There's a few things you could try. You might need some adapter cables, but...

If you have a home stereo system with a line in (maybe to plug an iPod or something in), try plugging the line-outs from your 3500 to the line in of the stereo and see if you get both channels.

Also, on the computer side, maybe plug an iPod or some other stereo audio device into your sound card, load an audio recording program, and see if you get stereo. That can tell you if you have a problem with the sound card. I did have an old Soundblaster that had a loose connection on the line-in and would intermittently go in and out when recording. So it's possible. Have you tried jiggling the wire while the keyboard is playing to see if that might be it.

Again, you would need an adapter cable, but you could also try connecting from the headphone output of the 3500 into the line-in on the sound card and see if you get stereo there as well. That, again, would tell you if the problem is the sound card or not.

Anyway, there's a few of things to check. Hope it helps.

- John