i just bought a yamaha ps-55 some weeks ago for 5€, it looks very
 similar to the mp-1. It is also from '83, same colours, it has the same 
buttons, faders and the transposer knob way on the left. Off-course it 
has no printer lol.. but it has some funky sounds though(like music box,
 harpsichord,vibes,marimba,pianos) and some cool monophonic lead 
sounds to(chimes,fantasy,funny) and its stereo with stereo effects built in
(tremolo and chorus with variations)
if any one wants to take a closer loock at the Yamaha mp-1 i found an 
auction on Ebay with some good pics:
http://cgi.ebay.de/YAMAHA-MP-1-Porta-So ... .m63.l1177