According to this fellow who lists a circuit-bent PSS-15 (same model as this but with a silver control panel), connecting part of the audio output to the keyboard matrix returns can re-trigger ...

This circuit bending hack takes advantage of ... All of those caterpillar feet are arranged in a keyboard matrix to detect button presses. After pulling out the oscilloscope for a bit of reverse ...

establishing the matrix circuit required for the keypresses to register. It’s a remarkable redesign of some fairly basic keyboard components, all sitting on top of the LCD screen. The 65-percent ...