You can play the chords without the rhythm (drums) running, but it won't sound very good. You can also set the Syncro-stop to only play the accomp. when you hold the keys down. I believe on the 400 that this is accomplished by turning the "memory" or "chord hold" button off. Haven't looked at one in a long time, but I had a PSR500 that was very similar.
DonM
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DonM