How badly out of tune is it?
There could be several reasons.
1 - The previous owner might have changed the master tuning. It's unlikely to be this because it should change the entire instrument.
2 - The previous owner might have changed the key scaling. This can be in effect for just one part, so this is a likely candidate.
For both of the above, refer to the user manual around page 134/ The manual is available on this site at http://www.synthzone.com/midi/technics/kn5000/
or at my website (in my signature below).
3 - Maybe the pitch bend wheel is slightly moved or defective. Try playing about with it. If it is loose you might have to tape it in position.
I suppose also that there could be a detune effect being applied to one of the parts but this is a bit unlikely.