Updated schematic!
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A common ground from pin 1 of the remote greatly improves reliability when the recorder is on AC power.
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Single-channel mode is sufficient for reliable operation. This makes it easier to use other parts for the digital pot.
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Previously, the schematic did not correspond to the picture of the breadboard. On the breadboard, the remote was incorrectly connected to terminals B and W and the code actually followed that. Now the picture is removed and code reflects the schematic so the default wiper values needed to be reversed.