Thanks for the suggestions.

I have a another PSU which supplies the appropriate voltage and current, but its plug is the wrong size and polarity so I was planning on rigging up and adapter so I can use it. (I've done this in the past successfully).

The root of the problem is the central pin in the socket which is rather thinner than most. Actually, I may just contact Roland and ask them to tell me the dimensions: They should know!

The thread you linked is certainly useful: I used the info in there previously when I had to work out the correct polarity. In a bizarre oversight, Roland neglected to stamp it on the back of the 707.

MDA.