Seeing how it's a trainee post it would be unfair to pose too many things medical related.
Basic electronic principles and circuit theory should be the order of the day. Mechanics cold include general engineering, hydraulics, motors, fluid mechanics etc but again should be fairly basic.
Electrical safety might be worth genning up on as are the standards which apply.
As a trainee a practical test should be basic and could involve circuit identification, soldering, test equipment usage.
You say you come from an electronic engineering background so your experience should be most useful.
Being aware of the type of equipment you will be expected to begin your career on will give you an advantage at interview and also demonstrate that you have taken an interest.
If you haven't already I would recommend you ask for an informal visit to where you will be working. This is normally accepted as a good idea when applying for any position.
Finally, good luck, sell yourself and mention that you have looked at sites like this, it can only help
