Franc Zabkar wrote:

I don't think the largest caps are your most likely culprits. Instead
I think that the one near R12 is probably the root cause of your
problems. As a general rule I always check the smaller electrolytics
on the primary side of switchmode PSUs (eg the one near R2) as these
often go high ESR. But as others have advised, change all the caps on
the supply secondary for good measure.

- Franc Zabkar

Thanks Franc.

If you have a minute spare I wonder if you would look at the second picture
again and tell me if you can work out what is next to the capacitor you
recommend I replace; the one near R12?

http://www.arcl40.dsl.pipex.com/roland/roland.html

I know it's difficult to make out but the power board is back in the
keyboard now until I can find a source of capacitors, so I can't take more
pictures yet. It looks to me like another capacitor but it's covered in
moulded black plastic with a pinched top. It looks to me like some kind of
sleeving to protect the component from heat, or interference maybe, as it's
located next to the transformer? I will remove the component when I change
the caps but at present I've no real idea what it is!

Any ideas?