Is not DC85, just the "form". This consist in an oscillator (search coil) and ferrite passive receiver, or instead the passive ferrite receiver a air coil as passive receiver.
Here inside. This use a BFO and the same coil of BFO for passive (long range). BFO just is for pinpointing the unburied object mixed in the sand. A switch disconnect the BFO but connect the BFO coil to the passive detector (long range). This BFO is extraordinary stable. This is the same that my avatar, except the color!
Regards
Esteban