RTC problems

Can't seem to get the return to cut to work properly on our 23 .IT seems to work on the straight head but when I put the pickup head on I can adjust it so the little arrow shows then the head runs up about a foot . Even with the straight head on it never seems to want to lower to the same height that well . have changed the potentiometer with another one and same thing . And where is the control module located as I've got another combine that I can use to switch parts . Any ideas ???