From Robert Mandara:
I built Andy Flind's "Magnum" detector back in 1984 and it still works just fine! Sometimes it has been slightly temperamental but I've never isolated the cause. It may just be a case of the detector responding to low batteries or not liking rechargable batteries. But when it's working it's a great machine!
You may like to note that the version of the Magnum that you have on your web site is not the same as, or as good as, the one I built. When Maplin Electronics produced instructions for making the Magnum (with Practical Electronic's permission) they also included part numbers for their ready-made circuit boards and there was a small additional board (I believe it was designed by Andy Flind) which makes the Magnum easier to use. The board makes it possible to use the pushbutton on the handle to switch between the search and discriminate modes and, in my opinion, was well worth adding. If you want, I can send you a photocopy of the Maplin version of the instructions. They're tatty but still usable. They also include some notes about possible problems constructors may face which were discovered after the PE article was published.
With the small additional board it can be difficult to know whether you are in search or discriminate mode. When the pushbutton is pressed the needle of the meter swings one way for discriminate and the other way for search - it's up to the user to learn which way it swings! If I were to make a modification to the Magnum it would be to add an LED indicator somehow to show that it was in discriminate mode. It is well worth using a good quality pushbutton on the front of the handle by the way! Poor contacts in the switch will cause the circuit to flit randomly between the modes rather than giving a decisive action.
I have not used my magnum a great deal - mainly because I like beachcombing but don't live near a beach! When I have been searching, on regularly searched beaches in the UK, I have been surprised by the depth of my finds and the apparent age of them. For example I have found many coins that had gone out of circulation, and were presumably lost, years before. Sadly I haven't found anything of great value but then I've not been looking in the right places!
I would be delighted to hear from other Magnum owners - especially if they have made good improvements to the design!