VCA switch- Synthesizer Expander Module's VCA has only one exciting control, but it's important to understand. It's located in the middle, directly above the master Output knob. When set to Env 1 position, amplitude is controlled by envelope 1; this where you'll generally leave it when playing with a keyboard or sequencer controller. The On/Ext position latches the VCA open; this useful for drones (or times when you're too lazy to actually press down keys. We've all been there.)
The red LED to the left of the switch will glow when the VCA is open. We think it's a funny spot for an LED, but that's how the original is.