Deben MK1 ferret finder

Hi im new here.
Does anyone have any knowledge of the workings of a deben mk1 ferret finder?? i have one of these and it has gone wrong, deben will not repair ait and want me to buy a new non compatable model - which i cannot afford. can anyone out there help me?? any information would be greatly beneficial and a circuit diagram a definate bonus.

Cheers for any help offered.

**broken link removed**

the trouble with ebay is;
1) dont know if he thing works before i buy it
2) I CANT AFFORD IT