The following is the recommended power adapter for the RC-300. 

Roland PSB-1U (PSB-120) 

http://shop.rolandus.com/p/ac-adapter-psb-1u-equivalent-replaces-acb-120-acf-120-ack-120-aci-120

Caution: The use of any other power adapter may cause malfunction and/or damage to the product.