Would be worth tracing the connections from the 3 pin connector to the relevant chip to see what inputs they are connected to (not easy on 3-4 layer pcbs!!!) , then grabbing the datasheet of the relevant chip and what level they operate at. If there is a xtal, worth checking what frequency it is operating at. (and be careful of cheap meters as the buzzer could potentially blow the low voltage circuits, probably stating the obvious now!)
Great work guys, keep it up !