As mentioned detecting is not the easiest hobby in the world, but very rewarding once you have put the hard yards in to recognise good targets.
You also never stop learning, I have been detecting for a few years now, and I certainly don't profess to being an "expert", I still dig junk, and have good days and some real bad ones. Just have a look at my posts to see how much junk I manage to detect on every outing. I don't purposely dig junk, the fact is that VDI numbers and tones can still be deceiving, and can actually be good targets - especially with deeper targets.
The x-terra series of detectors can be a bit of a hand full to use as a learner, the multi tones can drive you nuts at times if set up wrong - basically information overload. Once you learn your tones properly, you will dig less junk, but not completely devoid of junk, that is just about impossible.
When you get home, grab several of the more common pieces of junk, and bury them in your yard. Do a few runs over them with the 705 and take note of the tones and VDI numbers, and maybe plant a few coins amongst them to see if you can accurately identify one target from the next. Foil, aluminium and bottle caps can have quite a sharp and abrupt tone, whereas coins tend to be a more mellow "rounded" tone. If when pinpointing the target and you find the target is quite wide, give it a miss, coins tend to have a reasonably small surface area, and pinpoint as such. Anything larger is likely to be a can, part of, or foil wrapper flat in the ground. Also try burying some coins on edge vs flat and take note of the tone and VDI, this can confuse the detector at times with varying tones, and incorrect VDI's to match.
As SA Bogan mentioned, the best way to learn is by running in all-metal, and dig targets till you are familiar with the tones and VDI numbers. Try detecting in some tot lots for coins, they can deliver some good finds at times, and less junk to deal with. Starting off detecting in areas riddled with junk will be demoralising, and hard to deal with. I had the exact same issue using the x-terra 305, enough to sell the detector and move to a single tone Tesoro Vaquero. That was the best move I have ever made, as it forced me to learn detecting by tones alone, whilst still retaining Tesoros excellent discrimination ability to cut out most junk, with coins still coming through nice and clear. The Tesoro Silver Umax would also make a great coin/jewellery detector.
The Ace series of Garrett detectors are also good units to learn on, they give off very prominent bell tones when over most coin targets, and you can dumb down the sensitivty enough not to see too much of the in ground junk. The Ace is mostly at home on playgrounds, dry sand, and general coin detecting. As you have probably seen, some of the guys on here, including myself, have had good success with the Ace250 with not too much detector time up our sleeves,
There are no real "easy" fixes or "hidden" secrets to detecting, good finds come with experience, knowledge, and plenty of research into the areas you plan to detect. The 705 will be fine once you come to grips with it, you just have to persevere.
