hi SverreH,
space,the final frontier..and welcome to theremin world, the place you get picked up by friendly people and served with all about your theremin-issues.
theremin tuning is a bit difficult. and can give a lot of troubles. especially when you already turned a bit here and a bit there too look what happens. bad idea. but fear not.
therethereminworld is full of information about it. you simply look to that nice "search" field (top left),type something "etherwave, tuning, coils", press search..voila! guess you have something to read until next eastern ! but don't ignore ALL warnings.:http://www.thereminworld.com/Forums/T/27704/is-this-how-it-should-be-tuned
so you're not the first one with troubles. it's electric and therefore for many people difficult to understand.
but: do you have the manual? and followed all the steps? i don't speak english either and have sometimes difficulties to unterstand whats what in manuals. it's almost all in there. read it carefully. but: IF you bought your theremin second hand you can simply follow this : http://www.wolinlabs.com/blog/etherwave.tune.html .but read it first. i used a iphone app for the 3.8kHz (3800Hz) so you can hear that noise constantly. this helped me personally to get back in reasonable range. you will see, when you're more familiar with that most incredible instrument, that finer tuning can be done. be careful with it. it's precious.
hope that helped and cheer up it's YOUR instrument. stay tuned on thereminworld.