MONO PA Hookup
 

Many keyboards have intelligent outputs. If you only plug a 1/4" plug into the Left output, the keyboard and the stereo effects will be collapsed into a MONO signal. If you plug a 1/4" plug into each Left and Right outputs, the keyboard keeps the 2 channels separate.

What do you do with synths that don't collapse the signals to MONO when connecting to MONO PA systems? Like the Access Virus TI

Through mixer. Both channels panned to center.