Vintage Synth Repair

hello, I have a problem with my jx-8p. i think one of the 6 voices has a problem with osc2. every 6th sound is playing osc1 with the correct vokume, osc2 is also playing but lower. and this only happens when psc2 is switched to sawtooth. when i switch to square, osc1+2 are equal and there�s no problem. so this should not be a problem of the osc? any idea where to start the search? thank you, regards, michael