Vintage Synth Repair

the problem im having is ....

when i turn the unit on..and play the lower "c" it plays 
fine ....UNTIL i play another note ....then it no longer plays a "c" 
it plays what ever note was last played... for instance...if i press 
high "a" then low "c" i hear the high "a" agin (instead of the low 
c)....it does this no matter WHAT key i play....low "c" will always 
repeat whatever note was played before it.

any suggestions on how to fix this????