Phil's right.. a coil is a step-up transformer and the primary & secondary should be completely seperate. They have a bath of oil inside them that keeps the whole thing cool and this can degrade over time (more heat = faster degradation = more heat, vicious circle) which can cause part of it to melt which stops the thing working completely. You can also have parts of the coil short together, effectivley reducing the number of "loops" inside the coil, which reduces the maximum output.
