Hey Muzz !
Think i have worked out my issue whats yours ? lol.
DetectorMods
The trouble with coils
6 August 2014
The most obvious sign of a bad coil is if you get a signal from wet grass or green vegetation, this indicates a break down of the shielding inside the coil, but it can also indicate that the lead to the detector coil has some broken strands in the outer shielding. The breakage is caused by overly tight cable glands cutting into the cable over a long period of time, this is mainly applicable to older style coils as the coil manufacturers these days are changing the way the cable mounts into the coil housing.
Nearly all coils have shielding made from Graphite or fine Carbon mixed into a carrier material to make a partially conductive paint, depending on the way the coils are made this paint is either applied inside a mould and an expanding foam surrounds the coil wire winding and then sticks to the paint on the mould or the paint is applied directly to the foam with a spray gun or paint brush. Some coils have the shielding done with paper cut outs painted with the shielding paint and glued to the foam and coil assembly. In all these coils there is a difference in the rate of thermal expansion of the coil housing and the foam core with the shielding paint, over time with thermal cycling, the two parts rub and can reduce the conductive paints effectiveness and cause ground balance issues and general coil noise.
I have included some pictures of coil cable cut by too much pressure on the cable gland. A coil that has had part of the bottom shielding paint worn away by rubbing against the coil housing, and a coil where the shielding on the side of the coil was never there in the first place, this would of sounded off on grass and greenery.
Most coils can be pulled apart and inspected for issues, most coil housings are made from ABS plastic and are glued together with MEK that melts the plastic parts together.