Im no expert at these, but the way I see it is this,
the mk1 as an older model worked very much like a metal detector, picking up the signal from the collar(either 8ft or 15ft) then it has a simple wheel control with depth's marked and on turning down, gives you a read out of depth of ferret, very simple design, simple to use, and simply made thus less bits to go wrong
now I only know what Ive read on the mk3, but it seems it has 2 switch setters, the first a search/locate mode, then a depth mode, this seems to me, to be the bit causing most confussion with people using the wrong mode