in the middle/bottom of the MainStage window choose MIDI Input. The right side now should have a list with just one item in it saying “SysEx” and “Filter”. Click the + to add a new item to the list. It defaults to “Pitch Bend”, but you want to change that to “11 Expression”. Then change the right side from “Unchanged” to “Filter”. Verify that the volume pedal no longer affects the RD300SX.
You’ll need to do this for each concert file that you want to use in this way.
I’ll mention that instead of doing this at the concert level you could do it in each patch in the list separately. This way you can do two additional things: 1. Leave out the channel strip on patches where you don’t want the RD300SX to sound at all. 2. Under the MIDI Output tab you could send patch changes to the RD300SX so it would automatically go to the desired sound for that patch rather than calling it up manually on the RD300SX panel.
Hope this helps,
--Kurt
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